What is the total number of electrons in a $ M{g^{2 + }} $ ion?

Hint :An atom comprises a nucleus having neutrons and protons which are surrounded by electrons in orbits. Number of protons (atomic number i.e. ‘ $ Z $ ’) can be considered to be equal to the total number of electrons in a neutral atom. The summation of the number of neutrons and number of protons equals the mass number of an atom (denoted as’ $ M $ ’). Thus, the number of neutrons = $ M - Z $ .

Complete Step By Step Answer:
In the given question, we have to find out the number of electrons in $ M{g^{2 + }} $ ion. First of all, find out the atomic number of Magnesium from the periodic table which is equal to 12.
As atomic number is equal to the number of protons thus:
 $ number{\text{ }}of{\text{ }}protons = 12 $
In case of neutral atom, number of electrons is equal to the number of protons so:
 $ number{\text{ }}of{\text{ electr}}ons = 12 $
Now, in case of $ M{g^{2 + }} $ , it indicates that the magnesium atom might have lost 2 electrons to become a cation i.e. $ M{g^{2 + }} $ . In this case:
 $ number{\text{ }}of{\text{ electr}}ons = 12 - 2 = 10 $
Alternatively, we can write the electronic configuration of $ Mg $ . This will be $ 1{s^2}2{s^2}2{p^6}3{s^2} $
We know that superscripts indicate the number of electrons occupied in each orbital, so summation of all superscripts tell us the total number of electrons which is equal to 12 i.e. $ 2 + 2 + 6 + 2 = 12 $ .
For $ M{g^{2 + }} $ it will be $ 12 - 2 = 10\; $ electrons.

Note :
Each atom of a given element has the similar number of protons while atoms of distinct elements possess distinct numbers of protons. An atom consists of the same number of protons as well as electrons. As protons as well as electrons possess equal and opposite electrical charges, atoms have no overall electrical charge (i.e. neutral).