AS Chemistry

GETTING HELP WITH CHEMICAL FORMULAE

Formulae of Ionic compounds:

An ionic compound is generally formed when metal atoms give their outer electrons to non-metallic atoms in order to achieve a more stable electron configuration. The metal atoms always give away electrons and form positive ions; the non-metallic atoms will accept electrons and form negative ions

The formulae of the resulting compound represents the smallest possible repeating unit of that compound and is known as the empirical formula.

The formula can be worked out in detail, by drawing out the electron configurations of the different atoms, tracking where the electrons have been transferred and drawing the resulting ions. This would take too long to do every time you wanted to check the formula of a compound and a simpler method can be used.

You need to learn this table which shows the most common ions:

The sign on the positive ions tells you how many electrons have been lost when the metal has formed the compound. The sign on the negative ion tells you how many electrons must be accepted when the non-metal or larger radical forms a compound.

The number of electrons lost or gained must always be the same.

Thus the number of positive (+) charges must always

equal the number of negative (-) charges.

This rule is always used to work out the number of the (+) and (-) ions present.