1. ATOMS Ancient Greece to Now

2. DEFINED Atom (n.)- A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons. Atom (n.)- A unit of matter, the smallest unit of an element, consisting of a dense, central, positively charged nucleus surrounded by a system of electrons, equal in number to the number of nuclear protons, and characteristically remaining undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.

3. THE GREEK ATOMISTS The word "atom" comes from the Greek "atomos" and signifies "indivisible" or “Not to be cut”. The word "atom" comes from the Greek "atomos" and signifies "indivisible" or “Not to be cut”.

4. THE GREEK ATOMISTS Leucippus of Milet invented this notion in 420 B.C. His disciple, Democritus of Abdere (around 460-370 BC.), asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Leucippus of Milet invented this notion in 420 B.C. His disciple, Democritus of Abdere (around 460-370 BC.), asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further?

5. THE GREEK ATOMISTS Democritus thought that it ended at some point, a smallest possible bit of matter. He explained that matter was made up of particles in perpetual motion and endowed with ideal qualities: Democritus thought that it ended at some point, a smallest possible bit of matter. He explained that matter was made up of particles in perpetual motion and endowed with ideal qualities:

6. Democritus’ Atomic Theory  invisible because of their extremely small size  invisible because of their extremely small size  indivisible as their name indicates  indivisible as their name indicates  solid (no void inside)  solid (no void inside)

7. Democritus’ Atomic Theory  eternal because they are perfect  surrounded by an empty space (to explain their movement and changes in density)  having an infinite number of shapes (to explain the diversity observed in nature)

8. End of Atoms??? Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. Unfortunately, the atomic ideas of Democritus had no lasting effects on other Greek philosophers, including Aristotle. Aristotle dismissed the atomic idea as worthless. Aristotle dismissed the atomic idea as worthless.

9. Aristotle Aristotle believed that matter was made up of only four elements (the Four Element Theory) fire, air, earth, and water. People considered Aristotle’s opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also Aristotle believed that matter was made up of only four elements (the Four Element Theory) fire, air, earth, and water. People considered Aristotle’s opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also

10. Aristotle For more 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter. For more 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter.

11. THE ALCHEMY OF THE MIDDLE AGES Originating in the Middle Ages, alchemy was born from the progress of metallurgy and from the inadequacy of the theory of the 4 elements for representing the diversity of matter. Originating in the Middle Ages, alchemy was born from the progress of metallurgy and from the inadequacy of the theory of the 4 elements for representing the diversity of matter.

12. THE ALCHEMY OF THE MIDDLE AGES The "grand plan" of alchemy was to achieve the transmutation of lowly metals (like copper) into "noble" metals such as gold. Without doubt because the success of such "Grand Works" (transmutation) opened up prospects of wealth and power, the activities of alchemists were surrounded by secrecy and were performed using extremely ancient processes of the esoteric and of the occult. The "grand plan" of alchemy was to achieve the transmutation of lowly metals (like copper) into "noble" metals such as gold. Without doubt because the success of such "Grand Works" (transmutation) opened up prospects of wealth and power, the activities of alchemists were surrounded by secrecy and were performed using extremely ancient processes of the esoteric and of the occult.

13. THE ALCHEMY OF THE MIDDLE AGES In spite of their esoteric beliefs, alchemists developed the observation, experimentation, measurement and classification of the elements: alchemy is therefore a respectable precursor of chemistry. In spite of their esoteric beliefs, alchemists developed the observation, experimentation, measurement and classification of the elements: alchemy is therefore a respectable precursor of chemistry.

14. THE ALCHEMY OF THE MIDDLE AGES Anyway, don't forget that Newton was adept at alchemy and that today's physics has turned the old dream of transmutation into reality by transforming certain atoms into other atoms Anyway, don't forget that Newton was adept at alchemy and that today's physics has turned the old dream of transmutation into reality by transforming certain atoms into other atoms

15. 1803 John Dalton - first modern atomic theory 1799 Joseph Louis Proust - Law of Definite Proportions (matter combines in a definite proportion consistently) 1799 Joseph Louis Proust - Law of Definite Proportions (matter combines in a definite proportion consistently) Dalton - Law of Multiple Proportions (matter combines in small, fixed, whole number ratios) Dalton - Law of Multiple Proportions (matter combines in small, fixed, whole number ratios)

16. 1803 John Dalton - first modern atomic theory Dalton applied these ideas to form his "atomic theory" Dalton applied these ideas to form his "atomic theory" *Atoms are small, indivisible spheres *Atoms are small, indivisible spheres *Atoms of a given element are identical *Atoms of a given element are identical

17. 1803 John Dalton - first modern atomic theory *Atoms cannot be created, destroyed or transformed *Atoms cannot be created, destroyed or transformed *Compounds are the results of small whole number ratios of atoms *Compounds are the results of small whole number ratios of atoms *Relative numbers and kinds of atoms in a compound are constant *Relative numbers and kinds of atoms in a compound are constant

18. DISCOVERY OF NATURAL RADIATION On November 8, 1895, at the University of Würzburg, Wilhelm Roentgen’s attention was drawn to a glowing fluorescent screen on a nearby table. On November 8, 1895, at the University of Würzburg, Wilhelm Roentgen’s attention was drawn to a glowing fluorescent screen on a nearby table.

19. DISCOVERY OF NATURAL RADIATION Roentgen immediately determined that the fluorescence was caused by invisible rays originating from the partially evacuated glass Hittorf-Crookes tube he was using to study cathode rays (i.e., electrons). Roentgen immediately determined that the fluorescence was caused by invisible rays originating from the partially evacuated glass Hittorf-Crookes tube he was using to study cathode rays (i.e., electrons).

20. DISCOVERY OF NATURAL RADIATION Surprisingly, these mysterious rays penetrated the opaque black paper wrapped around the tube. Roentgen had discovered X- rays, a momentous event that instantly revolutionized the field of physics and medicine. Surprisingly, these mysterious rays penetrated the opaque black paper wrapped around the tube. Roentgen had discovered X- rays, a momentous event that instantly revolutionized the field of physics and medicine.

