Models of the Atom The Greeks, Alchemists, Dalton, Thomson and Rutherford

Greek Philosophers Greek Philosophers More than 2000 years ago, Democritus and Aristotle contributed to the first thought of atomic particles More than 2000 years ago, Democritus and Aristotle contributed to the first thought of atomic particles Aristotle believed in four elements: Earth, Air, Wind and Fire Aristotle believed in four elements: Earth, Air, Wind and Fire Democritus believed that matter was made of small particles called atoms Democritus believed that matter was made of small particles called atoms Aristotle and Democritus used observations and inferences to explain the existence of everything Aristotle and Democritus used observations and inferences to explain the existence of everything

Alchemists European Scientists, years ago European Scientists, years ago Their developed into what is now known as Chemistry Their developed into what is now known as Chemistry Their work was inspired by the quest to turn ordinary objects into Their work was inspired by the quest to turn ordinary objects into

John Dalton (1803) Dalton was the first to find experimental support for the following: Atoms were the smallest possible Atoms were the smallest possible particle of matter. particle of matter. Each element had its own atoms that Each element had its own atoms that differed from others in mass differed from others in mass Atoms were the fundamental building blocks of matter and could not be split Atoms were the fundamental building blocks of matter and could not be split In chemical reactions, atoms would rearrange themselves and combine with other atoms in new ways In chemical reactions, atoms would rearrange themselves and combine with other atoms in new ways

The Billiard Ball Model of Atom Dalton’s model built on the ideas of Democritus that the atom was indivisible Dalton’s model built on the ideas of Democritus that the atom was indivisible

J.J. Thomson’s Cathode Ray A gaseous element is in the end of a tube A gaseous element is in the end of a tube When the power source is connected, a shadow is observed at the end of the tube When the power source is connected, a shadow is observed at the end of the tube Conclusion: an invisible beam of “something” must be “fired” from the cathode and is being blocked by the “mica” (a thin sheet of mineral) Conclusion: an invisible beam of “something” must be “fired” from the cathode and is being blocked by the “mica” (a thin sheet of mineral)

Atoms Are Not the Smallest Observation: In Thomson’s experiments, the “invisible beam” always behaved the same regardless of what type of element was used for the gas or cathode. Observation: In Thomson’s experiments, the “invisible beam” always behaved the same regardless of what type of element was used for the gas or cathode. Conclusion: The ray or particle is common to all atoms of the elements so the atom is not the smallest fundamental building block of matter Conclusion: The ray or particle is common to all atoms of the elements so the atom is not the smallest fundamental building block of matter

What is a cathode ray? Thomson “fired” cathode rays again in his modified cathode ray tube (CRT) Thomson “fired” cathode rays again in his modified cathode ray tube (CRT) He noticed that the “ray” bent towards the positive magnet (plate) He noticed that the “ray” bent towards the positive magnet (plate) Conclusion: the “ray” must be negatively charged because opposite charges attract Conclusion: the “ray” must be negatively charged because opposite charges attract

Above is an example of a cathode ray (negative charge) which is “bent” by a positively charged plate Above is an example of a cathode ray (negative charge) which is “bent” by a positively charged plate

Thomson’s “Plum Pudding” Model The particles making up the rays acted as if they were negatively charged. The particles making up the rays acted as if they were negatively charged. Since elements are neutral, there must be positive particles balancing out the negative particles. Since elements are neutral, there must be positive particles balancing out the negative particles. Thomson theorized that an atom consisted Thomson theorized that an atom consisted of a “pudding” of evenly distributed positive and negative particles.

Plum Pudding Model Thomson called the negative particles corpuscles. Thomson called the negative particles corpuscles. Later, English physicist G.J. Stoney coined the name electron Later, English physicist G.J. Stoney coined the name electron

Thomson – used a modified cathode ray tube to prove what he had hypothesized; the existence of a positively charged subatomic particle called a proton. It has equal but opposite charge to the electron (+1) and a mass 1840 times heavier than an electron.

Robert Millikan (1909) used his oil drop experiment to prove that the charge on of an electron is -1.

Ernest Rutherford (1911) Ernest Rutherford (1911) Bombarded a thin metal foil Bombarded a thin metal foil with positively charged with positively charged alpha particles. alpha particles.

