Chart for the valency of elements
We can write the chemical formulae for compounds knowing their symbols and valencies.
Valency of elements chart
| Name of Element | Symbol | Valency |
| Hydrogen | H | 1 |
| Helium | He | 0 |
| Lithium | Li | 1 |
| Beryllium | Be | 2 |
| Boron | B | 3 |
| Carbon | C | 4 |
| Nitrogen | N | 3 |
| Oxygen | O | 2 |
| Fluorine | F | 1 |
| Neon | Ne | 0 |
| Sodium | Na | 1 |
| Magnesium | Mg | 2 |
| Aluminium | Al | 3 |
| Silicon | Si | 4 |
| Phosphorus | P | 3 |
| Sulphur | S | 2 |
| Chlorine | Cl | 1 |
Writing the chemical formula for compounds (Containing Polyatomic ions)
Calcium hydroxide
| Calcium | Hydroxide | |
| Symbol | Ca | OH |
| Valency | 2 | 1 |
| Charge | 2 + | 1 – |
The valency of calcium is 2 and that of hydroxide is 1.
This means that 1 calcium ion combines with 2 hydroxyl ions to form calcium hydroxide.
We also know that hydroxyl ion is polyatomic;, thus according to the rules of writing chemical formulae, it will be written in brackets.
Calcium Hydroxide = 𝑪𝒂(𝑶𝑯)𝟐
The chemical formula for calcium hydroxide
Ammonium sulphate
| Ammonium | Sulphate | |
| Symbol | 𝑁𝐻4 | 𝑆𝑂4 |
| Valency | 1 | 2 |
| Charge | 1 + | 2 – |
The valency of ammonium is 1 while that of the sulphate is 2.
This means 2 ammonium ions combine with 1 sulphate ion to form ammonium sulphate.
Here, both ammonium and sulphate are polyatomic ions. Hence, according to the rules of writing chemical formulae, they will be written in brackets. But as there is only one sulphate ion present in ammonium sulphate, we can get rid of its brackets.
Ammonium Sulphate = (𝑵𝑯𝟒)𝟐𝑺𝑶𝟒
The chemical formula for ammonium sulphate