21. DISCOVERY OF NATURAL RADIATION However, prior to his first formal correspondence to the University Physical- Medical Society, Roentgen spent two months thoroughly investigating the properties of X rays. For his discovery, Roentgen received the first Nobel Prize in physics in 1901. However, prior to his first formal correspondence to the University Physical- Medical Society, Roentgen spent two months thoroughly investigating the properties of X rays. For his discovery, Roentgen received the first Nobel Prize in physics in 1901.

22. DISCOVERY OF NATURAL RADIATION He rejected a title (i.e., von Roentgen) that would have provided entry into the German nobility, and donated the money he received from the Nobel Prize to his University. He rejected a title (i.e., von Roentgen) that would have provided entry into the German nobility, and donated the money he received from the Nobel Prize to his University.

23. DISCOVERY OF NATURAL RADIATION Roentgen did accept the honorary degree of Doctor of Medicine offered to him by the medical faculty of his own University of Würzburg. Roentgen did accept the honorary degree of Doctor of Medicine offered to him by the medical faculty of his own University of Würzburg. However, he refused to take out any patents in order that the world could freely benefit from his work. At the time of his death, Roentgen was nearly bankrupt from the inflation that followed WW I. However, he refused to take out any patents in order that the world could freely benefit from his work. At the time of his death, Roentgen was nearly bankrupt from the inflation that followed WW I.

24. DISCOVERY OF NATURAL RADIATION One day in 1896, Henri Becquerel (by chance according to legend?) arranged in his cupboard, a packet of uranium salt beside an unexposed photographic plate. Several days later, he took out the plate and developed it. One day in 1896, Henri Becquerel (by chance according to legend?) arranged in his cupboard, a packet of uranium salt beside an unexposed photographic plate. Several days later, he took out the plate and developed it.

25. DISCOVERY OF NATURAL RADIATION To his surprise, he noticed that the photographic plate had been exposed without having been exposed to the light. Having repeated this experiment, he concluded that Uranium spontaneously emits what he called "uranic rays". To his surprise, he noticed that the photographic plate had been exposed without having been exposed to the light. Having repeated this experiment, he concluded that Uranium spontaneously emits what he called "uranic rays".

26. DISCOVERY OF NATURAL RADIATION Marie and Pierre Curie, together, began investigating the phenomenon of radioactivity recently discovered in uranium ore, pitchblende. In 1898, Marie Curie discovered after chemical extraction of uranium from the ore, the residual material was more "active" than the pure uranium. Marie and Pierre Curie, together, began investigating the phenomenon of radioactivity recently discovered in uranium ore, pitchblende. In 1898, Marie Curie discovered after chemical extraction of uranium from the ore, the residual material was more "active" than the pure uranium.

27. DISCOVERY OF NATURAL RADIATION Pitchblende emits more radiation than uranium itself. She deduced that this ore contains, in very small quantities, one or more elements much more active that uranium. Pitchblende emits more radiation than uranium itself. She deduced that this ore contains, in very small quantities, one or more elements much more active that uranium.

28. DISCOVERY OF NATURAL RADIATION With the assistance of her husband Pierre Curie (shortly after his marriage to Marie in 1895, Pierre subjugated his research to her interests.) and after two years of effort, she arrived at isolating two new elements: Polonium (named thus in tribute to her homeland) and Radium. With the assistance of her husband Pierre Curie (shortly after his marriage to Marie in 1895, Pierre subjugated his research to her interests.) and after two years of effort, she arrived at isolating two new elements: Polonium (named thus in tribute to her homeland) and Radium. It took four more years of processing tons of ore under oppressive conditions to isolate enough of each element to determine its chemical properties. It took four more years of processing tons of ore under oppressive conditions to isolate enough of each element to determine its chemical properties.

29. DISCOVERY OF NATURAL RADIATION Although Henri Becquerel discovered the phenomenon, Marie coined the term “radioactivity”. For their work on radioactivity, the Curies were awarded the 1903 Nobel Prize in physics. Although Henri Becquerel discovered the phenomenon, Marie coined the term “radioactivity”. For their work on radioactivity, the Curies were awarded the 1903 Nobel Prize in physics. Marie was awarded the 1911 Nobel Prize in chemistry for her discoveries of radium and polonium, thus becoming the first person to receive two Nobel Prizes. Marie was awarded the 1911 Nobel Prize in chemistry for her discoveries of radium and polonium, thus becoming the first person to receive two Nobel Prizes. For the remainder of her life she tirelessly investigated and promoted the use of radium as a treatment for cancer. For the remainder of her life she tirelessly investigated and promoted the use of radium as a treatment for cancer.

31. DISCOVERY OF THE ELECTRON In 1897, J.J. Thompson discovered the first component part of the atom: the electron, a particle with a negative electric charge. In 1897, J.J. Thompson discovered the first component part of the atom: the electron, a particle with a negative electric charge.

32. DISCOVERY OF THE ELECTRON In 1904, he proposed an initial model of an atom, since nicknamed "the plum pudding model". He imagined the atom as a sphere full of an electrically positive substance mixed with negative electron "like the plums in a pudding"

33. QUANTA In 1900 Max Planck, a professor of theoretical physics in Berlin showed that when you vibrate atoms strong enough, such as when you heat an object until it glows, it emits radiation in separate bursts of energy. He called these energy bursts quanta. In 1900 Max Planck, a professor of theoretical physics in Berlin showed that when you vibrate atoms strong enough, such as when you heat an object until it glows, it emits radiation in separate bursts of energy. He called these energy bursts quanta.

34. QUANTA Physicists at the time thought that light consisted of waves. But the quanta behaved like particles and, in turn, these particles of light received the name photons. Atoms not only emit photons, but they can also absorb them. Physicists at the time thought that light consisted of waves. But the quanta behaved like particles and, in turn, these particles of light received the name photons. Atoms not only emit photons, but they can also absorb them.

