Question

What rules are to be kept in mind while writing a chemical formula?

Answer 1

Chemical formula is a symbolic representation of a molecule's composition. It indicates the elements that are present in the compound and the ratio of these elements in the compound. A chemical formula is also known as a molecular formula.

Complete answer:
$\bullet$ Write down the symbols of the elements/ions, which combine to form a molecule of the compound, side by side. While writing the formula of a compound containing a metal and a nonmetal, the symbol of the metal is written first followed by that of the nonmetal. Example, for water the symbols are $H$ and $O$.
$\bullet$ Write the valency of each element/ion. The valency of hydrogen is one and oxygen is two.
$H - 1$ and $O - 2$
$\bullet$ Interchange the valences of the elements/ions and write as the subscript. Write them close together and ignore $'1'$ to obtain the formula. Example, interchanging the valency of hydrogen and oxygen we get: ${H_2}{O_1}$or ${H_2}O$
$\bullet$ the radical/polyatomic ions must be written in brackets, before the subscript is written. In case the number of polyatomic ions is one, the bracket is not required. For example, $NaOH$.
$\bullet$ whenever applicable, write simple ratios of the valencies. Example, in carbon oxide the valency of carbon is four $(C - 4)$ and oxygen is two $(O - 2)$, the simple ratio will be $2:1$. Thus, the formula will be $C{O_2}$.
$\bullet$ the final formula must be written without any sign of the charge. Example, the final formula of water is ${H_2}O$.

Note:
We can represent the chemical formula in two ways: as a molecular formula and as an empirical formula. Molecular formula tells us the exact number of atoms [resent in a compound whereas the empirical formula only represents the ratio in which the elements are present.