Chemical formulae

• Chemical formula: A way to express the elements (atoms or ions) in a compound, showing the number of atoms/ions of each element using symbols and numbers.

• Ion formation:

  • Ions are charged atoms or groups of atoms formed by gaining or losing electrons.

  • Metal and hydrogen atoms lose electrons to form positively charged ions (cations).

  • Non-metal atoms gain electrons to form negatively charged ions (anions).

Group 4 elements DON'T FORM IONS!

This is because they will become so positively charged they just gain all the electrons back again.

Valency:

• The number representing the combining power of an atom (or group of atoms).

• It is equal to the number of electrons an atom needs to lose or gain to achieve a stable, full outer electron shell.

Diatomic molecules:

Diatomic molecules are molecules formed when 2 atoms chemically join together. Some elements exist as diatomic molecules. E.g.