A Journey in the realm of the unseen The History of the Atom A Journey in the realm of the unseen

Introduction What does an atom look like? It is so small that it cannot be seen. Yet we know there are particles like protons, neutrons and electrons that make up the atom. How did scientists discover these subatomic particles? This presentation will take you through the scientists who contributed to the discovery of the make-up of the atom. What does an atom look like? Does anyone know what the atom looks like? It is so little and it cannot be seen so how can anyone know the make-up of the atom? When you receive a present are you able to take an educated guess to determine what is inside? Sure you can. You feel the weight of the present, shake it a little and check out the size. Scientists have determined the size, shape, make-up, etc. of the atom by performing many different experiments and making inferences about what they learned, just as if it were a present. This unit is composed of the scientists who contributed to the discovery of the make-up of the atom, the experiments that they performed and the atomic models that they developed.

The early Greeks defined all matter As being rooted into the Four Elements. Earth – all things that are dense solids Wind – all things that float above. Water – all things that are wet. Fire – Special stuff that have both earth and wind in them. What does an atom look like? Does anyone know what the atom looks like? It is so little and it cannot be seen so how can anyone know the make-up of the atom? When you receive a present are you able to take an educated guess to determine what is inside? Sure you can. You feel the weight of the present, shake it a little and check out the size. Scientists have determined the size, shape, make-up, etc. of the atom by performing many different experiments and making inferences about what they learned, just as if it were a present. This unit is composed of the scientists who contributed to the discovery of the make-up of the atom, the experiments that they performed and the atomic models that they developed.

Greek philosopher Democritus (460-371 B.C.) Proposed that all matter is composed of small bits of matter too small to be seen. These atoms CANNOT be further split into smaller portions. There is a void, which is empty space between the small bits. He called the bits of matter “ATOMOS” Greek for indivisible. Greek philosopher Democritus (460-371 B.C.) Around 440 BC, Leucippus of Miletus originated the atom concept. He and his pupil, Democritus (c460-371 BC) of Abdera, refined and extended it in future years. There are five major points to their atomic idea. Almost all of the original writings of Leucippus and Democritus are lost. About the only sources we have for their atomistic ideas are found in quotations of other writers. Democritus [16K GIF] is known as the "Laughing Philosopher" because of his joyous spirit. He was a big man (relatively speaking) and enjoyed life tremendously. He also was very widely traveled. In Greek, the prefix "a" means "not" and the word "tomos" means cut. Our word atom therefore comes from atomos, a Greek word meaning uncuttable. Atoms are completely solid. Atoms are homogeneous, with no internal structure. Atoms are different in ... 1) ...their sizes. See the Democritus quote just below. 2) ...their shapes. According to Aristotle: "Democritus and Leucippus say that there are indivisible bodies, infinite both in number and in the varieties of their shapes...." (p. 110) Democritus says of atoms: "They have all sorts of shapes and appearences and different sizes.... Some are rough, some hook-shaped, some concave, some convex and some have other innumerable variations." (p. 110-111) 3) ...their weight. Again from Aristotle: "Democritus recognized only two basic properties of the atom: size and shape. But Epicurus added weight as a third. For, according to him, the bodies move by necessity through the force of weight." (p. 111)

Aristotle Greek philosopher Opposed Democritus’ beliefs Believed that all matter was continuous. Aristotle (384 BC – 322 BC) Aristotle (384 BC – 322 BC) The idea of the atom was strongly opposed by Aristotle and others. Because of this, the atom receded into the background. Although there is a fairly continuous pattern of atomistic thought through the ages, only a relative few scholars gave it much thought. Due to complex circumstances beyond the scope of this lesson, the Catholic Church accepted Aristotle's position and came to equate atomistic ideas with Godlessness. For example, "Democritus said that there is no end to the universe, since it was not created by any outside power." It was not until 1660 that Pierre Gassendi succeeded in separating the two and not until 1803 that John Dalton put the atom on a solid scientific basis. The atom concept is often presented as laying fallow between Democritus and Dalton. Aristotle

Antoine Lavoisier Father of Modern Chemistry Law of Conservation of Mass In a chemical reaction, the total mass of the reactants will equal the total mass of the products Antoine Laurent Lavoisier, (1743-1794), French chemist

Joseph Proust Law of Definite Composition (1799) A compound always contains the same elements in definite proportions Example: pure sugar is composed of: 42.1% carbon, 51.4% oxygen, & 6.5% hydrogen by mass regardless of where you find it. Joseph Proust (1754-1826) Joseph Proust (1754-1826) showed that copper carbonate (CuCO3), whether prepared in the laboratory or obtained from natural sources, always contains the same elements – copper, carbon, and oxygen – in the same proportion by mass.

