Greek philosopher

discovered idea of atom

(compared to grain of sand)

around 400 B.C.

compounds always have a definite proportion

(ratio) of each element by mass

(Proust)

atoms - like different sized marbles (solid)

atoms of different elements vary in size and mass only

came up with Atomic theory

first x-ray pictures -

produced by cathode ray tube

first x-ray - his wife's hand

experiment to test cathode ray

- ray made of high speed, negative particles

- called them electrons

modified Dalton's model -

plum pudding model  with raisins (electrons) embedded in it

oil drop experiment

charge from electron was negative

discovered radioactive elements

Polonium (Po)

Radium (Ra)

alpha particles

beta particles

gamma rays (most dangerous)

Gold Foil Experiment

1) atom is mostly empty space

2) dense nucleus in middle with a positive charge

3) electrons on outside - negative charge

identified another particle in nucleus - neutron

about same mass as proton

neutral charge

supported Democritus' theory of atoms - but no proof

atom with an electrical charge other than zero

(formed by a change in number of electrons - if the number of protons changed, it would be a different element)

      mass #        23          1+     electrical charge

                               Na

     atomic #       11