Explain the method of writing a formula to a compound using criss-cross method, with the help of an example.

Explanation

The following steps should be taken while attempting to write a chemical formula using criss-cross method.
Ex: take sodium carbonate as an example.
1) Write the symbols of atoms or group side by side, usually the cation first.
`NaCO_3`
2) Write the valency of each atoms or group of atoms on the top of its symbol.
`Na^1(CO_3)^2`
3) Divide the valency number by their highest common factor if any to get the simple ratio.
`Na^-1(CO_3)^2`
4) Intercharge tha valency and write the numbers to the lower right of the constituents as subscript.
`Na_2(CO_3)_1`
5) If any constant receives the number 1, ignore it while writing the formula.
`Na_2CO_3`
6) If group of atoms received the number more than 1, enclose it within brackets. Hence the formula for the sodium carbonate is `Na_2CO_3`.