Law of Definite Proportions Regardless of how its combined, lead (IV) sulfide will always have the same composition.

John Dalton Developed the Law of Multiple Proportions First to recognize that atoms could explain the laws of: conservation of mass, definite composition and multiple proportions Proposed the Atomic Theory in 1803 John Dalton was born in Cumberland County, England in 1766. He started teaching at the age of 12 and continued teaching throughout his life. He published work on various topics such as: color blindness, grammar, and meterorological observations before beginning his scientific career in earnest. He spent time studying the properties of gases and in 1803 Dalton first presented his theory of atoms at a lecture.

Five Part Atomic Theory All matter is composed of extremely small particles called atoms. Two or more atoms from the same element are identical. Atoms cannot be subdivided, created or destroyed.

Five Part Atomic Theory 4. The Law of multiple proportions: Atoms from different elements combine in simple, whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms can be combined, separated, or rearranged.

Dalton’s Model of the Atom Dalton developed a model of the atom based on his atomic theory. He felt the atom was an extremely small, indivisible particle. His model of the atom is called the Billiard Ball Model. Solid Sphere

JJ Thomson Experimented with a CRT (cathode ray tube) A CRT is an evacuated glass bulb containing two ends: the cathode and the anode. Joseph John Thomson was born near Manchester England in 1856. He began attending Owens College at the age of 14 and came under the influence of a physics professor. Three years later Joseph received a scholarship which honored the memory of John Dalton. He graduated from Trinity College in 1880 in mathematics and then worked at Cavendish Laboratory. A CRT can be found in televisions and computer monitors, but a hundred years ago a CRT was an experimental piece of equipment. A CRT is an evacuated glass bulb containing two ends: a cathode and an anode. An electrical current passes through the tube from the cathode (this is the end connected to the negative end of the electrical source) to the anode (which is connected to the positive end of the electrical source). Thomson did many experiments with cathode ray tubes. Thomson studied the cathode ray that left the cathode and traveled toward the anode when electrical current was applied. He noticed that the cathode rays were the same regardless of the element or metal used to make-up the cathode. A magnet was applied to these rays and always with the same results. The negative end of the magnet would repel the cathode ray and the positive end of the magnet would attract the ray. Thomson concluded that the cathode ray was not a beam of light, but consisted of particles. Through his experiments, Thomson was able to determine the ratio of the particle's electrical charge and mass, <i>e/m</I>. He found the ratio to be very large and he inferred that the particle was very small, at least one thousand times smaller than a hydrogen atom. After testing many materials and elements, Thomson found the <i>e/m</I> ratio to always be the same. His conclusion was that the cathode ray is made up of an extremely small particle that is common to all matter. It also has a negative charge since the negative end of the magnet would cause the particle stream to repel. These particles discovered by Thomson are the ELECTRONS. The Plum Pudding model is the name of Thomson's model of the atom. Plum pudding is an English dessert that contains raisins on the outside of it. The atom was thought to be a solid particle (the pudding) with negative charges or electrons(the raisins) attached to the surface.

Cathode Ray Tube An electrical current passes through the tube from the cathode (negative end) to the anode (positive end). Thomson studied the cathode ray that traveled from the cathode to the anode and noticed that the cathode rays were the same regardless of the element or metal used to make-up the cathode.

Cathode Ray Tube A magnet was applied to these rays and always with the same results: Negative end of magnet repelled cathode ray; Positive end of magnet attracted cathode rays.

