Atoms, elements and compounds

Everything is made of atoms — the smallest part of an element that can exist. Each atom is shown by a symbol, such as O for oxygen, Na for sodium or Fe for iron.

An element contains only one type of atom. There are about 100 elements, and they are listed in the periodic table.

A compound contains two or more elements chemically combined in fixed proportions. For example, water (H₂O) always has two hydrogen atoms for every oxygen atom. Compounds can only be separated into their elements by chemical reactions, not by physical methods such as filtering.

A compound's properties are usually very different from its elements'. Sodium is a reactive metal and chlorine is a toxic gas, but together they make sodium chloride (NaCl): table salt.

A chemical reaction makes new substances. The substances you start with are the reactants (on the left of the arrow) and the new substances made are the products (on the right).

A word equation uses names:

magnesium + oxygen → magnesium oxide

A symbol equation uses formulae:

2Mg + O₂ → 2MgO

In a formula, the small number after a symbol tells you how many of that atom there are. CO₂ has one carbon atom and two oxygen atoms. Brackets multiply everything inside them: Ca(OH)₂ has one calcium, two oxygen and two hydrogen atoms.

Atoms are never created or destroyed in a reaction, so a symbol equation must have the same number of each type of atom on both sides. You balance an equation by writing numbers in front of formulae. Never change the small numbers inside a formula, because that would change the substance.

Example: methane burning in oxygen.

1. Unbalanced: CH₄ + O₂ → CO₂ + H₂O
2. Hydrogen: 4 on the left, so make 2H₂O on the right.
3. Oxygen: now 2 + 2 = 4 on the right, so use 2O₂ on the left.
4. Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O

Check: 1 C, 4 H and 4 O on each side.