35. QUANTA Any list of the greatest thinkers in history contains the name of the brilliant physicist Albert Einstein. His theories of relativity led to entirely new ways of thinking about time, space, matter, energy, and gravity. Einstein won his only Nobel Prize for physics in 1908 for discovering that light absorption can release electrons from atoms. Any list of the greatest thinkers in history contains the name of the brilliant physicist Albert Einstein. His theories of relativity led to entirely new ways of thinking about time, space, matter, energy, and gravity. Einstein won his only Nobel Prize for physics in 1908 for discovering that light absorption can release electrons from atoms.

36. QUANTA This phenomenon has the name “photoelectric effect”. A heated controversy occurred for many years on deciding whether light consisted of waves or particles. The evidence appeared strong for both cases. This phenomenon has the name “photoelectric effect”. A heated controversy occurred for many years on deciding whether light consisted of waves or particles. The evidence appeared strong for both cases.

37. QUANTA Later, physicists showed that light appears as either wave-like or particle-like (but never both at the same time) depending on the experimental setup. This is called the dual nature of light. He also showed that energy and mass were interconvertible. His famous equation E=mc2 related energy and mass. E= energy, m=mass, c=speed of light. Later, physicists showed that light appears as either wave-like or particle-like (but never both at the same time) depending on the experimental setup. This is called the dual nature of light. He also showed that energy and mass were interconvertible. His famous equation E=mc2 related energy and mass. E= energy, m=mass, c=speed of light.

38. Oil Drop Experiment Robert Andrews Millikan, (1868–1953) was a U.S. physicist who made the first determination of the charge of the electron by using his famous “oil drop experiment”. For these achievements, he was awarded the 1923 Nobel Prize for Physics. Robert Andrews Millikan, (1868–1953) was a U.S. physicist who made the first determination of the charge of the electron by using his famous “oil drop experiment”. For these achievements, he was awarded the 1923 Nobel Prize for Physics.

39. DISCOVERY OF THE NUCLEUS In 1912, Ernest Rutherford (New Zealand physicist) discovered the atomic nucleus. In 1912, Ernest Rutherford (New Zealand physicist) discovered the atomic nucleus. Using his famous “gold foil experiment”, Rutherford used alpha particles to bombard atoms. Using his famous “gold foil experiment”, Rutherford used alpha particles to bombard atoms.

40. DISCOVERY OF THE NUCLEUS He used radium as the source of the alpha particles and shined them onto the atoms in gold foil. Behind the screen sat a fluorescent screen for which he could observe the alpha particles impact. The results of the experiment were unexpected. He used radium as the source of the alpha particles and shined them onto the atoms in gold foil. Behind the screen sat a fluorescent screen for which he could observe the alpha particles impact. The results of the experiment were unexpected.

44. DISCOVERY OF THE NUCLEUS Most of the alpha particles went smoothly through the foil. Only an occasional alpha veered sharply from its original path, sometimes bouncing straight back from the foil. This surprised Rutherford so much he said it was like firing a cannon at a sheet of tissue paper and having the shell bounce back to you. Most of the alpha particles went smoothly through the foil. Only an occasional alpha veered sharply from its original path, sometimes bouncing straight back from the foil. This surprised Rutherford so much he said it was like firing a cannon at a sheet of tissue paper and having the shell bounce back to you.

45. DISCOVERY OF THE NUCLEUS Rutherford reasoned that the alpha particles must get scattered by tiny bits of positively charged matter. His new model of the atom showed that its positive electric charge and the majority of its mass were concentrated in an almost point sized nucleus. Rutherford reasoned that the alpha particles must get scattered by tiny bits of positively charged matter. His new model of the atom showed that its positive electric charge and the majority of its mass were concentrated in an almost point sized nucleus.

46. DISCOVERY OF THE NUCLEUS Rutherford thought most of the space around the positive center had nothing in it. It is worth noting that in contrast to the atom of the Greeks, Rutherford's is neither indivisible (because it's a composite structure), nor is it solid as it contains mostly empty space. Rutherford thought most of the space around the positive center had nothing in it. It is worth noting that in contrast to the atom of the Greeks, Rutherford's is neither indivisible (because it's a composite structure), nor is it solid as it contains mostly empty space.

47. DISCOVERY OF THE NUCLEUS It was not until 1919 that Rutherford finally identified the particles in the nucleus with the discrete positive charges. He named them protons, from the Greek for first, because they were the first identified building blocks of the nuclei. He found the protons mass was 1,836 times greater than the mass of the electron. It was not until 1919 that Rutherford finally identified the particles in the nucleus with the discrete positive charges. He named them protons, from the Greek for first, because they were the first identified building blocks of the nuclei. He found the protons mass was 1,836 times greater than the mass of the electron.

48. DISCOVERY OF THE NUCLEUS

49. DISCOVERY OF NUCLEONS Rutherford understood that the nucleus is itself composed of nucleons. These nucleons are of two types: Rutherford understood that the nucleus is itself composed of nucleons. These nucleons are of two types: Positively charged, it's a proton. Neutrally charged, it's a neutron Positively charged, it's a proton. Neutrally charged, it's a neutron

50. DISCOVERY OF NUCLEONS James Chadwick effectively discovered the neutron in 1932. James Chadwick effectively discovered the neutron in 1932.

51. Moseley, Proton and atomic number Henry Gwyn Jeffreys Moseley, (1887– 1915) was a British physicist who first established the atomic numbers of the elements by studying their X-ray spectra. This led to a complete classification of the elements, and also provided an experimental basis for an understanding of the structure of the atom. Henry Gwyn Jeffreys Moseley, (1887– 1915) was a British physicist who first established the atomic numbers of the elements by studying their X-ray spectra. This led to a complete classification of the elements, and also provided an experimental basis for an understanding of the structure of the atom.

52. Moseley, Proton and atomic number All atoms are characterized by their Atomic Number represented by Z: this is the number of protons in the atom. For example, for hydrogen Z = 1, for carbon Z = 6, for uranium Z = 92 etc. All atoms are characterized by their Atomic Number represented by Z: this is the number of protons in the atom. For example, for hydrogen Z = 1, for carbon Z = 6, for uranium Z = 92 etc.

