Question

Question 3.16 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?

Class XI Classification of Elements and Periodicity in Properties Page 93

Answer 1

i) Be has elctrnic conf. 1s2 2s2 whereas B has E.C 1s2 2s2 2p1....as we can see that Be has its last orbital cmpltly filled therefore it is more stable than B ...Hence more energy will be required to remove n electron from Be than from B and it will have high ionisation enthlpy
...

ii) EC of O is 1 s2 2s2 2p4...
EC of N is 1s2 2s2 2p3..
EC of F is 1s2 2s2 2p5

since O neither hs neither hlf filled nor fully filled orbital it is the least stable among these three ....therefore less energy will b required and hnce it will less I.E than O n F...

Answer 2

we have given order of Ionisation enthalpies
Li < B < Be < C < O < N < F < Ne

(i) Be has higher Ionisation enthalpy than B :
electronic configuration of Be = 1s², 2s²
electronic configuration of B = 1s², 2s² , 2p¹

you can see that last electron of Be is in 2s while last electron of B is in 2p . but we know one thing that penetration of 2s - electron to the nucleus is more than that of 2p - electron.
it means 2s - electron strongly attracted by the nucleus than 2p - electron. it is clear that energy required to remove 2s - electron is higher than 2p - electron .
hence, Ionisation enthalpy of Be is higher than B .


(ii) O has lower ionisation enthalpy than N and F :
electronic configuration of O = 1s², 2s², 2p⁴
electronic configuration of N = 1s², 2s², 2p³
electronic configuration of F = 1s²,2s², 2p⁵

if you remembered ,
Ionisation enthalpy increases when we move left to right due to decreases atomic size.
but here you see N has higher Ionisation enthalpy than O . it is because of more stable { half filled or full filled orbitals shows more stablity } electronic configuration of N.so, it is difficult to remove an electron from N than O.
That's why Oxygen has lower Ionisation enthalpy than Nitrogen and fluorine.