ATOMS

DEMOCRITUS 400BC

ATOMOS THEORY

• First person to come up with the idea of the atoms
• Kept dividing matter until he could not divide anymore
• Atoms are indestructable
• Atoms are solid but invisible
• Atoms differ in size, shape, mass, position, and arrangement

ARISTOTLE 380BC

4 ELEMENTS THEORY

• All substances to be compounds of 4 elements: earth, water, air and fire
• These are a combination of 2 of 4 opposites, hot & cold, & wet & dry
• Discussed properties of real substances in terms of “elemental” composition
• Ex: water evaporates on heating because it goes from cold & wet to hot & wet

DALTON 1800'S

BILLIARD BALL THEORY

• Atoms make up all matter
• Cannot be created, destroyed, or subdivided
• Elements can be distinguished based on weights
• Combine with/separate from other atoms to form compounds in reactions.
• Atoms are not destroyed during chemical reactions, they are just rearranged

THOMSON 1897-1904

PLUM PUDDING MODEL THEORY

• Discovered the electron, a negatively charged particle more than 2,000x times lighter than a hydrogen atom.
• The electrons were thought to be positioned throughout the atom
• Electrons surrounded by a soup of positive charge to balance the electrons' negative charges,
• Abandoned after discovery of the atomic nucleus
• Did the cathode ray experiment

RUTHERFORD 1911

POSITIVELY CHARGED NUCLEUS MODEL

• Gold foil experiment
• Found that the nucleus was positively charged
• Nucleus contained most of the atomic mass of the atom
• Proved that Thomson's model was incorrect

MILLIKAN 1908-1917

MASS/CHARGE OF ELECTRON MODEL

• Oil drop experiment
• Confirmed Thomson's hypothesis
• Proved the mass of an electron is at least 10000x smaller than lightest atom
• Showed that the charge on a drop was always an integral multiple of a small,finite value.

CHADWICK 1932

PROTON NEUTRON MODEL

• Discovery of neutron
• Nucleus is made of protons AND neutrons
• Mass of neutron is about 0.1% more than a proton's
• Neutrons contain no charge
• Prepared the way for the creation of the atomic bomb