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Compounds 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

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.

Molecule name Image
Oxygen oxygen atoms in pairs
Hydrogen hydrogen atoms in pairs
Sulfur 8 sulfur atoms arranged in a circle

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.


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.

Compound name Image
Water molecule Water - two hydrogen atoms and one oxygen atom joined together
Carbon dioxide molecule Two oxygen atoms and one carbon atom joined together
Alcohol molecule 5 hydrogen atoms attached to 2 carbon atoms, and 1 oxygen atom with 1 hydrogen atom


Compounds and mixtures activity

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Can you conquer the elements?

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