ATOMIC THEORY TIMELINE

DEMOCRITUS 400 BC

ATOMOS THEORY

• Here is Democritus theory that he adopted from Leucippus
• Atoms are indestructible, are solid but not invisible and are homogenous
• Atoms differ in size, shape, mass, position, and arrangement
• All matter consists of invisible particles called atoms

ARISTOTLE 384 BC

EARTH,WIND,FIRE,AND WATER ELEMENTS

• Aristotle supported and added to the idea that everything was mace up of..
• 4 elements, earth, wind, fire, and water
• The elements were "pure" but could not be found in that state on earth.
• Every visible thing was made up of some combination of earth, water, air, and fire.

JOHN DALTON 1803

BILLIARD BALL MODEL

• Where Dalton and Democritus would have agreed...
• is that atoms were the smallest, most basic unit of matter.
• Where Dalton advanced atomic theory was by saying we had many different atoms out there.
• His model basically says you can't divide the atom into smaller pieces.

JJ THOMPSON 1897

PLUM PUDDING MODEL

• Thomson originally believed that the hydrogen atom must be made up of more than two thousand electrons
• Later Thomson suggested that atoms contained far fewer electrons, a # roughly equal to the atomic number.
• Thomson proposed a 'plum pudding' model...
• With positive and negative charge filling a sphere only one ten billionth of a metre across.
• This plum pudding model was generally accepted.

CATHODE RAY EXPERIMENT

• First experiment was to build a cathode ray tube with a metal cylinder on the end
• It had two slits in it, leading to electrometers, which could measure small electric charges
• He found that by applying a magnetic field across the tube.....
• There was no activity recorded by the electrometers and so the charge had been bent away by the magnet
• This proved that the negative charge and the ray were inseparable and intertwined.

ERNEST RUTHERFORD 1911

POSITIVELY CHARGED NUCLEUS MODEL

• Rutherford bombarded gold foil with alpha particles
• A source which undergoes alpha decay is put in a lead box with a hole in it
• Any of the alpha particles which hit the inside of the box are stopped
• Only those which pass through the opening are allowed to escape, they follow a straight line to the gold foil
• The atom is 99.99% empty space. Nucleus contains postitive charge. Nucleas is 100,000 X smaller than atom

ROBERT MILIKAN 1909

MASS/CHARGE OF ELECTRON

• An experiment performed by Robert Millikan determined the size of the charge on an electron.
• Put a charge on a drop of oil, measured how strong an electric field had to be to stop the drop from falling
• He could calculate the force of gravity on one drop
• He could then determine the electric charge that the drop must have

JAMES CHADWICK 1935

PROTON NEUTRON MODEL

• Chadwick reported the first accurate measurement of the mass of the neutron
• finding it to be slightly greater than the mass of the proton
• He then predicted that an unbound neutron is unstable and would undergo beta decay