Question

What is the correct formula of calcium chloride?
A.${\text{C}}{{\text{a}}_{\text{2}}}{\text{C}}{{\text{l}}_{\text{2}}}$
B.${\text{CaCl}}$
C.${\text{CaC}}{{\text{l}}_{\text{2}}}$
D.${\text{CaC}}{{\text{l}}_{\text{3}}}$
E.${\text{C}}{{\text{a}}_{\text{2}}}{\text{Cl}}$

Answer 1

The valency of Calcium is 2 and chlorine is 1. And thus 2 atoms of chlorine will be required to bond with 1 atom of calcium so that octet of both the species is completed and a stable compound is formed. All the atoms try to get octet in order to gain extra stability.

Complete step by step answer:
As we all know Calcium has an atomic number of 20. Thus, having an electronic configuration of (2, 8, 8, 2 or ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{6}}}{\text{4}}{{\text{s}}^{\text{2}}}$) which implies calcium has two electrons in its outermost shell (Thus, valency of Calcium is 2). Therefore, to complete its octet calcium will lose two electrons and gain a positive charge of 2, +2.
Similarly, we can look for chlorine the atomic number of Chlorine is 17, thus, having an electronic configuration of (2, 8, 7 or ${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{5}}}$). This implies that chlorine will need one electron to complete its octet (thus, valency of chlorine is 8-7 =1).
Hence, we can say that chlorine after gaining 1 electron will attain a negation charge of 1.
Considering the fact that valency of Ca is 2 and Cl is 1, we will have 1 atom of Ca getting attached to two atoms of Cl to complete the octet, giving one electron to each Cl atom.
One thing that you can keep in mind while trying to predict the formula of a compound having two heteroatoms is that you can always cross-multiply the elements with each other’s valency.
We take Calcium chloride for example, multiply Ca by 1 (valency of Cl) and Cl by 2 (valency of Ca), this will give you two atoms of Cl and 1 of Ca i.e. ${\text{CaC}}{{\text{l}}_{\text{2}}}$.
In the last we can conclude that the correct formula for calcium chloride is ${\text{CaC}}{{\text{l}}_{\text{2}}}$.
Hence option (C) is the correct answer.

Note:
Always remember that if the outermost shell in the electronic configuration of any element is having less than 4 electrons it will lose those electrons to complete its octet and its valency will be equal to the number of electrons in the outermost shell (e.g. Ca). But if an element is having more than 4 electrons in the outermost shell it will gain electrons as it is easier to gain electrons in this case rather than to lose all the electrons. And the valency of the element will be 8 - number of electrons in the outermost shell (e.g. Cl).