What Rutherford Observed Called the Gold Foil Experiment

Analysis of Rutherford’s Experiment Observation: Most particles flew right through the foil as if there were nothing there Observation: Most particles flew right through the foil as if there were nothing there Conclusion: The foil was mostly empty space Conclusion: The foil was mostly empty space Observation: A small number of particles were bounced back to their source Observation: A small number of particles were bounced back to their source Conclusion: There must be a small, dense nucleus with a positive charge Conclusion: There must be a small, dense nucleus with a positive charge

The Rutherford Model of the Atom A dense, positively-charged nucleus containing most of the total mass of the atom A dense, positively-charged nucleus containing most of the total mass of the atom Negatively-charged electrons orbiting the nucleus in the empty space of the atom Negatively-charged electrons orbiting the nucleus in the empty space of the atom

Rutherford’s Model: Not all it is cracked up to be! Rutherford’s model is referred to as the “Planetary Model” of the atom –PROBLEM!!!!!!! Rutherford’s model is referred to as the “Planetary Model” of the atom –PROBLEM!!!!!!! According to experiments dealing with electric charge, the negatively charged electron should have been attracted to the positively charged protons in the nucleus. According to experiments dealing with electric charge, the negatively charged electron should have been attracted to the positively charged protons in the nucleus. However, if this were true, the electrons would be pulled into the nucleus causing the atom to collapse. However, if this were true, the electrons would be pulled into the nucleus causing the atom to collapse. Scientists knew that the atom was stable and this suggested that a change in atomic structure was still needed Scientists knew that the atom was stable and this suggested that a change in atomic structure was still needed

Rutherford theorized that the nucleus although positively charged must contain other particles in it Rutherford theorized that the nucleus although positively charged must contain other particles in it He theorized this based on the fact that the mass of the total protons in the nucleus was less than the total mass of the nucleus He theorized this based on the fact that the mass of the total protons in the nucleus was less than the total mass of the nucleus Something else must be in the nucleus? But what? The mystery continues… Solution to the Problem?

The same mass as a proton The same mass as a proton Was not affected by electric or magnetic fields Was not affected by electric or magnetic fields Conclusion: the particles did not have a charge (neutral) Conclusion: the particles did not have a charge (neutral) Chadwick had discovered the NEUTRON Chadwick had discovered the NEUTRON Chadwick received the Nobel Prize for his discovery (the poor Curies) Chadwick received the Nobel Prize for his discovery (the poor Curies) In 1932 Fredric and Irene Joliot- Curie discovered a beam of particles when they bombarded beryllium with alpha radiation In 1932 Fredric and Irene Joliot- Curie discovered a beam of particles when they bombarded beryllium with alpha radiation They did not realize the importance of their discovery They did not realize the importance of their discovery James Chadwick: a student of Bohr James Chadwick: a student of Bohr He used the Curie’s work to demonstrate that the beam of particles had: He used the Curie’s work to demonstrate that the beam of particles had: Mystery solved?

Neutrons have no charge so how does their existence in the nucleus prevent protons from pulling electrons into the nucleus and having the atom collapse? Neutrons have no charge so how does their existence in the nucleus prevent protons from pulling electrons into the nucleus and having the atom collapse? Why do protons not repel each other? Both of these are valid questions that need answering! How do the existence of neutrons solve the problems with the structure of the atom?

Neutrons and protons are made up of subatomic particles called quarks Quarks are shown to the right held together by a strong force called gluons Quarks are shown to the right held together by a strong force called gluons The gluons are represented by the coiled springs The gluons are represented by the coiled springs

A proton comprises 2 up and 1 down quarks, while the neutron comprises 1 up and 2 down quarks Gluons hold the types of quarks together Gluons hold the types of quarks together Gluons also play a role in creating a force between neutrons and protons. Gluons also play a role in creating a force between neutrons and protons. This force is called the stong nuclear force This force is called the stong nuclear force

The hydrogen atom does not collapse even though it has no neutrons. Why? It was this question that many scientists struggled with It was this question that many scientists struggled with It led to a new theory of the atom based on a new way of thinking called Quantum Mechanics It led to a new theory of the atom based on a new way of thinking called Quantum Mechanics

Subatomic Particles ProtonElectronNeutron Symbolp+e-n Charge1+1-0 Mass11/18371