ATOM TIMELINE

DEMOCRITUS 400 B.C.E.

ATOMOS THEORY

• Matter could not be divided into smaller and smaller pieces forever.
• Eventually the smallest piece possible would he obtained.
• This piece would be indivisible.
• The smallest piece was named atomos, meaning "cannot be cut".

ATOMOS MODEL

• Democritus thought all atoms were small hard particles made of the same material.
• Could be different shapes and sizes.
• Atoms were infinite in number.

ARISTOTLE 350 B.C.E.

4 ELEMENTS THEORY

• Aristotle believed that everything was made of Fire, Earth, Air, and Water.
• He did not believe that atoms made up everything.
• Many people believed this idea instead of Democritus' for many years.

ATOM MODEL

• Aristotle did not really believe that atoms existed.
• He instead thought that everything had small ammoumts of each of the 4 elements.

DALTON 1803

DALTON'S THEORY

• All matter is composed of atoms.
• Atoms are indivisible and indestructable.
• All atoms of a given element are identical.
• A chemical reaction is a rearrangement of atoms.

BILLIARD BALL MODEL

• Atoms cannot be divided further.
• He knew nothing of what might be inside of an atom.

THOMSON 1897

PLUM PUDDING MODEL

• This provided the first hint that atoms were made up of even smaller paricles.
• He suggested that just as plum pudding has plum chunks in it, so does an atom have smaller things within it.
• He said that there were negatively charged particles scattered about the atom
• He did not yet know that the electrons had an orbit.
• Atoms were divisible.

CATHODE RAY EXPERIMENT

• Thomsons studied the passage of an electric current through a gas.
• As the current passed through the gas it gave off rays of negativity.
• This led Thomson to believe that atoms had negatively charged particles.

RUTHERFORD 1911

RUTHERFORD' MODEL

• Rutherford suggested that there was a "nucleus" at the center of the atom.
• He also thought that there was an electron cloud, the elctrons orbited the nucleus.
• The positive charge was centered in the nucleus.

GOLD FOIL EXPERIMENT

• Rutherford performed a series of experiments where he pointed a beam of alpha particles at gold foil.
• He expected the particles to all pass through, but some were bounced back towards the source.
• This suggested that the positive charge was concentrated in a smaller area than had been thought previously.

MILLIKAN 1909

OIL DROP EXPERIMENT

• The experiment balanced charged droplets of oil with upward drag.
• He measured the force on oil droplets in a glass chamber.
• Using this, Millikan determined the charge and mass of Electrons.

CHADWICK 1932

DISCOVERY OF NEUTRONS

• Chadwick bombarded beryllium atoms with alpha particles.
• An unknown radiation was produced.
• Chadwich interpreted this as a particle having a neutral charge.
• This particle became known as the neutron.