53. Moseley, Proton and atomic number For a neutral atom, the number of protons Z is equal to the number of electrons because the - charge of an electron cancels out the + charge of a proton. It is the number of protons Z that defines an atom. For a neutral atom, the number of protons Z is equal to the number of electrons because the - charge of an electron cancels out the + charge of a proton. It is the number of protons Z that defines an atom.

54. BOHRS ATOM In order to take account of atomic stability, in 1913 Niels Bohr created a new model of the atom: the Electron Shell Model, or Solar System Model. In order to take account of atomic stability, in 1913 Niels Bohr created a new model of the atom: the Electron Shell Model, or Solar System Model.

55. BOHRS ATOM This model explained why electrons’ negative charge did not spiral toward the nucleus’ positive charge. The orbits of the electrons can't be just anywhere but are "quantified"; only certain particular orbits are permitted for the electron. It's not until one jumps from one orbit to another that it can emit (or absorb) light. This model explained why electrons’ negative charge did not spiral toward the nucleus’ positive charge. The orbits of the electrons can't be just anywhere but are "quantified"; only certain particular orbits are permitted for the electron. It's not until one jumps from one orbit to another that it can emit (or absorb) light.

57. What is "classical" and continuous light ? Not so easy to represent such an immaterial concept! At the end of the 19th century, James Maxwell defined light as being a beam of electromagnetic waves moving at a constant speed in the vacuum: the famous speed c of 300,000 kilometers per second. Not so easy to represent such an immaterial concept! At the end of the 19th century, James Maxwell defined light as being a beam of electromagnetic waves moving at a constant speed in the vacuum: the famous speed c of 300,000 kilometers per second.

58. Where does light come from? Quantum physics allows us to better understand how light is emitted by matter... The world of the atom according to Niels Bohr was a model at the frontier of two ages: the classical age, pre- quantum and the quantum world. Quantum physics allows us to better understand how light is emitted by matter... The world of the atom according to Niels Bohr was a model at the frontier of two ages: the classical age, pre- quantum and the quantum world.

59. Where does light come from? This emission is explained then by the jump that an electron makes from an orbit E2 to an orbit E1. During this jump towards this less energetic orbit E1 (an inner orbit), the electron will lose part of its energy in the form of a photon emitted outwards. This emission is explained then by the jump that an electron makes from an orbit E2 to an orbit E1. During this jump towards this less energetic orbit E1 (an inner orbit), the electron will lose part of its energy in the form of a photon emitted outwards.

60. Where does light come from? Each photon of a radiation (light, radio waves, X rays...) carries a quantum of energy characteristic of its frequency (frequency of visible light = color) Each photon of a radiation (light, radio waves, X rays...) carries a quantum of energy characteristic of its frequency (frequency of visible light = color)

61. Where does light come from?

62. The higher the frequency of light, the greater the energy and the more the color will tend towards blue (and from there towards ultra-violet, X ray and Gamma rays). The higher the frequency of light, the greater the energy and the more the color will tend towards blue (and from there towards ultra-violet, X ray and Gamma rays).

63. Where does light come from?

64. An electron, making a "bigger jump" from one atomic orbit to another, will then emit a photon correspondingly more energetic and of a correspondingly higher frequency. An electron, making a "bigger jump" from one atomic orbit to another, will then emit a photon correspondingly more energetic and of a correspondingly higher frequency.

65. Where does light come from?

66. Conversely, an atom's electron could absorb a photon of a given energy and thus jump from a less energetic orbit to a more energetic orbit: It will thus become more excited than normal because it is in a more energetic orbit. It is in becoming less excited that it would subsequently re- emit a photon. Conversely, an atom's electron could absorb a photon of a given energy and thus jump from a less energetic orbit to a more energetic orbit: It will thus become more excited than normal because it is in a more energetic orbit. It is in becoming less excited that it would subsequently re- emit a photon.

67. The visible spectrum Each atom can only emit a precise and characteristic set of colors: Each color of light is in fact a particular frequency (and therefore a level of energy) of a photon. All of the possible jumps between orbits that an electron can make within a given atom translate into the emission (or absorption) of a characteristic spectrum of light: Each atom can only emit a precise and characteristic set of colors: Each color of light is in fact a particular frequency (and therefore a level of energy) of a photon. All of the possible jumps between orbits that an electron can make within a given atom translate into the emission (or absorption) of a characteristic spectrum of light:

68. The visible spectrum Here we have a veritable identity card for a given type of atom. It's because of this readily identifiable spectrum that we can know which atoms exist in stars in the firmament. Their light is captured by telescopes, analyzed and compared with the spectrums of hydrogen, helium etc Here we have a veritable identity card for a given type of atom. It's because of this readily identifiable spectrum that we can know which atoms exist in stars in the firmament. Their light is captured by telescopes, analyzed and compared with the spectrums of hydrogen, helium etc

69. The visible spectrum

70. TOWARDS QUANTUM PHYSICS What an intellectual pleasure (and what laziness!) to represent atoms in the form of little balls turning one around another... This model is, moreover, still the one that the general public has in their heads. In fact this model is false because at the atomic scale, new laws apply! These laws are part of a strange physics, very far from our current concepts: quantum physics. What an intellectual pleasure (and what laziness!) to represent atoms in the form of little balls turning one around another... This model is, moreover, still the one that the general public has in their heads. In fact this model is false because at the atomic scale, new laws apply! These laws are part of a strange physics, very far from our current concepts: quantum physics.quantum physicsquantum physics

71. TOWARDS QUANTUM PHYSICS Bohr's model is the last model obedient to classical physics, that is to say physics that explains movements and phenomena in terms of our human scale. These models of atoms are therefore easy to understand and to represent. Bohr's model is the last model obedient to classical physics, that is to say physics that explains movements and phenomena in terms of our human scale. These models of atoms are therefore easy to understand and to represent. Since the middle of the 1930s, the atom has become a mathematical description that is very difficult to transcribe into images Since the middle of the 1930s, the atom has become a mathematical description that is very difficult to transcribe into images

72. Wave-particle duality Louis Victor de Broglie (1892–1987) was a French physicist who first developed the principle that an electron or any other particle can be considered to behave as a wave as well as a particle. This wave- particle duality is a fundamental principle governing the structure of the atom, and for its discovery, de Broglie was awarded the 1929 Nobel Prize for Physics. Louis Victor de Broglie (1892–1987) was a French physicist who first developed the principle that an electron or any other particle can be considered to behave as a wave as well as a particle. This wave- particle duality is a fundamental principle governing the structure of the atom, and for its discovery, de Broglie was awarded the 1929 Nobel Prize for Physics.

