Mixtures, pure substances, elements, compounds and molecules
Pure substances can be classified as elements or compounds.
Elements have only one type of atom.
Compounds have two or more types of atom joined by chemical bonds.
Both elements and compounds may exist as molecules.
Molecules are small groups of atoms joined by chemical bonds.
Mixtures contain more than one substance.
Elements have only one type of atom.
Atoms can be modelled as spheres: 
Atoms are too small to be seen directly.
Models are mental pictures based on experimental evidence.
Atoms are too small to be seen directly.
Models are mental pictures based on experimental evidence.
Compounds have two or more types of atom joined by chemical bonds.
Compounds can be modelled as overlapping spheres of different colours.
Both elements and compounds may exist as molecules.
Molecules are small groups of atoms joined by chemical bonds.
Molecules of an element have the same type of atom joined by chemical bonds
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Molecules of a compound have different types of atoms joined by bonds.
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Molecules of a compound have different types of atoms joined by bonds.
Mixtures contain more than one substance.
A sample containing both and is a mixture.
This mixture contains two different molecular particles.
and is a mixture.This mixture contains two different molecular particles.
Different samples of a mixture may have different relative amounts of the substances.
A sample containing only or only or only is a pure substance.
Two of these pure samples contain molecular particles.
One of the pure samples contains particles that are atoms.
or only or only is a pure substance.Two of these pure samples contain molecular particles.
One of the pure samples contains particles that are atoms.
Every sample of a compound contains the same relative amounts of the component atoms.