An atom is the smallest particle of an element that takes part in a chemical reaction. Atoms are unimaginably small — their radius is measured in nanometres (1 nm = 10−9 m), and a single drop of water contains more atoms than there are stars we can see. Because writing out element names is clumsy, every element has a short symbol: the first (sometimes first two) letters of its name, with the first letter capital and the second small, for example H for hydrogen, Na for sodium (from natrium) and Fe for iron (from ferrum).
Atomic mass and the atomic mass unit
Atoms are far too light to weigh in grams, so chemists use the atomic mass unit (u). One u is defined as exactly one-twelfth of the mass of one carbon-12 atom. On this scale hydrogen is about 1 u, carbon 12 u, nitrogen 14 u, oxygen 16 u, sodium 23 u, sulphur 32 u, chlorine 35.5 u and calcium 40 u. The atomic mass of an element is a relative number — it tells us how heavy one atom is compared with this standard.
Molecules and atomicity
A molecule is a group of two or more atoms held together by chemical bonds. A molecule of an element contains atoms of the same kind, such as O2 (oxygen) or O3 (ozone). A molecule of a compound contains atoms of different elements, such as H2O or CO2. The number of atoms in one molecule of an element is called its atomicity — helium is monatomic, oxygen is diatomic and phosphorus (P4) and sulphur (S8) are polyatomic.
Ions and polyatomic ions
An ion is a charged atom or group of atoms. A positively charged ion is a cation (formed by loss of electrons, e.g. Na+, Ca2+), and a negatively charged ion is an anion (formed by gain of electrons, e.g. Cl−, O2−). A polyatomic ion is a group of atoms carrying a net charge that stays together as a unit, such as sulphate SO42−, nitrate NO3−, carbonate CO32− and ammonium NH4+.
Valency and writing formulae by criss-cross
Valency is the combining capacity of an element or ion — effectively the size of its charge ignoring the sign. To write a chemical formula by the criss-cross method: write the symbols side by side, write the valency of each above it, then cross over each valency to become the subscript of the other. For aluminium (valency 3) and oxygen (valency 2), crossing gives Al2O3. Always reduce the subscripts to the simplest whole-number ratio, and enclose a polyatomic ion in brackets if more than one is needed, e.g. calcium nitrate is Ca(NO3)2.