Compounds and mixtures - Compounds

Atoms of different elements can join together in chemical reactions to form compounds. For example hydrogen and oxygen are elements. They react together to form water, a compound.

There are countless different ways for the elements to join together, and millions of compounds are known.

Properties of compounds

The properties of compounds are usually very different from the properties of the elements they contain. For example hydrogen and oxygen are both gases at room temperature, but water is a liquid.

ElementElementCompound

iron

sulfur

iron sulfide

colour silvery grey yellow black is it attracted to a magnet? yes no no reaction with hydrochloric acid hydrogen formed no reaction smelly hydrogen sulfide formed

Chemical bonds

The atoms in a compound are chemically joined together by strong forces called bonds. You can only separate the elements in a compound using another chemical reaction. Separation methods like filtration and distillation will not do this.

Compounds form when atoms join together in a chemical reaction

ompounds and mixtures - Atoms and molecules

Remember that an element is one type of atom, like carbon, gold or chlorine. We will look at three ways that atoms can exist.

Single atoms

Helium atoms

The atoms of some elements do not join up with other atoms. They stay as single atoms.

The element helium is like this. Helium is an unreactive gas. Helium atoms do not join up with each other or any other element.

Molecules of elements

When atoms of the same element join together we get a molecule of that element.

Molecules Oxygen Hydrogen Sulfur

Oxygen is like this. Two oxygen atoms join together to make an oxyen molecule. Most of the oxygen in the air is in this form. Hydrogen and chlorine also have molecules with two atoms.

Some elements have molecules with more than two atoms. Sulfur atoms can make molecules of eight atoms joined together.

Compounds

A compound is made when atoms of different elements join together by chemical bonds.

This means that compounds will always exist as molecules, not separate atoms. The diagrams show some molecules of common compounds.

Compounds Water molecule Carbon dioxide molecule Alcohol molecule

Compounds and mixtures - Chemical formulae

Remember that we use chemical symbols to stand for the elements. For example, C stands for carbon, O stands for oxygen, S stands for sulfur and Nastands for sodium. For a molecule we use the chemical symbols of the atoms it contains to write down its formula.

For example the formula for carbon monoxide is CO. It tells you that each molecule of carbon monoxide consists of one carbon atom joined to one oxygen atom.

Take care when writing your symbols and formulae. Be careful about when to use capital letters. For example CO means a molecule of carbon monoxide butCo is the symbol for cobalt.

Formula and formulae

The word 'formulae' ("form-u-lee") is the plural of 'formula'. If…