73. Wave-particle duality The most important question that quantum physics has been attempting to address concerns the manner in which to represent physical objects and their properties. The most important question that quantum physics has been attempting to address concerns the manner in which to represent physical objects and their properties.

74. Wave-particle duality The old physics, known as classical, distinguished two types of fundamental entities: The old physics, known as classical, distinguished two types of fundamental entities: particles, which are sorts of microscopic balls, particles, which are sorts of microscopic balls, waves, which propagate in space a bit like the movement of a wave on the sea. waves, which propagate in space a bit like the movement of a wave on the sea.

75. Wave-particle duality Quantum physics doesn't hold on to this classification, convenient as it is. The objects which it considers are neither particles, nor waves, but "something else". Quantum physics doesn't hold on to this classification, convenient as it is. The objects which it considers are neither particles, nor waves, but "something else".

76. Wave-particle duality The following analogy should help us: The following analogy should help us: Look at a cylinder from two different angles, a cylinder appears sometimes as a circle, sometimes as a rectangle. When in fact it is neither one nor the other. Look at a cylinder from two different angles, a cylinder appears sometimes as a circle, sometimes as a rectangle. When in fact it is neither one nor the other.

77. Wave-particle duality

78. That’s the way the photon, the electron and all elementary particles are, thus the image of a particle is but one facet of a more complex entity. That’s the way the photon, the electron and all elementary particles are, thus the image of a particle is but one facet of a more complex entity. De Broglie's discovery of wave-particle duality enabled physicists to view Einstein's conviction that matter and energy are interconvertible as being fundamental to the structure of matter. De Broglie's discovery of wave-particle duality enabled physicists to view Einstein's conviction that matter and energy are interconvertible as being fundamental to the structure of matter.

79. Wave-particle duality The study of matter waves led not only to a much deeper understanding of the nature of the atom but also to explanations of chemical bonds and the practical application of electron waves in electron microscopes The study of matter waves led not only to a much deeper understanding of the nature of the atom but also to explanations of chemical bonds and the practical application of electron waves in electron microscopes

80. PAULI'S EXCLUSION PRINCIPLE This fundamental law of quantum physics was set out for the first time by the physicist Wolfgang Pauli in 1925. This fundamental law of quantum physics was set out for the first time by the physicist Wolfgang Pauli in 1925.

81. PAULI'S EXCLUSION PRINCIPLE All of the particles of the same type have an identical nature but can have different properties. All of the particles of the same type have an identical nature but can have different properties. For example, the electrons in an atom have different energies (associated with their orbits). Each particle thus possesses a certain number of its own properties that form "the state of the particle". For example, the electrons in an atom have different energies (associated with their orbits). Each particle thus possesses a certain number of its own properties that form "the state of the particle".

82. PAULI'S EXCLUSION PRINCIPLE No two electrons in the same atom can have the same set of four quantum numbers (energy level, sublevel, orbital, or spin). No two electrons in the same atom can have the same set of four quantum numbers (energy level, sublevel, orbital, or spin).

83. PAULI'S EXCLUSION PRINCIPLE In an atom, two electrons can have the same energy on the condition that their spins are different. This explains the progressive filling of the periodic table of Mendeleev, that is to say the electronic structure of atoms. In an atom, two electrons can have the same energy on the condition that their spins are different. This explains the progressive filling of the periodic table of Mendeleev, that is to say the electronic structure of atoms.

84. Heisenberg Uncertainty Principle In 1927 Werner Karl Heisenberg made the discovery for which he is best known -- that of the uncertainty principle. This states that it is impossible to specify precisely both the position and the simultaneous momentum (mass multiplied by velocity) of a particle. In 1927 Werner Karl Heisenberg made the discovery for which he is best known -- that of the uncertainty principle. This states that it is impossible to specify precisely both the position and the simultaneous momentum (mass multiplied by velocity) of a particle.

85. Heisenberg Uncertainty Principle Dare we have a new image to illustrate this principle: Deep in the woods at night, a nature lover hears the hooting of an owl. If he would like, at the same time, see the feathered creature, he would have to turn a torch on him: But then it's a good bet that the surprised owl will stop singing. This gives rise to the insoluble dilemma: We can't both hear and see the owl at the same time... Alas! Dare we have a new image to illustrate this principle: Deep in the woods at night, a nature lover hears the hooting of an owl. If he would like, at the same time, see the feathered creature, he would have to turn a torch on him: But then it's a good bet that the surprised owl will stop singing. This gives rise to the insoluble dilemma: We can't both hear and see the owl at the same time... Alas!

87. Heisenberg Uncertainty Principle Therefore electrons are better thought of as occurring in an electron cloud surrounding the nucleus than in orbits around the nucleus. Therefore electrons are better thought of as occurring in an electron cloud surrounding the nucleus than in orbits around the nucleus.

89. Heisenberg Uncertainty Principle If it is necessary to risk an image to illustrate this curious phenomena, one could imagine the electron as a sub- marine which emerges, long enough for a measurement, from its probabilistic ocean. If it is necessary to risk an image to illustrate this curious phenomena, one could imagine the electron as a sub- marine which emerges, long enough for a measurement, from its probabilistic ocean.

90. Heisenberg Uncertainty Principle Later, it submerges and it will be impossible to an observer from the surface to localize it with any precision: one could do no more that define the volume of the ocean where the submarine could probably be found. Later, it submerges and it will be impossible to an observer from the surface to localize it with any precision: one could do no more that define the volume of the ocean where the submarine could probably be found.

91. Orbitals and orbits When the a planet moves around the sun, you can plot a definite path for it which is called an orbit. A simple view of the atom looks similar and you may have pictured the electrons as orbiting around the nucleus. The truth is different, and electrons in fact inhabit regions of space known as orbitals. When the a planet moves around the sun, you can plot a definite path for it which is called an orbit. A simple view of the atom looks similar and you may have pictured the electrons as orbiting around the nucleus. The truth is different, and electrons in fact inhabit regions of space known as orbitals.

