(ii) I.E increases as we move across the period but Ionisation enthalpies (I.E) of second period of elements in the order. Li lt B lt Be lt C lt O lt N lt F lt Ne Explain why ? (1) Be has higher I.E and B (2) O has lower I.E than N & F

(ii) I.E increases as we move across the period but Ionisation enthalp

1.	(ii) I.E increases as we move across the period but Ionisation enthalpies (I.E) of second period of elements in the order. Li lt B lt Be lt C lt O lt N lt F lt Ne Explain why ? (1) Be has higher I.E and B (2) O has lower I.E than N & F
Answer» Solution :(ii) (1)` ._(4)Be-1s^(2)2S^(2),_(5)B-1s^(2)2s^(2)2p^(1)` The I.E. of Be is more than that of B though the nuclear charge of boron atom `(Z=5)` is greater than that of beryllium atom `(Z=4)` . This can be explained as follows : Boron atom `(Z=5,1s^(2)2s^(2), 2p_(x)^(1) 2p_(y)^(0)2p_(z)^(0))` is having one paired electrons in the 2p-subshell. Be-atom `(Z=4,1s^(2)2s^(2))` is having paired electrons in the 2s-subshell. As the fully filled 2s-subshell in Be-atom is more stable than B-atom due to symmetry, more energy would be needed to REMOVE an ELECTRON from Be-atom. Hence, Be has high I.E. Hence I.E of Be `gt` B. `._(7)N-1s^(2),2s^(2),2p_(x)^(1),2p_(y)^(1),2p_(z)^(1)` (2)`._(8)O-1s^(2),2s^(2),2p_(x)^(2),2p_(y)^(1),2p_(z)^(1)` `therefore` O has lower I.E than N.