Early Atomic Theory and Evolution

early ideas in atomic theory –

atomos Democritus’ idea that matter is made of tiny indivisible particles called atomos.

Democritus

Proposed that matter is composed of small, indivisible particles; properties come from atom shapes.

Leucippus

Early Greek philosopher who helped develop atomism; influenced Democritus.

Aristotle

Rejected atomism; believed matter was continuous and made of four elements.

John Dalton

Revived atomic theory with experimental evidence; created Dalton’s Atomic Theory.

Dalton’s Atomic Theory

All matter is made of atoms; atoms of same element are identical; atoms cannot be created or destroyed; atoms combine in whole-number ratios.

DAT modification (one type of atom) –

isotopes Atoms of the same element can have different masses due to isotopes.

DAT modification (divisible) –

splitting the atom Atoms are divisible into smaller particles (protons, neutrons, electrons).

Law of Conservation of Matter

Matter cannot be created or destroyed; total mass stays constant.

Law of Definite Proportions

A compound always contains the same elements in the same mass ratio.

Law of Multiple Proportions

When two elements form multiple compounds, their mass ratios combine in small whole numbers.

DAT and Law of Definite Proportions

Dalton used this law to support that atoms combine in fixed ratios.

J.J. Thomson –

discovery of the electron Used cathode ray tubes to discover the electron; proposed the plum pudding model.

Robert A. Millikan Oil-drop experiment;

determined the charge of the electron.

Ernest Rutherford Gold foil experiment;

discovered the nucleus and that atoms are mostly empty space.