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Odd-one-out

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The atoms in some elements join together to make molecules.

Molecules of elements

One of the elements that exists like this is hydrogen.

Each hydrogen atom joins up with one other atom to make a hydrogen molecule.

The formula of a hydrogen molecule is H2.

Other elements such as nitrogen, oxygen, and chlorine also exist as molecules. What are the formulae of these elements?

= hydrogen atomhydrogenmolecule

=

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When two or more elements react in a chemical reaction, the atoms do not just mix together, they become joined to one another to make compounds.

Atoms joining to make compounds

In a chemical reaction, the atoms in the reactants become joined in different ways and this is how

new compounds are formed.

hydrogen oxygen water+

+

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Some compounds exist as molecules:

Molecules of compounds

Water is made of hydrogen and oxygen atoms. What is the formula of water?

Carbon dioxide is made of carbon and oxygen atoms. What is the formula of carbon dioxide?

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Element or compound?

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Recognising elements and compounds