The foundations of chemistry
Chemistry is the science of what things are made of and how they change. Almost everything in it follows from a handful of ideas worked out between the 1780s and the 1930s. This site sets those ideas out in the order they depend on one another.
The arrangement of atoms in space — which is bonded to which, and in what shape — determines what a substance does. Kekulé’s structural theory (1858), van ’t Hoff and Le Bel’s tetrahedral carbon (1874) and X-ray crystallography (the Braggs, 1913) made structure something that could be drawn, then measured.
Thermodynamics says whether a reaction can go; kinetics says how quickly. Arrhenius’s equation (1889) ties rate to temperature through an activation energy, and a catalyst works by offering a path with a lower one.
A reaction runs in the direction that lowers Gibbs free energy, and stops at equilibrium. Gibbs set this out in a paper printed in parts between 1875 and 1878; Le Chatelier’s principle (1884) describes how an equilibrium shifts when it is disturbed.
Atoms bond by sharing or transferring electrons. Lewis proposed the shared electron pair in 1916; Heitler and London gave it a quantum-mechanical basis in 1927; Pauling’s The Nature of the Chemical Bond (1939) turned it into a working theory.
Arranged by atomic number, the elements repeat their properties at regular intervals. Mendeleev published his table in 1869 and left gaps for elements not yet found; Moseley showed in 1913 that the ordering principle is nuclear charge, not weight.
Mass is conserved (Lavoisier, 1789), compounds have fixed proportions, and equal volumes of gas hold equal numbers of molecules (Avogadro, 1811). Together these let chemists count atoms by weighing. Since 2019 the mole is defined as exactly 6.02214076 × 10²³ entities.
All ordinary matter is built from atoms of a limited number of elements — 118 are known. Dalton’s atomic theory (1808) made the idea quantitative: each element has atoms of a characteristic mass, and compounds are atoms combined in whole-number ratios.
Read from the bottom up. Each idea needs the one beneath it: nothing can be weighed out without atoms to count, and nothing can be predicted without knowing how they bond.