1. The Structure of the Atom
Section 3.2

2. Introduction
Atom: the smallest particle of an element that retains the chemical properties of that element
Nucleus: very small region located at the center; composed of proton(s) and usually one or more neutrons
Proton: a positively charged particle in the nucleus

3. More Neutron: a neutral particle found in the nucleus
Electron: a negatively charged particle found in a region surrounding the nucleus
Subatomic particles: protons, neutrons, and electrons

4. Discovery of the Electron
A glass tube was prepared with a gas in it with metal disks at each end
The cathode ray tube was connected to a vacuum pump because the charges will only flow through gases at low pressure
An electric current was passed through various gases

5. More
When the current was passed through the gas, the surface of the tube opposite the cathode (negative charge) glowed
Also, the ray of light traveled from the cathode to the anode
The rays were deflected away from a negatively charged object

6. More
In 1897, J.J. Thomson used the cathode ray tube to deduce the presence of a negatively charged particle: the electron

7. Charge and Mass of the Electron
Thomson’s experiment revealed that the electron has a very large charge-to-mass ratio
In 1909, Robert Millikan measured the charge of the electron
More accurate experiments were conducted later to determine the mass

8. Mass of the Electron Mass of the electron is 9.109 x 10-31kg
The oil drop apparatus
Robert Millikan

9. Thomson’s Model of the Atom
Thomson’s model is called the plum pudding model
His model is not valid
He believed that the negative electrons were spread evenly throughout the positive charge of the rest of the atom

10. J. J. Thomson and his Model

11. Ernest Rutherford’s Gold Foil Experiment
Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil

12. Rutherford’s Findings
Most of the particles passed right through
A few particles were deflected
Some were greatly deflected
“Like howitzer shells bouncing off of tissue paper

13. Conclusions of the Experiment
The nucleus is small
The nucleus is dense
The nucleus is positively charged
The atom is mostly empty space

14. Rutherford’s Model
All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a nucleus
The nucleus is composed of protons and neutrons (they make the nucleus!)
The electrons are distributed around the nucleus, and occupy most of the volume
His model was called a “nuclear model”

15. Rutherford’s Model

16. Other Discoveries
Eugene Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1836 times that of an electron)
1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

17. Subatomic Particles Particle Relative Charge Relative mass Location
Actual mass in kg
Electron (e-) -1
__1__
1836
electron cloud
9.109 x 10-31
Neutron (n0) 1
nucleus
1.675 x 10-27
Proton (p+) +1
1.673 x 10-27

18. Other Interesting Stuff
Atoms are electrically neutral because they contain an equal number of protons and electrons
The number of protons determines the atom’s identity
There are other subatomic particles, but they have little effect on the chemical properties of matter

19. Forces in the Nucleus
Protons, with their positive charge, would be expected to repel each other in the nucleus
When they are extremely close to each other, there is a strong attraction
A similar attraction exists when neutrons are present

20. More
Nuclear forces: the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus
Nuclei are very dense

21. Sizes of Atoms
Electrons occupy the outer region known as the electron cloud
Radius of an atom is the distance from the center of the nucleus to the outer portion of the electron cloud
Atomic radii are extremely small so they are usually measured in picometers
1 x 1012 pm = 1 m