Compounds: Names and Formulae

A mixture is two or more substances simply stirred together. They keep their own properties, can be present in any proportion, and can usually be separated physically — sand and iron filings come apart with a magnet.

A compound is different. Two or more elements are chemically bonded in a fixed ratio, forming a substance with completely new properties that can only be separated by another chemical reaction.

The classic demonstration: sodium is a metal so reactive it catches fire in water, and chlorine is a poisonous green gas. Bond them and you get sodium chloride — the table salt you shake onto your chips.

A chemical formula records exactly which atoms are in one unit of a compound.

• A subscript (the small low number) counts the atoms of the symbol directly in front of it. In $\text{H}_2\text{O}$ there are 2 hydrogen atoms and 1 oxygen atom.
• Brackets multiply everything inside them. In $\text{Ca(OH)}_2$ the subscript 2 applies to both the O and the H.
• A coefficient — a full-size number in front — multiplies the whole formula. $2\text{H}_2\text{O}$ means two complete water molecules.

Worked example — count the atoms in $\text{Ca(OH)}_2$.

Total $= 1 + 2 + 2 = 5$ atoms.

Naming. When a metal joins a non-metal, the non-metal's name is changed to end in -ide: sodium $+$ chlorine $\rightarrow$ sodium chloride; magnesium $+$ oxygen $\rightarrow$ magnesium oxide; iron $+$ sulfur $\rightarrow$ iron sulfide. When oxygen is part of a group of atoms the name usually ends in -ate: sulfate, carbonate, nitrate.