### Q.16:- Among the second period elements the actual ionization enthalpies are in the order Li < B < Be < C < O < N < F < Ne. Explain why

(i) Be has higher ΔiH than B

(ii) O has lower ΔiH than N and F?

**Answer:-**

(i) In case of Be (1s^{2} 2s^{2}) the outermost electron is present in 2s-orbital while in B (1s^{2} 2s^{2} 2p^{1}) it is present in 2p-orbital. Since 2s – electrons are more strongly attracted by the nucleus than 2p-electrons, therefore, lesser amount of energy is required to knock out a 2p-electron than a 2s – electron. Consequently, At of Be is higher than that ∆_{i}H_{1} of B.

(ii) The electronic configuration of

N_{7} = 1s^{2 }2s^{2} 2p_{x}^{1} 2p_{y}^{1} 2p_{z}^{1 }

O_{8} =1s^{2} 2s^{2} 2p_{x}^{1} 2p_{y}^{1} 2p_{z}^{1}

We can see that in case of nitrogen 2p-orbitals are exactly half filled. Therefore, it is difficult to remove an electron from N than from O. As a result ∆_{i}H_{1} of N is higher than that of O.