• Atoms of the same element with different mass number. 

• These have the same number of protons but a different number of neutrons.

• Atomic Mass Units

• 1/12 of mass of carbon-12 atom mass

(mass)(%) + (mass)(%)

100

Thomson

A sphere of positive charge with electrons able to be removed from it.

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Most of atom's mass was concentrated in the nucleus which lay in the center of atom.  Electrons revolved around the nucleus.

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• Bohr

• Electrons orbit the nucleus like planets around the sun.

Region around the nucleus where electrons may be found.

In the center of the atom.  Contains the protons and neutrons.

• Atom is a solid sphere and is indivisible (cannot be broken down into smaller pieces). 

• Atoms of the same element are identical.

Attributed to Schrodinger and Heisenberg

Consists of 2 regions

• Electron cloud (probability of where electrons might be found)

• Nucleus

• Emitted by some radioactive materials

• Helium nucleus

• Has mass and charge

• Can be blocked by a sheet of paper

• Emitted by some radioactive materials

• High speed electrons

• Has a charge but little mass

• At least a thin sheet of metal is necessary to block it

• Emitted by radioactive materials

• Highly energetic light

• No mass and no charge

• At a minimum, thick concrete is needed to block it

Discovered the electron

Model was the plum pudding model