1. matter is composed of tiny particles he named atoms

2. atoms are indivisible 

the proportions of elements in a compound are always the same

ex: water is always H₂O

1. atom is not indivisble. it is made up of subatomic particles
2. atoms of the same element are not exactly alike. they can have different masses (isotopes)

1. discovered electron using a cathode ray tube
2. electrons have a mass of 1/1837 amu
3. there are (+) and (-) charges
4. opposites attract, same repel

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"plum pudding model"

1. electrons and protons are scattered throughout

GOLD FOIL EXPERIMENT

1. atom is mostly empty space
2. nucleus has a positive charge because of protons
3. nucleus is small and dense
4. electrons move about the empty space

Neils Bohr

1. electrons are outside the nucleus
2. electrons move in a distinct pathway around the nucleus called orbitals
3. each orbit is a fixed distance away from nucleus
4. electrons cannot exist between energy levels
5. shells given letters K-Q or 1-7

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"Bohr's Model" or the "Planetary Model"

1. uncertainty principle

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"Electron Cloud Model" or "Wave Mechanical Model"

1. an electron's path cannot be predicted. it can only be estimated
2. the more precisely the position of a particle is given, the less precisely can one say what its momentum is and vice versa

1. shot alpha particles (+) through a thin gold foil
2. most went throught, but some bounce back