1. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
The Electron
Because of Dalton’s atomic theory, most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.
The first clue that this assumption was not correct came in the 1870s from the work of an English scientist named William Crookes.
Crookes developed a vacuum tube which he used to study the effect of electrical discharges on samples of pure gas.
Today, this tube is known as a Crookes’ Tube or Cathode Ray Tube.

2. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
The Electron
When Crookes applied electricity to a gas sample he noted that a green glowing effect was produced in the tube.
Upon closer inspection , he saw that the glow was produced from a diffuse beam of rays that originated at the cathode (the negative terminal or electrode of an electrical system). He called them cathode rays.
Crookes was not sure if his beam was a beam of light or if it was a beam of electrically charged atoms.
In order to determine the true composition of the beam, he placed a magnet close to the tube and noted that this deflected the beam away from its normal path.

3. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
The Electron
Because light beams cannot be deflected by a magnet, Crookes concluded that the beam must not be composed of light but, rather, that it must be composed of particles.
He also concluded that, because the particles were deflected by a magnet, the particles must carry some kind of electrical charge. He further concluded that the particles must have a negative charge due to their origination at the cathode and their interaction with the magnetic field.
But Crookes failed to connect his discovery to the atom…………A young apprentice working in his lab, JJ Thomson, would be the one who is credited with the discovery of electrons & protons.

4. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
The Electron
In 1877, a British physicist, J.J. Thomson, discovered and proved that Dalton’s solid-ball model was not accurate.
Thomson’s experiments were a based on Crookes’ CRT experiments.
Cathode-Ray Tube
When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.
Because these rays originate at the cathode, they are called cathode rays.

5. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Cathode-Ray Tube
Thomson found that the rays bent toward a positively charged electrical plate / magnetic pole and away from a negatively charged electrical plate / magnetic pole.
He knew that objects with like charges repel each other, and objects with unlike charges attract each other.
Thomson concluded that cathode rays are made up of invisible, negatively charged particles he called cathode ray particles.
Click box to view movie clip.

6. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Cathode-Ray Tube
Today we call cathode ray particles electrons.
From Thomson’s experiments, he had to conclude that atoms were not just neutral spheres, but somehow were composed of electrically charged particles.
Reason told him that there must be a lot more to the atom than electrons.
Matter is not net negatively charged, so atoms can’t be made exclusively from negatively charged particles either.

7. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Cathode-Ray Tube
Thomson reasoned that if atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles—probably with a much greater mass than electrons.
In 1886, Thomson discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode. These rays travel in a direction opposite to that of cathode rays.
Like cathode rays, they are deflected by electrical and magnetic fields, but in directions opposite to the way cathode rays are deflected.

8. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Protons
Thomson was able to show that these anode rays were composed of positively charged particles he called anode ray particles.
Today we call anode ray particles protons.
Thomson’s findings prompted him to suggest a new model of the atom he called the plum-pudding model.
Thomson’s new model proposed that the atom was a neutral, solid, uniformly dense, divisible sphere composed of two types of oppositely charged particles evenly distributed throughout the atom

9. Thomson’s Cathode-Ray Tube Experiment
Visual Concepts
Chapter 3
Thomson’s Cathode-Ray Tube Experiment

10. Chapter 3 Charge and Mass of the Electron
The Atom: From Philosophical Idea to Scientific Theory
Chapter 3
Charge and Mass of the Electron
Using data from his cathode ray experiments, JJ Thomson tried to determine the values for the amount of electrical charge (e ) and the mass ( m ) possessed by the electron.
Although he was unable to determine the values for e or m independent of each other, he was able to calculate the ratio of charge to mass (e/m) for the electron.
Thomson’s charge/mass ratio of the electron is x 108 coulombs/gram.

11. Chapter 3 Charge and Mass of the Electron
The Atom: From Philosophical Idea to Scientific Theory
Chapter 3
Charge and Mass of the Electron
In 1911, the American physicist, Robert A. Millikan was able to determine the amount of charge possessed by an individual electron from the results of his oil drop experiment.
e= x coulombs
With this information, he was able to determine the mass of an individual electron as well by using Thomson’s e/m ratio.
e/m= x 108 coulomb/gram
1.602 x coulombs/m= x 108 coulomb/gram
m= 9.11 x grams

12. Millikan’s Oil Drop Experiment
Visual Concepts
Chapter 3
Millikan’s Oil Drop Experiment

13. Discovery of the Atomic Nucleus
The Atom: From Philosophical Idea to Scientific Theory
Chapter 3
Discovery of the Atomic Nucleus
In 1909 the next major advance in unlocking the secrets of the atom’s structure resulted from the work of another English scientist named Ernest Rutherford..
The results of his gold foil experiment led to the discovery of a very densely packed, positively charged bundle of matter that was centrally located in the atom’s structure.
Rutherford named this positive bundle of matter the nucleus.

14. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Rutherford was involved in studying radioactivity and it was by accident that he changed the way that scientists pictured the atom.
Rutherford was interested in studying the deflection of alpha particles by atoms with large atomic masses.
An alpha particle (α) is a high speed, positively charged particle that has a mass equal to the mass of a helium nucleus. Alpha particles are considered to be radioactive emissions that are released when certain radioactive elements (in his case- Polonium )decay or disintegrate.
Rutherford’s observations of the deflection patterns of alpha particles could not be explained using the current Thomson model of the atom. This compelled Rutherford to generate a new atomic model ( Rutherford Nuclear Model ) that could logically explain the cause of the unexpected deflections that he witnessed.

15. Chapter 3 Gold Foil Experiment
The Atom: From Philosophical Idea to Scientific Theory
Chapter 3
Gold Foil Experiment

16. Gold Foil Experiment on the Atomic Level
The Atom: From Philosophical Idea to Scientific Theory
Chapter 3
Gold Foil Experiment on the Atomic Level

17. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Rutherford’s interpretations of results from gold foil experiment:
Undeflected particles- most of the atom’s volume is empty space outside the nucleus through which the electrons travel
Small angle deflections- the atom contains a very small, densely packed, central core of mass.
Large angle deflections- the central core of mass contains all of the protons and has positive charge.

18. Rutherford’s Gold Foil Experiment
Visual Concepts
Chapter 3
Rutherford’s Gold Foil Experiment

19. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Problems/Flaws with Rutherford model
Insufficient explanation of electron behavior - electrons moving in a circular path around nucleus should be giving off visible light and other forms of electromagnetic wave energy thus losing energy and eventually collapsing into nucleus.
Insufficient explanation of nuclear structure- due to strong repulsion forces between like charged protons, the nucleus would be extremely unstable and should not be able to stay intact.
In 1932, the English scientist, James Chadwick, would solve the nuclear issue by confirming the existence of a neutral particle in the nucleus he named the neutron.
In 1913, the Danish scientist, Niels Bohr would begin to solve the electron issue by providing an atomic model ( Bohr / Planetary Model )in which electrons would be able to move around outside the nucleus without losing energy.

20. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
Bohr Atomic Model
The key idea in Bohr’s model of the atom is that there are certain definite orbits in which an electron can travel around a nucleus without radiating or losing energy.
main points of the Bohr model:
1. Each of the electron orbits is a circular orbit at a fixed distance from the nucleus.
2. An electron in a given orbit has a certain, fixed, definite amount of energy.
3. The greater the radius of the orbit, or the distance of the electron from the nucleus, the greater the energy of an electron in that orbit will be. For this reason the possible electron orbits are known as energy levels.

21. Chapter 3 The Atom: From Philosophical Idea to Scientific Theory
4. The only way an electron can lose energy is by dropping from one energy level to a lower one. When this happens, the atom emits its energy Ioss in the form of light, that will be equal to the energy difference between the two levels.
5. As long as electrons remain in their orbits they do not lose energy.
6. EIectrons in the lowest energy level cannot lose any energy because they cannot move any closer to the nucleus.
7. Each energy level has a maximum number of electrons that it can contain.
[energy level/max # e- : 1/2, 2/8, 3/18, 4/32……]
8. EIectrons in higher energy levels cannot drop to a lower level if that lower level Is filled with its maximum number of electrons.
9. In every atom in its stable state, known as the ground state, the electrons will always be found in the lowest energy levels available.

22. The Charge-Cloud Model (current)
The Atom: From Philosophical Idea to Scientific Theory
Chapter 3
The Charge-Cloud Model (current)
By 1950 a new model of the atom replaced the Bohr model. Using computers, it is a mathematical probability based model that describes the locations of electrons based on a unique set of 4 quantum numbers possessed by each electron.
The main points of the charge-cloud model are as follows:
1. The charge cloud is composed of different principal energy levels (energy shells).
2. The further away from the nucleus an electron is, the more energy it will posses.
3. Each principal energy level contains sublevels of energy.
4. Each sublevel has a slightly different energy than the other sublevels in the same principal energy level.
5. Within each sublevel there are regions of space called orbitals.
6. Orbitals may be empty or occupied by a maximum of two electrons having the exact same amount of energy.
7. At the center of the atom is a positively charged nucleus that contains all of the atom’s protons and neutrons and where most of the mass of the atom is concentrated.

23. Chapter 3

24. Chapter 3 Multiple Choice
Standardized Test Preparation
Chapter 3
Multiple Choice
2. An important result of Rutherford’s experiments with gold foil was to establish that
A. atoms have mass.
B. electrons have a negative charge.
C. neutrons are uncharged particles.
D. the atom is mostly empty space.

25. Chapter 3 Multiple Choice
Standardized Test Preparation
Chapter 3
Multiple Choice
2. An important result of Rutherford’s experiments with gold foil was to establish that
A. atoms have mass.
B. electrons have a negative charge.
C. neutrons are uncharged particles.
D. the atom is mostly empty space.