Writing Ionic Chemical Formulae 5


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Ionic formulae are formula which show the charges on the ions present.

In other posts in this series, we developed the idea of valency number, Roman numerals, and complex ions which have charges.

Generally the charge an ion has is closely linked to its valency, which also means that the charge is also linked to the Roman numeral. The charge an ion has is the same as its valency number.

There are two types of ions, positive and negative. The charges on the complex ions are given in another post in this series. Also, as a general rule, metals form positive ions and non-metals form negative ions when joining to make an ionic formula

Sodium is an atom in Group 1 and will form an ion with 1 charge. As its electron arrangement is 2,8,1 it will lose one electron to form Na+ with a stable electron arrangement of 2,8. However, as it is a metal, we could work out that it would form a positive ion. Chlorine is an atom in Group 7 and will form a ion negative ion. As its electron arrangement is 2,8,7, it will gain one electron to form Cl- with a stable electron arrangement of 2,8,8. However, it is a non-metal which forms a negative ion.

       Na+         Cl- Ca2+            OH- Al3+          SO42- NH4+      CO32-
      1               1          2                  1         3                    2        1               2
Na+Cl- Ca2+(OH-)2  (Al3+)2(SO42-)3     (NH4+)2CO32-

If you want the ionic formula for silver(I) oxide, the valency of the metal silver is 1 so the formula of the ion will be Ag+ while oxygen, a non metal in Group 6 with a valency of 2 will be O2-. The formula will be (Ag+)2O2-. This is correct as two silver ions have a total charge of 2+ while one oxygen ion (or oxide ion) has 2- charges.

If you have written the formula correctly, there will be the same number of positive and negative charges.