Question

What is the valency of magnesium and chlorine in magnesium chloride?

Answer 1

We know that the valency of an element is defined as the number of hydrogen atoms that can combine with or replace one atom of the element, either directly or indirectly. In other words, it is the combining power of an atom of the element.

Complete answer:
We know that, valency of an element is basically its tendency to accept or lose electrons in order to complete its octet and get stable electronic configuration. It is the minimum number of electrons we have to add or remove in order to completely fill the outermost shell of an element. Mathematically we can say that if the outermost shell of an atom contains four electrons then the valency of an element is equal to the number of electrons present in the outermost shell and if it is greater than four, then the valency of an element is determined by subtracting the total number of electrons present in outermost shell from eight.
In an atom, the valence electrons are the electrons that participate in combination with other atoms; these are the electrons in the orbitals of the outermost shell which is also called valence shell. Alternatively, valency can also be defined as the number of electron(s) an element must lose in order to gain noble gas configuration. Octet Rule states that an atom is most stable when the valence shell is completely filled with eight electrons. If the atom has less than eight electrons in its valence shell then it tends to be unstable and tries to gain or lose electrons in order to have a completely filled valence electron.
The electronic configuration of magnesium is $\left[ Ne \right]3{{s}^{2}}$ can also be written as $2,8,2$ which shows magnesium has two valence electrons in its outermost shell. If the valence electron is less than 4, it is recognized as valency. As magnesium has two valence electrons, its valency is two.
The electronic configuration of chlorine is $\left[ Ne \right]3{{s}^{2}}3{{p}^{5}}$ can also be written as $2,8,7$ which shows chlorine has seven valence electrons in its outermost shell. If the valence electron is more than four, then the valency can be calculated as follows: $Valency=Valence\text{ }electron-8.$ Accordingly, the valency of chlorine is $7-8$ which is $-1.$
Thus the valency of $Mg$ is $2,$ whereas for chlorine it is $-1$ in $MgC{{l}_{2}}.$

Note:
Remember that the valence shell is completely filled, and then only the atom becomes stable. Noble gases are the most stable as they already have a completely filled valence shell while all the other elements try to lose, gain or share electrons to achieve the noble gas configuration that is fully filled valence shell.