Question

What is the oxidation number of $Mg$ and $N$ in magnesium nitride?
(A) $Mg = + 2;N = - 3$
(B) $Mg = + 1;N = - 3$
(C) $Mg = + 2;N = - 2$
(D) None of these

Answer 1

Hint: Electroprotine metals of group 1, 2 and 3 lose a specific number of electrons and always have a constant positive oxidation number. Oxidation number is the total number of electrons, an atom gains or loses to form a bond with another atom.

Complete Step by step answer:In simple language, oxidation state or oxidation number is the total number of electrons an atom gains or loses to form a bond with another atom.
If there are two molecules forming a bond, one of them will gain an electron and one of them will lose an electron. So both of them will have different signs of oxidation state.
In the above example, i.e. magnesium nitrate, $M{g_3}{N_2}$
There are three molecules of magnesium, Mg and one molecule of nitrogen, N
We know that magnesium belongs to the second group of periodic tables. And the elements of second group of periodic table have the oxidation state $ + 2$
Thus, the oxidation state of Mg is $ + 2$
Now, let us assume that the oxidation state of N be $x$
Now, since, the compound $M{g_3}{N_2}$ is neutral. We can write
$3( + 2) + 2 \times x = 0$
$ \Rightarrow 6 + 2x = 0$
By rearranging it, we get
$2x = - 6$
By dividing both the sides by $2$, we get
$x = - 3$
Therefore, in magnesium nitrate $(M{g_3}{N_2})$the oxidation number of $Mg$and $N$are $ + 2$ and $ - 3.$

Therefore, from the above explanation, the correct answer is, option (A) $Mg = + 2;N = - 3$

Note: From the above example, we understand that it is not necessary for the two molecules to have the same oxidation state to form a bond. In the above example, the compound is neutral, so the RHS of the equation we formed was zero. But again, it is not necessary that any compound given in the question will be neutral. Sometimes you may be asked to find an oxidation state of some oxides or something which might not be neutral. So decide the RHS after carefully observing the net charge on the compound given to you.