The Language of Chemistry

Every element has a symbol, a shorthand of one or two letters taken from its English or Latin name, such as O for oxygen, Na for sodium (Latin natrium) and Fe for iron (ferrum). Each element also has a valency, its combining capacity, defined as the number of hydrogen atoms it can combine with or displace. Hydrogen, chlorine and sodium have valency 1; oxygen, calcium and magnesium have valency 2; aluminium has valency 3. Valency tells you how many 'bonds' or 'hands' an atom holds out to join with others.

A radical is an atom or a group of atoms that carries a charge and behaves as one unit in reactions. Basic radicals are positively charged, mostly metals and the ammonium group (Na+, Ca2+, Al3+, NH4+). Acidic radicals are negatively charged, mostly non-metals and acid groups (Cl-, O2-, SO4 2-, NO3-, CO3 2-). In any formula the basic radical is written first and the acidic radical second. The size of the charge equals the valency of the radical.

To write the formula of a compound, place the basic radical first and the acidic radical second with their valencies above them. Then criss-cross: bring each valency down as the subscript of the other radical, and simplify the numbers to the lowest ratio. Aluminium (valency 3) and oxygen (valency 2) cross to give Al2O3. If a radical is a group of more than one atom and takes a subscript, enclose it in brackets, for example calcium nitrate is Ca(NO3)2.

Compounds are named basic radical first, acidic radical second, e.g. sodium chloride, calcium carbonate. When a metal shows two valencies we use suffixes: -ous for the lower (ferrous, FeCl2) and -ic for the higher (ferric, FeCl3). A chemical equation describes a reaction; it must be balanced so that the number of atoms of each element is the same on both sides, because matter is neither created nor destroyed (law of conservation of mass). We balance by placing whole-number coefficients in front of formulae, never by changing a formula's subscripts.