Question

Valency of magnesium and oxygen in MgO is?
(A) One and one
(B) Two and two
(C) One and one
(D) Two and one

Answer 1

Hint: You should know that for atoms, valency is the number of electrons gained, lost or shared by an atom to complete its octet i.e. to achieve its noble gas configuration

Complete step by step answer:
You can generally determine the valency of any transitional element or a radical in a particular compound by noting how it combines with elements with known valency. This strategy is based on gaining the octet rule. This implies elements and radicals combine so as to produce a stable outer shell of eight electrons.
As we know magnesium is an alkaline metal and it is in the 2nd group of periodic table.
In this case of magnesium it has an electronic configuration of 2,8. So we can see that valence electrons are 2 and hence the valency is 2.
Hence its oxidation number is +2.
As we have already seen magnesium has a valency of +2. So it has to give 2 electrons to attain stability. Oxygen has a valency of -2 so it needs electrons to attain stability.
Therefore, magnesium gives oxygen 2 electrons and attains a positive charge and oxygen attains a negative charge by gaining two electrons. Hence, they make an ionic bond and cancel out each other’s charges.
Therefore, the valencies magnesium and oxygen in MgO are two and two.

Hence, option B is the required answer.

Note: Remember that for polyatomic substances, the total oxidation state of its polyatomic molecule is equal to the sum of oxidation states of its constituent ions formed from bond fission.