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Science

Elements, compounds and chemical equations

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All matter is made of atoms. There are over a hundred different types of atom, called elements, and each one has a symbol. The atoms of a particular element are identical to each other.

When atoms of different elements join together they make a compound. Compounds are represented by formulae that show how many atoms of each element are in the compound. Formulae can be worked out from the charge of the ions that make up the compound.

Chemical reactions can be written down as balanced chemical equations.

Elements and compounds

All matter is made from atoms. Atoms are very, very small. A molecule is formed when atoms join together by chemical bonds.

There are over a hundred different types of atom, called elements. The atoms of a particular element are identical to each other. They cannot be changed chemically into any different element. For example a piece of pure sulfur consists only of sulfur atoms. No chemical process can break it down into a different substance.

Compounds are substances that contain atoms of at least two elements chemically combined. For example: magnesium oxide consists of magnesium atoms and oxygen atoms chemically bonded together.

Examples of compounds and elements

SubstanceTypes of atom chemically combinedSo it is...
hydrogenhydrogenan element
sodium chloridesodium and chlorinea compound
carboncarbonan element
carbon dioxidecarbon and oxygena compound

Notice how the name sometimes suggests whether a substance is an element or a compound. If you can see the names of two or more elements it's almost certainly a compound but beware – many substances with only one name are actually compounds – water and methane for example.

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