How to learn ions

Simple ions

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.)