Atomic theories

Definite composition

A law stating that a chemical compound always contains the same proportion of elements by mass.

Multiple proportions

When two elements form more than one compound, the ratio of the masses of one element that combine with a fixed mass of the other is a small whole number.

Conservation of mass

A fundamental principle stating that mass cannot be created or destroyed in a chemical reaction.

Billiard ball model

Dalton's model of the atom, which describes atoms as indestructible and indivisible spheres.

Cathode ray

A stream of electrons observed in vacuum tubes, demonstrating the existence of electrons.

Chocolate chip cookie model

Thompson's model of the atom, describing electrons as 'chocolate chips' embedded in a 'cookie' of positive charge.

Gold foil experiment

Rutherford's experiment that demonstrated that atoms are mostly empty space with a dense, positively charged nucleus.

Atomic model

Rutherford’s model proposing that the atom consists of a dense nucleus surrounded by orbiting electrons.

Neutrons

Neutral particles in the nucleus of an atom, discovered by Chadwick, that help explain atomic mass discrepancies.

Planetary model

Bohr's model that describes electrons orbiting the nucleus at fixed energy levels.

Electron Cloud model

Schrödinger's model which suggests that electrons exist in a cloud of probabilities rather than distinct orbits.