Cambridge O Level Chemistry · Syllabus 5070 · The Periodic Table
Ion Charge
What is Ion Charge?
The net electrical charge left on an atom after it has lost or gained electrons to reach a full outer shell. Losing n electrons gives a charge of n plus; gaining n electrons gives a charge of n minus. For main-group elements the number of electrons transferred, and therefore the charge, follows from the group number.
This definition is part of the The Periodic Table chapter in Cambridge O Level Chemistry.
Common mistakes with Ion Charge
- 3 · “Group number = ion charge” applied everywhere Defect Deducing a \(+4\) ion for carbon, or a \(+7\) ion for chlorine, by reading the group number as the charge. Fix The relationship holds for Groups I, II and III (charge \(=\) group number, positive) and for Groups V, VI and VII (charge \(=\) group number \(-\,8\), negative). Group IV and the transition elements do not follow it, and Group VIII normally forms no ions at all.
Questions students ask about Ion Charge
Why can't you predict a transition element's ion charge from its group position?
Transition elements show variable oxidation numbers, so no single charge follows from their position between Group II and Group III. Iron forms both \(\mathrm{Fe^{2+}}\) and \(\mathrm{Fe^{3+}}\), so the charge must be read from the Roman numeral in the compound's name or from its formula, never from a column count.