1. Atomic Structure

2. Democritus (400 B.C.)
Believed that matter was composed of tiny particles called atoms
His ideas were rejected by others because he had no evidence

3. John Dalton (1800’s) Dalton’s Atomic Theory
All matter is composed of atoms
All atoms of a given element are identical
Atoms cannot be created, divided or destroyed
Different atoms combine to form compounds
In chemical reactions atoms are rearranged.

4. Thompson (1890’s)
Identified the electron through use of a cathode ray tube.
Determined that electrons carry a negative charge and have an extremely small mass
Created the Plum Pudding Model (chocolate chip cookie dough model)
Electrons: charge = symbol = e-

5. Plum Pudding Model
Negatively charged electrons were distributed throughout a uniform positive charge

6. Rutherford (1900’s) Gold Foil Experiment
Shot alpha particles (like a laser beam) at a thin piece of gold foil
Expected the alpha particles to go through foil, but found that some particles bounced back

7. Gold Foil Experiment

8. Rutherford (cont’d)
Discovered the nucleus - tiny dense region which contains all of the atom’s positive charge and almost all its mass
Developed the Nuclear Model – nucleus in center and electrons found in empty space surrounding nucleus

9. Nuclear Model Plum Pudding Model
Because some of the alpha particles bounced back – they must have hit something with a positive charge

10. Inside the nucleus
Protons – subatomic particle with a charge equal and opposite to an electron
Discovered by Rutherford
Proton: charge = symbol = p+
Neutrons – subatomic particle with a mass almost equal to that of a proton but has no charge (neutral)
Discovered by Chadwick
Neutron: charge = symbol = n0

11. Bohr (1900’s)
Discovered that e- have energy of their own that keep them from being drawn into the nucleus
Believed that e- were found in paths (orbits) around the nucleus
Developed the Planetary Model

13. Schrodinger (1920’s)
Found that e- were not in orbits, but instead were found in 3D clouds (shells)
Developed the Modern Model of the atom

14. MODERN ATOMIC MODEL

15. How Atoms Differ
The number of protons determine the identity of the element
Atomic Number = number of p+
Atomic Number = number of e-
(in a neutral atom)
Mass Number = sum of p+ and no

16. Nuclear Symbol (Symbolic Notation)

17. Examples
p+ =
e- =
no =
p+ =
e- =
no =

18. ISOTOPES
Atoms of the same element that have the same number of protons, but a different number of neutrons.
Same atomic number, different mass number
Carbon The number after the name
Carbon is the mass number
Carbon-14

19. Using the Periodic Table

20. Atomic Mass
Weighted average of the masses for all isotopes of an element
If you round the atomic mass on the periodic table to the nearest whole number, you will get the mass number of the most abundant (common) isotope

21. Calculating Atomic Mass
Example