Compounds

What is a compound?

We have seen that some substances are elements. An element is made up of only one kind of atom. Many substances are made up of more than one kind of atom. If the different sorts of atoms are joined tightly together, then the substance is a compound.

The chemical term for two atoms joining tightly together is bonding. In a compound, two or more different kinds of atoms are bonded. For example, when sodium atoms bond with chlorine atoms, they form the compound sodium chloride.

Properties of elements and compounds

A compound is very different from elements from which it is made. Once two different elements are bonded, they completely lose the properties of the individual elements. The compound has totally new properties.

The above photograph shows two elements, sodium and chlorine. The photograph below shows the compound that is made when sodium atoms and chlorine atoms bond together. This compound-sodium chloride-is not at all like either sodium or chlorine.

You may have eaten some sodium chloride today. Sodium chloride is common salt. You would not want to eat any sodium or chlorine, though.

Naming compounds

Each compound has a chemical name. The chemical name usually tells us the elements that the compound is made from.

There are some important rules to remember when naming compounds.

• If the compound contains a metal, then the name of the metal comes first in the name of the compound. 
• If the compound contains a non-metal, the name of the non-metal is usually changed. For example, the compound made from sodium and chlorine is not sodium chlorine, but sodium chloride.
• When two elements form a compound the name often ends in ‘ide’.

Some compounds contain two different elements, plus a third element-oxygen. These compounds often have names ending with ‘ate’.

For example, a compound of calcium, carbon and oxygen is called calcium carbonate.

Sometimes, the name of a compound tells us how many of each kind of atom are bonded together.

Carbon dioxide is made up of molecules in which one carbon atom is joined to two oxygen atoms. ‘Di’ means ‘two’.

Carbon monoxide is made up of molecules in which one carbon atom is joined to one oxygen atom. ‘Mon’ or ‘mono’ means ‘one’.