Inorganic compounds, formulas and names

A formula is an accounting statement

A chemical formula identifies the elements present and their relative numbers. For a simple ionic solid it normally gives the smallest whole-number ratio of ions, rather than a separate molecule floating inside the lattice. The total positive and negative charges must balance. Begin with known ionic charges, find counts that cancel and then reduce those counts if necessary. The formula does not give permission to alter an ion internally: a sulphate ion remains a group containing one sulphur and four oxygen atoms.

For calcium and chloride, one calcium ion has charge +2 and each chloride has charge −1, so two chloride ions accompany one calcium ion. For aluminium and sulphate, the charges are +3 and −2. Two aluminium ions and three sulphate ions balance six units of each sign. Parentheses show that a subscript applies to a complete polyatomic ion. Without parentheses, the reader may interpret the subscript as applying only to the immediately preceding element, changing the composition.

Distinguish compound families by composition and behaviour

Oxides contain oxygen combined with another element. In many ordinary oxides, oxygen has oxidation number −2. Metal oxides often show basic behaviour, while many non-metal oxides react with water or bases in ways associated with acids. These are trends, not universal rules. Some oxides are amphoteric and can react with both acids and bases. Peroxides contain an oxygen–oxygen unit with a different oxidation-number pattern, so do not apply the ordinary-oxide rule to every oxygen-containing compound without checking its class.

Hydroxides contain the hydroxide ion, OH−, associated with a cation. For example, a +2 metal ion requires two hydroxide ions. The hydroxide group is not equivalent to an oxygen atom carrying two negative charges, and its hydrogen must not disappear when building the formula. Many hydroxides behave as bases, though their solubilities differ. A compound's basic character and the amount that dissolves in water are separate properties: poor solubility does not mean that its formula has ceased to contain hydroxide.

Acids can donate protons in the Brønsted description. Some contain oxygen, while binary acids such as hydrogen chloride do not. In aqueous solution, hydrogen chloride transfers a proton to water, forming hydronium and chloride. The molecular substance and its aqueous acidic solution require attention to context. Do not identify every hydrogen-containing compound as an acid solely because an H appears in its formula. The ability to transfer a proton depends on bonding and the surrounding reaction partners.

Salts consist of cations and anions and can be formed in acid–base reactions, among other routes. A simple neutralisation combines acid and base to produce salt and water. Balancing still requires conservation of every atom. For instance, an acid supplying one proton per molecule needs two molecules to neutralise a hydroxide containing two hydroxide groups. The word salt does not mean only sodium chloride, nor does it guarantee that every salt solution has pH seven. Some ions react with water and alter its acidity.

Read names as structural clues

An ionic name normally states the cation followed by the anion. A Roman numeral in a metal name specifies its oxidation state, not the number of metal atoms. Iron(III) chloride therefore combines iron in the +3 state with chloride in the −1 state. Three chloride ions balance one iron ion. Iron(II) chloride uses a different ratio because the iron state is +2. Recover the charges from the name before writing subscripts.

Molecular compound names can use prefixes to show atom counts, as in carbon dioxide. These prefixes and Roman oxidation-state numerals do different jobs. When a question supplies a formula, count atoms directly, treating a bracketed group as a unit multiplied by its outer subscript. When it supplies charges, derive the ratio. Finally check neutrality and the stated compound family. This two-way translation between name, formula and charge is a stronger method than remembering a list of isolated formulas.