1. Origin of the idea of atom…
GREECE : Democritus ( b.c.)
matter is made of tiny particles atomos
Atoms are solid, homogeneous, indestructible and indivisible
Different kinds of atoms have different sizes and shapes
Size, shape and movement of atom
determine the properties of matter.

2. John Dalton’s Atomic Theory- 1803
Five notions about atoms…
1. All matter is made of atoms (Democritus)
2. All atoms of given element are identical
3. The atoms of one element are different from other elements
4. Atoms combine in whole ratios to make compounds: LAW OF CONSTANT COMPOSITION (Joseph Proust)
5. In chemical reactions atoms are separated, combined, or rearranged: Conservation of Mass (Antoine Lavoiser)

3. Atomic Structure Atoms are composed of 3 subatomic particles-
Electron- (-) Charge
Proton- (+) Charge
Neutron- Neutral Charge

4. Electrons: J.J. Thomson- 1856-1940
Using a cathode ray tube he found that atoms emit tiny negative particles “corpuscle”
Found that atoms have particles that move AWAY from the negative end, so they must have a negative charge!

6. Electrons- Robert Millikan – 1868-1953
Used the “oil drop experiment” to determine the mass and charge of a single electron.

8. Protons- Thompson-1897 “Plum Pudding” Model
Electrons have a negative (-) charge
But atoms are neutral (no charge)
Therefore there must be a positive (+) charge too!

9. Protons- Ernest Rutherford - 1911
He shot (+) charged alpha particles at gold foil

11. Protons- Ernest Rutherford - 1911
Only a few positive particles were repelled
This disproved his hypothesis
There must be a VERY small positive central core (nucleus) , and the rest of the atom % is empty space.

12. -1919: positive particle = proton -1932: neutral particle = neutron

13. Bohr model 1915 Protons and Neutrons are in the nucleus
Electrons are outside the nucleus in orbitals
Electrons can move from one orbital to another, and release energy as electromagnetic radiation.

14. Electron cloud Model Electrons are in clouds (Schrodinger)
Quantum Model

16. 3 subatomic particles 1.007 1.009 -1 5.486x10-4
Name
(symbol)
Charge
Mass (amu)
(atomic mass unit)
Actual Mass
location
Proton
(p) +1
1.007
1.673x10-24 g
Nucleus
Neutron
(n)
1.009
1.675x10-24 g
Electron
(e-) -1
5.486x10-4
(so small that its not counted!)
9.109 x10-28g
Outside Nucleus