Question

What is the valency of calcium carbonate?

Answer 1

Calcium carbonate is made up of calcium and carbonate in their ionic forms. Each ion has a specific charge on it, determining the charge for both calcium and carbonate in order to find out the valency of the molecule.

Complete answer:
The structure of an atom is such that each element has a specific number of electrons in its outermost shell called valence electrons. Thus valency can be determined by knowing the outer electronic configuration of the concerned element.
Calcium is an alkaline earth metal belonging to the second group of the modern periodic table. It has an atomic number of $20$and its electronic configuration is $[Ar]4{s^2}$. Since there are only two extra electrons present in the s-orbital of Calcium, it can easily lose these electrons to achieve a perfect octet. Thus calcium has a valency of two and its forms a $C{a^{2 + }}$ cation.
The carbonate molecule is anionic in nature and has a charge of $- 2$ on it.
Both calcium and carbonate have equal yet opposite charges and hence these ions bind together to form calcium carbonate which is a neutral ionic compound.
The concept of valency is applicable only to elements that have a fixed electronic configuration and not to an ionic compound like calcium carbonate.
But it can be said that, since both the ions of the compound contribute two electrons (calcium loses them and carbonate accepts them), the valency of calcium carbonate is two.

Note:
Charge balancing in an ionic compound is essential. Even though ionic compounds are made up of ions, yet the overall molecule remains neutral. If the charges of the cation and anion do not come out to be equal then their stoichiometric numbers are changed to bring neutrality.