92. Orbitals and orbits Orbits and orbitals sound similar, but they have quite different meanings. It is essential that you understand the difference between them. Orbits and orbitals sound similar, but they have quite different meanings. It is essential that you understand the difference between them.

93. The impossibility of drawing orbits for electrons To plot a path for something you need to know exactly where the object is and be able to work out exactly where it's going to be an instant later. You can't do this for electrons. To plot a path for something you need to know exactly where the object is and be able to work out exactly where it's going to be an instant later. You can't do this for electrons.

94. The impossibility of drawing orbits for electrons The Heisenberg Uncertainty Principle says - loosely - that you can't know with certainty both where an electron is and where it's going next. That makes it impossible to plot an orbit for an electron around a nucleus. Is this a big problem? No. If something is impossible, you have to accept it and find a way around it. The Heisenberg Uncertainty Principle says - loosely - that you can't know with certainty both where an electron is and where it's going next. That makes it impossible to plot an orbit for an electron around a nucleus. Is this a big problem? No. If something is impossible, you have to accept it and find a way around it.

95. The impossibility of drawing orbits for electrons Suppose you had a single hydrogen atom and at a particular instant plotted the position of the one electron. Soon afterwards, you do the same thing, and find that it is in a new position. You have no idea how it got from the first place to the second. Suppose you had a single hydrogen atom and at a particular instant plotted the position of the one electron. Soon afterwards, you do the same thing, and find that it is in a new position. You have no idea how it got from the first place to the second.

96. The impossibility of drawing orbits for electrons You keep on doing this over and over again, and gradually build up a sort of 3D map of the places that the electron is likely to be found. You keep on doing this over and over again, and gradually build up a sort of 3D map of the places that the electron is likely to be found.

97. The impossibility of drawing orbits for electrons 95% of the time (or any other percentage you choose), the electron will be found within a fairly easily defined region of space quite close to the nucleus. Such a region of space is called an orbital. You can think of an orbital as being the region of space in which the electron lives. 95% of the time (or any other percentage you choose), the electron will be found within a fairly easily defined region of space quite close to the nucleus. Such a region of space is called an orbital. You can think of an orbital as being the region of space in which the electron lives.

98. The impossibility of drawing orbits for electrons What is the electron doing in the orbital? We don't know, we can't know, and so we just ignore the problem! All you can say is that if an electron is in a particular orbital it will have a particular definable energy. What is the electron doing in the orbital? We don't know, we can't know, and so we just ignore the problem! All you can say is that if an electron is in a particular orbital it will have a particular definable energy.

99. Orbitals Each orbital has a name. Each orbital has a name. The orbital occupied by the hydrogen electron is called a 1s orbital. The "1" represents the fact that the orbital is in the energy level closest to the nucleus. The "s" tells you about the shape of the orbital. s orbitals are spherically symmetric around the nucleus - in each case, like a hollow ball made of rather chunky material with the nucleus at its center. The orbital occupied by the hydrogen electron is called a 1s orbital. The "1" represents the fact that the orbital is in the energy level closest to the nucleus. The "s" tells you about the shape of the orbital. s orbitals are spherically symmetric around the nucleus - in each case, like a hollow ball made of rather chunky material with the nucleus at its center.

100. s orbital The orbital on the left is a 2s orbital. This is similar to a 1s orbital except that the region where there is the greatest chance of finding the electron is further from the nucleus - this is an orbital at the second energy level.

101. s orbital If you look carefully, you will notice that there is another region of slightly higher electron density (where the dots are thicker) nearer the nucleus. ("Electron density" is another way of talking about how likely you are to find an electron at a particular place.)

102. s orbital 2s (and 3s, 4s, etc) electrons spend some of their time closer to the nucleus than you might expect. The effect of this is to slightly reduce the energy of electrons in s orbitals. The nearer the nucleus the electrons get, the lower their energy.

103. p orbitals Not all electrons inhabit s orbitals (in fact, very few electrons live in s orbitals). At the first energy level, the only orbital available to electrons is the 1s orbital, but at the second level, as well as a 2s orbital, there are also orbitals called 2p orbitals. Not all electrons inhabit s orbitals (in fact, very few electrons live in s orbitals). At the first energy level, the only orbital available to electrons is the 1s orbital, but at the second level, as well as a 2s orbital, there are also orbitals called 2p orbitals.

104. A p orbital is rather like 2 identical balloons tied together at the nucleus. The diagram on the right is a cross-section through that 3-dimensional region of space. Once again, the orbital shows where there is a 95% chance of finding a particular electron. At any one energy level it is possible to have three absolutely equivalent p orbitals

105. d and f orbitals In addition to s and p orbitals, there are two other sets of orbitals which become available for electrons to inhabit at higher energy levels. At the third level, there is a set of five d orbitals (with complicated shapes and names) as well as the 3s and 3p orbitals. At the third level there are a total of nine orbitals altogether In addition to s and p orbitals, there are two other sets of orbitals which become available for electrons to inhabit at higher energy levels. At the third level, there is a set of five d orbitals (with complicated shapes and names) as well as the 3s and 3p orbitals. At the third level there are a total of nine orbitals altogether

106. d and f orbitals At the fourth level, as well the 4s and 4p and 4d orbitals there are an additional seven f orbitals - 16 orbitals in all. s, p, d and f orbitals are then available at all higher energy levels as well At the fourth level, as well the 4s and 4p and 4d orbitals there are an additional seven f orbitals - 16 orbitals in all. s, p, d and f orbitals are then available at all higher energy levels as well

107. d and f orbitals You have to be aware that there are sets of five d orbitals at levels from the third level upwards, but you will not be expected name them. Apart from a passing reference, you won't come across f orbitals at all. You have to be aware that there are sets of five d orbitals at levels from the third level upwards, but you will not be expected name them. Apart from a passing reference, you won't come across f orbitals at all.

