We can determine the valency from the group number in the periodic table.
In general, the normal valency of an atom is equal to the smaller of the following two numbers
Group Number in the Periodic Table |
Example of element |
Valency |
I |
Li, Na |
1 |
II |
Be, Mg |
2 |
III |
B, Al |
3 |
IV |
C, Si |
4 |
V |
N, P |
3 |
VI |
O, S |
2 |
VII |
F, Cl |
1 |
Transition metallic elements have variable valencies. Their valencies are indicated by a Roman numeral in the bracket.
Example of transition metallic element |
Valency |
Copper (II) |
2 |
Iron (III) |
3 |
Lead (IV) |
4 |
Manganese (VI) |
6 |
There are radicals which consist of a group of atoms known as polyatomic ions. The following table shows the valencies of some common polyatomic ions.
Common polyatomic ions |
|||
Positive |
Valency |
Negative |
Valency |
Ammonium ion, NH4+ |
1 |
Nitrate ion, NO3- |
1 |
Hydroxide ion, OH- |
1 |
||
Carbonate ion, CO32- |
2 |
||
Sulphate ion, SO42- |
2 |
1. Write the respective valency for each element and criss-cross.
2. If the valency is equal then just write out the formula.
3. If a number is required for polyatomic ions then a bracket must be used.
4. Reduce the ratio by a common factor if possible.
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