Origin of the idea of atom… GREECE : Democritus (460 -370 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.

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)

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

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!

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

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!

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

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 99.999% is empty space.

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

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.

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

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