108. Fitting electrons into orbitals You can think of an atom as a very bizarre house (like an inverted pyramid!) - with the nucleus living on the ground floor, and then various rooms (orbitals) on the higher floors occupied by the electrons. You can think of an atom as a very bizarre house (like an inverted pyramid!) - with the nucleus living on the ground floor, and then various rooms (orbitals) on the higher floors occupied by the electrons.

109. Fitting electrons into orbitals On the first floor there is only 1 room (the 1s orbital); on the second floor there are 4 rooms (the 2s, and three 2p orbitals); on the third floor there are 9 rooms (one 3s orbital, three 3p orbitals and five 3d orbitals); and so on. But the rooms aren't very big... Each orbital can only hold 2 electrons. On the first floor there is only 1 room (the 1s orbital); on the second floor there are 4 rooms (the 2s, and three 2p orbitals); on the third floor there are 9 rooms (one 3s orbital, three 3p orbitals and five 3d orbitals); and so on. But the rooms aren't very big... Each orbital can only hold 2 electrons.

110. Fitting electrons into orbitals A convenient way of showing the orbitals that the electrons live in is to draw "electrons-in-boxes". A convenient way of showing the orbitals that the electrons live in is to draw "electrons-in-boxes".

111. "Electrons-in-boxes" Orbitals can be represented as boxes with the electrons in them shown as arrows. Often an up-arrow and a down-arrow are used to show that the electrons are in some way different. Orbitals can be represented as boxes with the electrons in them shown as arrows. Often an up-arrow and a down-arrow are used to show that the electrons are in some way different.

112. "Electrons-in-boxes" A 1s orbital holding 2 electrons would be drawn as shown on the right, but it can be written even more quickly as 1s 2. This is read as "one s two" - not as "one s squared". A 1s orbital holding 2 electrons would be drawn as shown on the right, but it can be written even more quickly as 1s 2. This is read as "one s two" - not as "one s squared".

113. "Electrons-in-boxes" You mustn't confuse the two numbers in this notation: You mustn't confuse the two numbers in this notation:

114. The order of filling orbitals Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible. Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible.

115. The order of filling orbitals The diagram (not to scale) summarizes the energies of the orbitals up to the 4p level. The diagram (not to scale) summarizes the energies of the orbitals up to the 4p level.

116. s- and p-block elements The elements in group 1 of the Periodic Table all have an outer electronic structure of ns 1 (where n is a number between 2 and 7). All group 2 elements have an outer electronic structure of ns 2. Elements in groups 1 and 2 are described as s-block elements. The elements in group 1 of the Periodic Table all have an outer electronic structure of ns 1 (where n is a number between 2 and 7). All group 2 elements have an outer electronic structure of ns 2. Elements in groups 1 and 2 are described as s-block elements.

117. s- and p-block elements Elements from group 3 across to the noble gases all have their outer electrons in p orbitals. These are then described as p- block elements. Elements from group 3 across to the noble gases all have their outer electrons in p orbitals. These are then described as p- block elements.

118. s- and p-block elements

119. d-block elements d-block elements are elements in which the last electron to be added to the atom is in a d orbital. d-block elements are elements in which the last electron to be added to the atom is in a d orbital. Remember that the 4s orbital has a lower energy than the 3d orbitals and so fills first. Once the 3d orbitals have filled up, the next electrons go into the 4p orbitals as you would expect. Remember that the 4s orbital has a lower energy than the 3d orbitals and so fills first. Once the 3d orbitals have filled up, the next electrons go into the 4p orbitals as you would expect.

120. d-block elements

121. QUARKS Up until 1964, it was believed that there only existed three elementary particles making up the atom: the electron, the proton and the neutron. However, numerous unstable particles (with a lifetime of the order of 10-23 seconds) have been detected, either in cosmic rays, or in the high-energy impacts created in particle accelerators constructed after the 1939-1945 war. Up until 1964, it was believed that there only existed three elementary particles making up the atom: the electron, the proton and the neutron. However, numerous unstable particles (with a lifetime of the order of 10-23 seconds) have been detected, either in cosmic rays, or in the high-energy impacts created in particle accelerators constructed after the 1939-1945 war.

122. QUARKS Set out for the first time in 1964 by Murray Gell-Mann and independently by George Zweig, the theory of quarks progressively established its pedigree and won acclaim as advances corroborated the theory by experiment; it was not until 1975 that quarks were detected experimentally. Set out for the first time in 1964 by Murray Gell-Mann and independently by George Zweig, the theory of quarks progressively established its pedigree and won acclaim as advances corroborated the theory by experiment; it was not until 1975 that quarks were detected experimentally. The strange name of Quark comes from a Roman phrase of James Joyce in "Finnegan’s Wake": Three Quarks for Muster Mark! The strange name of Quark comes from a Roman phrase of James Joyce in "Finnegan’s Wake": Three Quarks for Muster Mark!

123. QUARKS Quarks have a unique property: they are incapable of existing alone, unaccompanied! It is absolutely impossible to observe a quark in isolation. Quarks have a unique property: they are incapable of existing alone, unaccompanied! It is absolutely impossible to observe a quark in isolation. Quarks are the constituents of nucleons. There exist two types in ordinary matter : Quarks are the constituents of nucleons. There exist two types in ordinary matter : up quark (symbol = u) up quark (symbol = u) down quark (symbol = d) down quark (symbol = d)

125. Antimatter There exists a mirror universe where matter is transformed into anti-matter. There exists a mirror universe where matter is transformed into anti-matter. Antimatter is composed of antiparticles: antiquarks, anti-electrons, and antineutrinos. Antimatter is composed of antiparticles: antiquarks, anti-electrons, and antineutrinos. An antiparticle is simply a particle with opposing quantum numbers An antiparticle is simply a particle with opposing quantum numbers

126. How many atoms are there? How many atoms are there? In nature, about 90 different atoms exist which combine to form an infinite variety of compounds. It is necessary to add to this natural radioactive atoms (and therefore unstable) and those created artificially by man by nuclear reactions. In nature, about 90 different atoms exist which combine to form an infinite variety of compounds. It is necessary to add to this natural radioactive atoms (and therefore unstable) and those created artificially by man by nuclear reactions.