Thomson’s Conclusion Cathode ray is made up of an extremely small particles that are common to all matter. The particle have a negative charge. Thomson discovered the ELECTRONS. The Plum Pudding model is the name of Thomson’s model of the atom. Plum pudding is an English dessert that contains raisins on the outside of it. The atom was thought to be a solid particle (the pudding) with negative charges or electrons(the raisins) attached to the surface. Or think of a raisin bun.

Robert Millikan Oil Drop Experiment (1909) Work contained excellent precision Determined the exact charge and exact mass of an electron Robert Millikan was born in 1868 and died in 1953. Millikan's work contained excellent precision. In 1909 Millikan determined the exact charge and the exact mass of an electron through his oil drop experiment. illikan set up a metal canister that contained two metal plates inside. The plate at the bottom was connected to the negative end of an electrical source. The plate toward the top contained a hole and was connected to the positive end of an electrical source. Near the top of the canister there was a place for a spray bottle containing oil to be attached. A microscope was attached in the middle of the canister in order to view the oil drop. There was also a source of x-rays coming in from the side. Oil was sprayed at the top of the canister. A few drops of oil fell through the hole in the top plate and traveled downward because of gravity. The drops became negatively charged from the extra electrons that were dislodged from gases in the air by x-rays. As the drops approached the plate at the bottom, the electrical charge was turned on. The negatively charged oil drops would be repelled upward from the negatively charged plate and also attracted upward toward the positively charged plate. By controlling the electrical current, Millikan could suspend the oil drop in mid-air in order to study it. The charge on each oil drop could be calculated from the voltage between the two plates. The mass of the oil drop could be determined by the amount of time it took to fall. Millikan discovered the exact charge of an electron to be 1.6 x 10-19 coulombs and the exact mass to be 9.109 x 10-28 grams.

Oil Drop Experiment Millikan set up a metal canister that contained two metal plates inside. The plate at the bottom was connected to the negative end of an electrical source. The plate toward the top contained a hole and was connected to the positive end of an electrical source. Near the top of the canister there was a place for a spray bottle containing oil to be attached. A microscope was attached in the middle of the canister in order to view the oil drop. There was also a source of x-rays coming in from the side. Oil was sprayed at the top of the canister. A few drops of oil fell through the hole in the top plate and traveled downward because of gravity. The drops became negatively charged from the extra electrons that were dislodged from gases in the air by x-rays. As the drops approached the plate at the bottom, the electrical charge was turned on. The negatively charged oil drops would be repelled upward from the negatively charged plate and also attracted upward toward the positively charged plate. By controlling the electrical current, Millikan could suspend the oil drop in mid-air in order to study it. The charge on each oil drop could be calculated from the voltage between the two plates. The mass of the oil drop could be determined by the amount of time it took to fall. Millikan discovered the exact charge of an electron to be 1.6 x 10-19 coulombs and the exact mass to be 9.109 x 10-28 grams.

How the Oil Drop Experiment Worked A fine mist of oil is sprayed into the chamber. A few oil drops will fall through the hole in the positively charged plate at the top. As the oil drops fall due to gravity, they acquire extra electrons which are dislodged from gases in the air by X rays. As the charged oil drops descend, the electrically charged plates are turned on.

How the Oil Drop Experiment Worked The oil drops now have two forces acting on them. Gravity and electrical charge. Using the microscope to observe the oil drops, Millikan could determine the charge needed to suspend the drops in mid-air. Millikan calculated the: exact mass (9.109 x 10-28 grams) and charge (-1.6 x 10-19 coulombs) of an electron.

Results of CRT and Oil Drop Experiment Proved that atoms are divisible. Atoms are electrically neutral therefore they must have a positive charge equal to the negative charge. Since electrons have such a small mass, atoms must have additional particles to account for most of their mass. The Plum pudding model was created & confirmed.

