What are the rules for writing chemical formulae of compounds?
Rule 1 – In a compound formed by metal and non-metal, the metal component is written first and the non-metal later.
Rule 2 – Polyatomic ions in the compounds are represented using brackets.
Rule 3 – The net charge on a complete molecule should be balanced. In order to balance the net charge on a molecule, we require the valency of each element present in that molecule.
What is Valency of an element?
We know that Valency is the combining power of an element, that is related to the number of electrons present in its outer shell or valence shell. The valencies for some elements are shown below:
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 |
We will now use the above rules to write the chemical formulae of hydrogen chloride and hydrogen sulphide.
Writing the chemical formula for compounds (Sharing of electrons)
Hydrogen chloride
The symbol for hydrogen is ‘H’ and chlorine is ‘Cl’. Both have a valency of one. Also, both
the hydrogen and chlorine have a tendency to acquire one electron.
Thus, one hydrogen atom combines with one chlorine atom to form hydrogen chloride by sharing one electron each.
Balancing the net charge, we get the chemical formula as ‘𝐻1𝐶𝑙1’.
But we need not write one in the chemical formula, hence the chemical formula for
hydrogen chloride as ‘HCl’
The chemical formula for hydrogen chloride
Hydrogen sulphide
The symbol of hydrogen is ‘H’ and sulphur is ‘S’. The valency of sulphur is 2 and hydrogen is
- This means that sulphur needs two electrons to complete its outer shell and hydrogen needs one electron.
In this case, two atoms of hydrogen will share one electron each, with a sulphur atom. Thus we get the chemical formula of hydrogen sulphide as ‘𝐻2𝑆 ’.
The chemical formula for hydrogen sulphide