What are materials made of?
       4. Elements and compounds
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Symbols and compounds
We can put the symbols for different elements together to make a compound. This shows how many atoms are combined together in a molecule of that compound. We call this group of symbols the formula of the compound.
Water and methane molecules
Picture 4.9 Molecules of water and methane.

Photo of helium airship
Picture 4.10 Helium is one of the noble gases. It is used to fill airships because it is less dense than air and doesn't burn.

Water in symbols
The molecule of water contains two hydrogen atoms and one oxygen atom. It is given the formula H2O. The suffix '2' after the 'H' shows that there are 2 hydrogen atoms in the molecule.

Natural gas is largely made up of the compound methane. A molecule of methane contains four hydrogen atoms and one carbon atom. It has the formula CH4.

Diatomic elements
In some elements, small numbers of atoms of the same kind combine together to form a molecule. For example, oxygen in air is made up of molecules that contain two oxygen atoms. This diatomic molecule has the formula O2.

In fact, it is very unusual to find atoms that don’t combine with other atoms. Helium, argon and neon are examples of a group of elements called the Noble Gases that do exist as separate atoms.

Salt lattice
Picture 4.11 Salt is a lattice of sodium and chloride ions.

Salt of the earth
Water is an example of a molecular compound. The hydrogen and oxygen atoms combine to form molecules.

Many elements combine in a very different way. For example, the compound sodium chloride is not molecular. A single atom of sodium and a single atom of chlorine do not combine to form a molecule.

Instead, many atoms of sodium and chlorine combine together to form sodium chloride particles. We use the word lattice rather than molecule to describe the collection of sodium chloride atoms. The same number of sodium atoms as chlorine atoms combine together to form the sodium chloride lattice so we can still use the formula, NaCl.

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Question 21
Look at the paragraph below. Some of the words are missing. Fill in the missing words using the drop down menus.
The atoms of some elements, like the don't combine with other atoms at all.

Some atoms do combine making . Some of these, like water, are made up of lots of similar particles called . Each of these is made from atoms of more than one element - sometimes in different ratios. For example, each water particle is made from atoms but only different elements.