University of Edinburgh
 

BSL Chemistry Glossary - compound - definition

Definition: In a compound, different types of atoms – metal and non-metal - are chemically joined together. 

I'll explain how they are linked. You know that an atom is made of a nucleus with electrons moving around it in different levels or shells.

The first layer has two electrons in it.  The next layer has eight electrons, and then the next one has eight electrons too.

atom

If we look at an example - table salt (which is a compound of sodium and chlorine (NaCl)). In sodium (Na), there are two electrons on the first layer, then eight on the second layer but only one on the outer layer. An atom is most stable if it has a complete outer shell but if the outer shell is not full, the atom will be unstable and reactive. In chlorine (Cl), there are two electrons on the first layer, then eight on the second layer but only seven on the outer layer – so it is not complete and will be unstable. When sodium and chlorine combine, the electron will move from the sodium atom to a chlorine atom so that both sodium and chlorine each now have a stable outer shell of 8 electrons – forming a compound of sodium chloride.

sodium chloride