CBSE Class 9 Answered

(i)The combining capacity of an element is called its valency.

Magnesium has atomic number 12 and electronic configuration is 2, 8, 2. It can lose 2 electrons to get the octet configuration in the outermost shell thus its valency is 2.

Oxygen has atomic number 8 and its electronic configuration is 2, 6.

It can gain 2 electrons to get the octet configuration in the outermost shell thus its valency is 8 - 6 = 2.

(ii) K = 2,   L = 8

Now, M i.e. the outermost shell can have only eight electrons and since S has -2 charge hence the number of electrons in M shell = 8.

So, the atomic number is equal to number of electrons thus atomic number of S^2- ion is 18.