For example, sodium reacts with chlorine to form sodium chloride. The transfer of electrons can be modelled using dot and cross diagrams. The electrons from one atom are shown as dots, and the electrons from the other atom are shown as crosses. The dots and crosses do not need to be coloured.
Sodium ions and chloride ions form in the reaction.
For elements in group numbers 1, 2 and 3 (IUPAC groups 1, 2 and 13) the number of electrons lost is the same as the old group number.
|IUPAC group number||1||2||13|
|Old group number||1||2||3|
|Charge on ions||+||2+||3+|
Non-metal atoms gain electrons when they form ions. These are negatively charged because they have more electrons than protons.
For elements in old group numbers 6 and 7 (IUPAC groups 16 and 17) the number of electrons gained is equal to (8 - old group number).
|IUPAC group number||16||17|
|Old group number||6||7|
|Charge on ions||2-||-|