Brief Summary of Atomic Models and Laws

Democritus Model of the Atom

• Proposed in the 4th century B.C. by Greek philosophers Democritus, Aristotle, Leucippus, and Epicurus.
• Believed all matter is made of atoms.
• Atoms were considered tiny, indivisible bodies of different sizes, shapes, and materials.

Dalton's Atomic Theory

• John Dalton (1766 – 1844) discovered that matter is made up of atoms of different weights.
• All matter is made of atoms.
• Atoms of a certain element are identical; atoms of different elements are different.
• Atoms cannot be subdivided, created, or destroyed.
• Atoms combine in simple, whole number ratios to form compounds.
• In chemical reactions, atoms are rearranged.

Thomson's Atom

• J.J. Thomson (1856 – 1940) discovered electrons in 1897.
• Experimented with electric current through gas at low pressure using a cathode ray tube.
• Concluded that electrons are parts of the atoms of all elements.

Rutherford's Atom

• Ernest Rutherford (1871 – 1937) discovered the atom has a nucleus in 1911.
• Experimented using a thin sheet of gold foil.
• An atom is mostly empty space.
• All the positive charge of an atom is concentrated in a small region called the nucleus.
• The nucleus is composed of protons and neutrons.
• Nearly all mass of an atom is in its nucleus

First Modern Atomic Ideas

• Law of Definite Proportions: A compound always contains the same elements in the same proportion by mass.
• Law of Multiple Proportions: If two (or more) elements can make more than one compound, then the compounds made always have mass ratios of small whole numbers. Examples – H2O, NH3