1. CHAPTER 4
ATOMIC STRUCTURE

2. 4.1 Early Theories of Matter
In about 400 B.C., Democritus proposed the earliest recorded atomic theory. He believed that matter was made up of tiny particles, or atoms.

3. In the 1800’s, John Dalton proposed that “matter can be changed in many ways, but it cannot be created or destroyed”.

4. 4.2 Subatomic Particles and the Nuclear Atom
In 1897, J.J. Thomson used a cathode ray to study subatomic particles. He discovered the existence of electrons.

5. Electrons Have a negative charge.
Are the smallest and fastest subatomic particles
Are located on the outside of the nucleus
The number of electrons are determined by an atom (element’s) atomic number.
Electrons can sometimes be lost or gained

7. Earnest Rutherford discovered the proton in 1920.
James Chadwick discovered the neutron in 1932.

8. Protons Have a positive (+) charge
Are slightly smaller and less dense than a neutron
Are located inside the nucleus
The number of protons are determined by an atom’s (element’s) atomic number.
This number never changes for an element

9. Neutrons Have a neutral (0) charge
Are the biggest and heaviest subatomic particle
Are located inside the nucleus
Accounts for most of the atom’s mass
Neutrons are determined by taking the
Mass # - Atomic #
This number can vary

10. In 1910, Robert Millikan was the first scientist to obtain an accurate measurement of an electron’s charge (-1).

11. Gold-Foil Experiment
The Gold-Foil Experiment was performed in The experiment launched radiation into a thin strip of gold.

12. Gold-Foil Experiment
The Gold-Foil Experiment proved that atoms are made of mostly empty space.
Also, most of the mass of the atom is contained in the nucleus.

14. What is What

15. Example 1:
P E N Symbol
Actinium ___ ___ ___ ____
Magnesium ___ ___ ___ ____

16. PP1:
P E N Symbol
Carbon ___ ___ ___ _____
Chlorine ___ ___ ___ _____
Hydrogen ___ ___ ___ _____
Yttrium ___ ___ ___ _____

17. 4.3 How Atoms Differ
Isotopes are atoms that have the same number of protons but a different number of neutrons.