The atomic number of an element 'X' is 20. Write

(a) its valency,

(b) whether it is a metal or non-metal,

(c) the formula of compound formed when the element 'X' reacts with an element 'Y' of atomic number 8.

Justify your answer in each case.

The electronic configuration of X is 2,8,8,2

Since it has 2 valence electrons in its valence shell and it will lose 2 electrons to gain nearest stable gas configuration. So, the valency of X is 2.

(b) Since it loses electrons to gain inert gas configuration, so it is a non-metal.

(c) Atomic no. of element Y is 8

Electronic configuration will be 2,6

Valency of Y will be 2

The formula of compound can be calculated by cross-multiplying the valences as follows:

Therefore, formula is XY.