127. How many atoms are there? How many atoms are there? At the moment, about 115 atoms exist which have been discovered or created, but the list could grow. The last atom created is element 114, baptized with a provisional name of ununquadium (symbol Uuq): It was created in 1998 at the Nuclear Institute of Dubna in Russia. At the moment, about 115 atoms exist which have been discovered or created, but the list could grow. The last atom created is element 114, baptized with a provisional name of ununquadium (symbol Uuq): It was created in 1998 at the Nuclear Institute of Dubna in Russia. Late breaking news: Elements 116 and 118 have been synthesized in 1999 in California at the Lawrence Berkeley National Laboratory Late breaking news: Elements 116 and 118 have been synthesized in 1999 in California at the Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryLawrence Berkeley National Laboratory

128. The 3 radiations There exist three varieties of radioactivity characterized by the emission of different rays emitted by the nucleus of the atom: There exist three varieties of radioactivity characterized by the emission of different rays emitted by the nucleus of the atom:

129. The 3 radiations  α (alpha) rays are stopped by 6 cm of air. They are composed of alpha particles made up of two protons and two neutrons (in fact a helium nucleus). The particle is therefore positively charged. These alpha particles are nothing more than fragments of unstable heavy nuclei that reorganize themselves to become lighter and more stable nuclei (thus non radioactive!).

130. The 3 radiations  β (beta) rays are stopped by an aluminum screen. They are notably composed of electrons and are therefore negatively charged. Beta radiation is identical to the cathode radiation in your TV!

131. The 3 radiations γ (gamma) rays are extremely penetrating and can pass straight through a safe. They are composed of high-energy photons (particles of light). They are nothing but pure energy without any mass. γ (gamma) rays are extremely penetrating and can pass straight through a safe. They are composed of high-energy photons (particles of light). They are nothing but pure energy without any mass.

133. The 3 radiations These three varieties of radioactivity are not emitted simultaneously. Each nuclear reaction of an atom emits only one single type of radiation at a time! For example, radioactive Uranium- 238 emits an alpha ray and thus loses 4 nucleons (2 protons + 2 neutrons): U 238 thereby transforms itself into Thorium- 234 (because 2 protons less - that changes an atom!). These three varieties of radioactivity are not emitted simultaneously. Each nuclear reaction of an atom emits only one single type of radiation at a time! For example, radioactive Uranium- 238 emits an alpha ray and thus loses 4 nucleons (2 protons + 2 neutrons): U 238 thereby transforms itself into Thorium- 234 (because 2 protons less - that changes an atom!).

134. Nuclear fission Nuclear fission Whenever the nucleus of a heavy atom (like uranium 235) fissions (fragments) into two smaller nuclei, it produces a remarkable event: the sum of the masses of these two remaining nuclei is less than the mass of the original large nucleus. Where has the missing mass gone? It has transformed itself into pure energy (Einstein's mass-energy equivalence), an enormous quantity of energy. Whenever the nucleus of a heavy atom (like uranium 235) fissions (fragments) into two smaller nuclei, it produces a remarkable event: the sum of the masses of these two remaining nuclei is less than the mass of the original large nucleus. Where has the missing mass gone? It has transformed itself into pure energy (Einstein's mass-energy equivalence), an enormous quantity of energy.

135. Nuclear fusion In broad terms it's the inverse of fission. In broad terms it's the inverse of fission. Two light atomic nuclei (like hydrogen) crash into each other and fuse together into a single bigger nucleus. Now the final mass of this big nucleus is smaller than the sum of the masses of the two initial nuclei, which is where we get an enormous release of energy produced by the annihilation of this difference of mass. Two light atomic nuclei (like hydrogen) crash into each other and fuse together into a single bigger nucleus. Now the final mass of this big nucleus is smaller than the sum of the masses of the two initial nuclei, which is where we get an enormous release of energy produced by the annihilation of this difference of mass.

136. Nuclear fusion In order to be able to provoke such a fusion reaction, it is necessary to force the nuclei, all positively charged, to move together and to overcome their mutual repulsion : This is not possible except at very high temperatures (the temperature corresponding to the intensity necessary to get the particles to crash into each other). This is why the nuclear fusion reaction is also called a thermonuclear reaction (thermo = heat). In order to be able to provoke such a fusion reaction, it is necessary to force the nuclei, all positively charged, to move together and to overcome their mutual repulsion : This is not possible except at very high temperatures (the temperature corresponding to the intensity necessary to get the particles to crash into each other). This is why the nuclear fusion reaction is also called a thermonuclear reaction (thermo = heat).

137. Nuclear fusion This uncontrolled reaction is used in the hydrogen bomb or H-bomb. This reaction is also seen in the heart of our Sun where temperatures reach hundreds of millions of degrees. This uncontrolled reaction is used in the hydrogen bomb or H-bomb. This reaction is also seen in the heart of our Sun where temperatures reach hundreds of millions of degrees.

138. Natural or artificial radioactivity? Man has not invented radioactivity. It has existed since the beginning of the universe: We speak of natural radioactivity when it is due to the durable radio-elements formed in the stars which have not yet found their stable state: they will end up transforming themselves into stable atoms. Man has not invented radioactivity. It has existed since the beginning of the universe: We speak of natural radioactivity when it is due to the durable radio-elements formed in the stars which have not yet found their stable state: they will end up transforming themselves into stable atoms.

139. Natural or artificial radioactivity? We speak of artificial radioactivity when referring to elements fabricated by man. In this case, these atoms are very heavy (with a high atomic number Z), very unstable and therefore have a very short half-life. Physicists create these artificial radioelements by bombarding natural atoms with protons or alpha particles: the nuclei of these atoms acquire additional protons that transform them into new heavier atoms. We speak of artificial radioactivity when referring to elements fabricated by man. In this case, these atoms are very heavy (with a high atomic number Z), very unstable and therefore have a very short half-life. Physicists create these artificial radioelements by bombarding natural atoms with protons or alpha particles: the nuclei of these atoms acquire additional protons that transform them into new heavier atoms.