Glossary
The vocabulary of chemistry, in plain words.
- Atom
- The smallest unit of an element: a nucleus of protons and neutrons surrounded by electrons.
- Element
- A substance whose atoms all have the same number of protons.
- Isotope
- Atoms of one element that differ in their number of neutrons.
- Ion
- An atom or molecule carrying a net electric charge.
- Molecule
- Two or more atoms held together by covalent bonds.
- Compound
- A substance made of two or more elements in fixed proportion.
- Mole
- The SI unit of amount of substance: exactly 6.02214076 × 10²³ entities.
- Stoichiometry
- The arithmetic of reactions — how much of each reactant and product.
- Valence
- The number of bonds an atom typically forms.
- Electronegativity
- How strongly an atom attracts the electrons in a bond; Pauling’s scale is the usual measure.
- Oxidation state
- The charge an atom would have if all its bonds were fully ionic. Book-keeping, but useful.
- Redox
- Reduction and oxidation: reactions in which electrons pass from one species to another.
- Acid and base
- In the Brønsted–Lowry sense (1923), an acid gives up a proton and a base accepts one.
- pH
- The negative logarithm of hydrogen-ion activity; introduced by Søren Sørensen in 1909.
- Catalyst
- A substance that speeds a reaction without being consumed by it.
- Enthalpy
- Heat content at constant pressure.
- Entropy
- A measure of how many microscopic arrangements match a given state.
- Gibbs free energy
- Enthalpy minus temperature times entropy; its sign tells which way a reaction can go.
- Equilibrium constant
- The ratio of products to reactants once a reaction has stopped changing.
- Isomer
- Compounds with the same formula but different structures.
- Chirality
- The property of a molecule that cannot be superimposed on its mirror image.
- Functional group
- A cluster of atoms that reacts in a characteristic way wherever it appears.
- Polymer
- A large molecule built from many repeated units.