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A compound is a substance made up of two or more types of atoms. Atoms can bond together to form a large number of different compounds.

Unlike elements, compounds can be broken down into simpler substances.

Compounds often exist as molecules or as ionic lattice structures.

Water (formula: $$\ce{H2O}$$) is a molecule made of one oxygen ($$\ce{O}$$) atom and two hydrogen ($$\ce{H}$$) atoms.

The bonds between atoms in a compound can be broken if energy is added (for example: in the form of heat or electricity).

An input of energy (usually in the form of electricity) can separate atoms in a water molecule, converting it into oxygen and hydrogen gas.

$$$\ce{2H2O -> O2 + 2H2}$$$

Molecular diagram of water
Molecular diagram of water

The different types of atoms in a compound are always present in a fixed ratio.

Each water molecule always consists of one oxygen atom and two hydrogen atoms. Any volume of water will contain hydrogen and oxygen atoms in a 2:1 ratio.