Ncert Exercises & Solutions

Arrange the elements N, P, O and S in the order of

(i) increasing first ionisation enthalpy.

(ii) increasing non-metallic character.

Give reason for the arrangement assigned.

The placing of elements are as

Period	Group 15	Group 16
2^nd period	N	O
3^rd period	P	S

(i) Ionisation enthalpy of nitrogen $({}_{7}N = 1 s^{2}, 2 s^{2}, 2 p^{3})$ is greater than oxygen $({}_{8}O = 1 s^{2}, 2 s^{2}, 2 p^{4})$ due to extra stable exactly half-filled 2p-orbitals. Similarly, ionisation enthalpy of phosphorus $({}_{15}P = 1 s^{2}, 2 s^{2}, 2 p^{6}, 3 s^{2}, 3 p^{3})$ is greater than sulphur $({}_{16}S = 1 s^{2}, 2 s^{2}, 2 p^{6}, 3 s^{2}, 3 p^{4})$ .

On moving down the group, ionisation enthalpy decreases with increasing atomic size. So, the order is

S<P<O<N $\to$ First ionisation enthalpy increases

(ii) Non-metallic character across a period (left to right) increases but on moving down the group it decreases. So, the order is

P<S<N<O $\to$ Non-metallic character increases

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