The molecular nature of matter

Feynman singled out the atomic hypothesis as the idea most worth passing on if all other science were lost: everything is made of atoms that never stop moving, pull on each other when slightly apart and push apart when squeezed together.

The guess that matter is not continuous is old. Kanada in India (Vaiseshika school) and Democritus in Greece both proposed indivisible building blocks, but these ideas were never tested by measurement, so they did not grow into a science.

John Dalton is credited with the scientific atomic theory. He used it to explain two laws of chemical combination: a compound always has a fixed proportion of its elements by mass (definite proportions), and when two elements form several compounds, the masses of one that combine with a fixed mass of the other are in small whole-number ratios (multiple proportions).

Dalton's picture: an element's smallest parts are atoms, identical for one element and different from element to element; a few atoms join to form a molecule of a compound.

Gay Lussac's law: when gases react to give another gas, their volumes are in small whole-number ratios. Avogadro's law: at the same temperature and pressure, equal volumes of all gases hold the same number of molecules. Avogadro's law together with Dalton's theory explains Gay Lussac's law.

Scale: an atom is about 1 Å (10⁻¹⁰ m) across. In solids atoms sit about 2 Å apart, and in liquids at about the same spacing but free to move past one another, which is why liquids flow. In gases the spacing is tens of angstroms.

Mean free path, the average distance a molecule travels between collisions, is of the order of thousands of angstroms in a gas. That freedom is why an unenclosed gas spreads away.

The force between atoms attracts at a few angstroms and repels at shorter range.

A gas at rest only looks static. Its molecules keep colliding and changing speed; only the averages stay constant, so the equilibrium is dynamic.

Atoms are not the end of the story: they have nuclei and electrons, nuclei hold protons and neutrons, and those are made of quarks. This chapter stays with gases (and a little about solids) seen as collections of molecules in ceaseless motion.