Ernest Rutherford Thought that the atom was all empty space. Used the Gold Foil Experiment to test his hypothesis. (1908 and 1909) Rutherford from New Zealand, working in England with his associates Hans Geiger and Ernest Marsden studied the bombardment of thin metal foils with fast-moving positively charged particles. Rutherford set up a source of alpha particles (tiny, positively charged particles) that was directed toward a very thin (~20,000 gold atoms thick) sheet of gold foil. He placed a detector screen around the foil. What Rutherford found was that most of the alpha particles passed straight through the foil to the detector screen. Only a few alpha particles (1 in 8,000) were deflected very sharply; some even bounced straight back toward the source of the alpha particles. Rutherford said this was "about as believable as if you fired a 15-inch shell into a piece of tissue paper, and it came back and hit you." Rutherford discovered the nucleus of the atom. He determined that the nucleus must be small and positively charged, which would account for so few of the particles being deflected. The empty space in the atom must be taken up by electrons moving around the nucleus at very fast speeds creating an electron cloud. Due to Rutherford's discovery, Thomson's plum pudding model of the atom was replaced by the nuclear model. This model contains a positively charged nucleus that contains almost all of the mass of the atom and very little of the atoms volume and an electron cloud that surrounds the nucleus.

Gold Foil Experiment Almost all of the particles pass through with a slight deflection BUT some particles came back. 1 in 8,000 particles ricocheted back to the source Geiger and Marsden checked for the possibility of wide-angle deflections. Much to their surprise, they found that 1 in 8000 came back.

Gold Foil Experiment Rutherford said it was “as if you had fired a 15-inch (artillery) shell at a piece of tissue paper and it came back and hit you.” Why did this happen? Rutherford reasoned that the fast-moving particles must be repelled by some powerful force within the atom. Also, whatever caused this repulsion must occupy a very small amount of space since only a very few particles ran into it.

The Nucleus

So how small is the nucleus? How large is an atom’s volume compared to its nucleus? Think of a football field and place a dime in the center of the 50 yard line.

Rutherford’s Atom The dime represents the nucleus of the atom while the outer edge of the football field would represent the outer edge of the atom. Rutherford concluded that the atom is mostly empty space. Rutherford’s model of the atom is the nuclear model.

A Puzzle If an atom has a positive center and the negative electrons are on the outside of the atom, why don’t the electrons fall into the center? ………………..Centrifugal force {an inertia force due to e- traveling in circles.}

Niels Bohr Developed the Planetary Model in 1913 Electrons move around the nucleus like planets move around the sun.

Bohr Bohr suggested that electrons travel in a specified path around the nucleus which he called energy levels. These energy levels are designated distances from the nucleus in which electrons may be found. The maximum number of electrons found in an energy level can be determined by the formula 2n2, where n = energy level. Electrons could gain energy to jump to an energy level further away from the nucleus or give off energy when returning to an energy level closer to the nucleus.

Werner Heisenberg 1927 – Heisenberg Uncertainty Principle It is not possible to know both the velocity and the position of an electron at the same time. German physicist

Erwin ShrÖdinger Austrian Physicist who developed an e- formula.             Austrian Physicist who developed an e- formula. His theory was able to determine the most likely AREA an e- is to be found. These areas are called Orbitals

James Chadwick Discovered the neutron in 1932 The neutron is a particle in the nucleus that has about the same mass as a proton, but has no charge.

Modern Day Model Two Main Parts The Nucleus Positively Charged PROTONS Neutral NEUTRONS Held together by the STRONG NUCLEAR FORCE The Electron Cloud e- moving about the nucleus in 3-D ORBITALS (s,p,d,f). The e- ORBITALS are positioned in ENERGY LEVELS

Properties of Subatomic Particles Charge Mass # Relative mass (a.m.u.) Actual mass (grams) Electron e- -1 0.0005486 9.109 x10 -28 Proton p+ +1 1 1.007276 1.673x 10 -24 Neutron n 1.008665 1.675x 10 -24

Fuzzy Blob of Uncertainty The Modern Model

Modern Day Model of the ATOM The modern day model is a collection of all the contributions of the previous scientists, from Dalton to Chadwick. Today we would have to include quarks which make-up the protons and neutrons in the nucleus. MURRAY GELL-MANN named the 6 Quarks after a line in the play “Finnegan’s Wake.” A good bonus questions might be to name these six flavors of quarks.

THE END Are you ready for the history of the Atom quiz?