The periodic table is very helpful when learning the formulae of simple ions. These are the rules:
Metals and hydrogen form positive ions (cations).
Non-metals form negative ions (anions).
Group 1 metals (alkali metals) form +1 ions.
Group 2 metals (alkaline earth metals) form +2 ions.
Group 3 metals form +3 ions. (Boron, a non-metal does not form a simple ion.)
Group 4 metals (tin and lead) can each form ions with charges +2 and +4. (The non-metals above them, such as carbon and silicon, do not form simple ions.)
Group 5 non-metals (nitrogen and phophorus) form 3- ions.
Group 6 non-metals (such as oxygen and sulfur) form 2- ions.
Group 7 (all non-metals) form -1 ions.
Transition metals and zinc form positive ions, often several, and the charges are not easily predictable from the periodic table position.
When an element can form ions with several different charges, the name of the ion includes the charge in roman numerals.
For example, iron(III) is Fe3+ and iron(II) is Fe2+.
Group 0 (Group 8) do not form ions.
Molecular ions
These are very important. The periodic table is not helpful and they just need to be memorised.
Naming ions
A positive simple ion has the same name as its element (e.g. Na+ is sodium). If the element can form several ions, then the name includes the charge in
roman numerals. (e.g. Fe2+ is iron(II) and Fe3+ is iron(III)).
A negative simple ion's name begins with the first part of the name of the element and ends with _ide. (e.g S2- is sulfide.)
A negative molecular ion's name containing oxygen usually ends in _ite or _ate.
(e.g. SO32- is sulfite and SO42- is sulfate.)