We know that when we combine two or more nonmetal atoms or combine a metal atom with a non-metal atom, molecules or compounds are formed. Molecules or compounds are usually represented by a chemical formula. Writing chemical formulae is quite a task but when you understand its fundamentals, it becomes easy to represent any chemical equation.
The chemical formula of a compound is the symbolic representation of its composition.
It represents the actual number of atoms of different elements present in one molecule of a compound. e.g.
- Water (H2O): contains 2 atoms of hydrogen and 1 atom of oxygen.
- Ammonia (NH3): contains 1 atom of nitrogen and 3 atoms of hydrogen.
- Carbon dioxide (CO2): contains 1 atom of carbon and 2 atoms of oxygen.
In this blog, we will learn to write the chemical formula of a compound. To begin with we must know the combining capacity of an element i.e. valency. For writing the chemical formula of ionic compounds valency is defined as follows:- Valency of an ion is defined as the units of positive or negative charge present on the ion.
Rules for writing chemical formulae
In the case of simple molecules, those compounds which are made up of only two types of elements are called binary compounds. The symbols of the 2 elements are written side by side & their respective valencies are written below their symbols e.g. in the case of Carbon Tetra-chloride.
If there is a common factor between the valencies of two elements, the valency is divided by the common factor. Then crisscross the valencies so that they appear on the lower right-hand side of the symbol. One (1) appearing in the formula can be omitted, then the chemical formula of carbon tetrachloride becomes CCl4.
In the case of Ionic Compounds such as Calcium Oxide, the symbol of metal atom is written first, followed by the symbol of nonmetal atom. Their respective valencies are written below their symbols.
Divide the valencies by any common factor. Now criss-cross the valencies or charges appearing in the formula. 1 can be omitted and the Chemical formula of Calcium Oxide will be CaO.
In the case of Ionic Compounds containing polyatomic ions such as Sodium Sulphate, polyatomic ions are written in brackets. If there is a common factor between the valency of the cation and the anion, the valency is divided by the common factor & then crisscrossed. Omitting �1� in the Chemical Formula of Sodium Sulphate gives us the formula Na2SO4.
Read More: Atoms and Molecules: Ions - Explanation | Science Class 9
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