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The molecular world
The molecular world

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1.3 Chemical compounds

Chemical elements contain atoms of the same atomic number. But most materials consist of chemical compounds. These are a combination of the atoms of two or more chemical elements. Such combinations often occur in simple numerical ratios. Thus, when sodium metal (Figure 2b) and chlorine gas (Figure 2e) are brought into contact, they react vigorously, and white crystals of common salt (sodium chloride) are formed. In these crystals, there are equal numbers of sodium and chlorine atoms; that is, the sodium and chlorine atoms are combined in the simple ratio 1:1. This is expressed by writing sodium chloride as NaCl. In this formula, there is one chlorine atom (Cl) for every sodium atom (Na).

Likewise, aluminium (Figure 2a) and liquid bromine (Figure 2d) will react violently after a short interval, and yield a white solid called aluminium bromide. In this solid there are three bromine atoms for every aluminium atom.

Question 4

Write a chemical formula for aluminium bromide.


AlBr3; the subscript three following the bromine marks the fact that the Al : Br atomic ratio is 1 : 3.

Formulae such as NaCl and AlBr3 tell us the ratios in which atoms are combined in compounds. When they are written down, the ratio is reduced to the lowest possible whole number, and the chemical formulae obtained in this way are then called empirical formulae. Most chemical elements are metals, and the formulae quoted for compounds of these metallic elements are usually empirical formulae. But they tell us nothing about the way that the atoms are grouped within the compound. For this, we need formulae of a different type.