•inned the term "atom" for the smallest substance

•Indivisible & indestructible

Democritus

"Dalton’s Atomic Theory"

1.All elements made of atoms –indivisible

2.Atoms of the same elements are identical & different elements are different

3.Atoms of different elements can combine in simple whole number ratios

4.Atoms can’t change into other elements in a chemical reaction

•Discovered the electron & the charge of an electron by using a "cathode ray tube"

Cathode

( -)

Anode

( + )(+)

(-)

–Referred to as: the "plum pudding" model

Sphere of positive electricity with negative electrons floating around

WHY?

Protons had not been discovered!

•Determined the quantity of charge of the electron and the ratio of charge to mass of an electron

•HOW?

–OIL DROP EXPERIMENT

–Millikan oil-drop experiment

•iscovered the proton and it’s charge using the cathode ray tube

•Confirmed the existence of the neutron

p+

p+

p+

p+

p+p+

p+e-

e-e-

e-

e-

e-

e-

•Discovered that the atom is mostly made of empty space with a small dense positively charged core

•HOW? Using theGOLD FOIL EXPERIMENT

•gold foil simulation

Nucleus

(containing protons and neutrons)e-

e-

e-

e-

e-

e-e-

e-

•Believed that electrons travel in definite orbits around the nucleus

•eferred to as "planetary model"

•** theory based solely on Hydrogen atom

BOHR MODEL

QUANTUM MECHANICAL MODEL

-

-

-

-

-

-