ATOMIC THEORY

Democritus- 460-370 BC
Nothing exists except atomss and empty space;everything else is opinion."
Was the first to suggest atoms made up things and his theory was never backed up until Dalton.

JOHN DALTON 1776-1844

• Each element is composed of tiny, indestructible atoms.
• All atoms of an element are identical.
• Atoms combine in simple, whole # ratios to form molecules of compounds.
• Dalton was able to determine relative masses of atoms based on the scale:
• H=1 amu (only works for hydrogen)

JOSEPH JOHN THOMSON 1856-1940

PLUM PUDDING MODEL FINDINGS

• Electricity has no mass
• There is space between the electroms because they are negative.
• The atom is breakable
• The atoms structure has electrons suspended in a positively charged electric field.

ERNEST RUTHERFORD 1871-1937

ANSWER: Put something through it.

• Use large target atoms
• Use thin sheets of target
• Use small particles as "bullets"
• Target - gold foil
• Bullet - alpha particles (a)

RUTHERFORD'S RESULTS

• Over 98% of "a" particles went right through.
• Around 2% of "a" particles went through, but were deflected by angles.
• About 0.01% of the "a" particles bounced off the foil.

RUTHERFORD'S INTERPRETATION

• Atoms contain a tiny dense center called "nucleus"
• Nucleus takes up only 1/10 trillionth the volume of the atom
• Nucleus has essentially the mass of the atom.
• The nucleus is positively charged
• Electrons are dispersed around nucleus in empty space of atom. (Wrong)

THE NUCLEUS

RUTHERFORD PROPOSED..

• Nucleus has a particle that had the same amount of charge as an electron, but opposite sign.
• These are called "protons"
• There is also another type of particle in the nucleus called a neutron
• They have no charge.
• The mass of the proton and neutron are approximately 1 AMU.

NEILS BOHR

THE BOHR MODEL

• Electrons travel in orbits around the nucleus.
• The farther away the electron is from the nucleus, the more energy it has.
• Each orbit has a specific amount of energy, and the energy is characterized by a #.

When an atom gains energy, electron leaps to a higher energy orbit: called an excited state. When the opposite happens, light is emitted from the atom.