Question

Give chemical formulae of the following compounds:
A.Potassium Carbonate
B.Calcium Carbonate

Answer 1

Hint: Before writing any formula the first thing which you should keep in mind is that the overall ionic formula for a compound must be electrically neutral and when we write the formula the cation is written before the anion .

Complete step by step answer:
Compounds are formed when different elements combine together in different ratios.
The chemical formula of a compound is determined by the valency of an element , that is the combining capacity of the element.
The valency of an atom = the no. of electrons present in the valence shell of the atom when the number of valence electrons is = 1, 2, 3, 4
and , it is 8- number of valence electrons when the number of valence electrons is 5, 6, 7,8
For example , potassium $(K)$ has 1 valence electron in its outer shell so its valency is= 1 which means $K$ can give 1 electron to another atom.
Similarly, carbonate ion has -2 valency, that is the oxygen accepts 2 electrons from another element.
So the formula for potassium carbonate becomes $K_{2}C{O}_3$
Similarly , the valency of calcium is +2 as it has 2 electrons in the valence shell which can be donated and that of carbonate ion is -2 so the chemical formula of calcium carbonate is $CaC{O}_3$ .

Note:
Another method to find the formula of a compound is by transferring the charge .
For example , In case of potassium carbonate charge on potassium is +1 and charge on carbonate is -2 .
Therefore $K^{+1}\times C{O_3}^{-2}\to K_{2}C{O}_3$ As you can see the charge on carbonate becomes the subscript of potassium and vice versa .