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Q.Arrange the following in increasing order with respect to the parameter given in the brackets: (1x2=2)

(i) C, N, O, F (First ionization enthalpy)
(ii) Li, Na, K, Cs (Electronegativity)
Assam AhsecAHSEC Higher Secondary (HS) 1st Year Examination 2024Subjective· 2mImportance★★★★★
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(i) First ionization enthalpy order: C < O < N < F (N is anomalously higher than O due to its stable half-filled 2p3 configuration). (ii) Electronegativity order: Cs < K < Na < Li (electronegativity decreases down a group).

(i) Ionization enthalpy of C, N, O, F: Moving left to right across Period 2, nuclear charge increases and atomic radius decreases, so first ionization enthalpy generally increases. Approximate values: C (1086 kJ/mol), N (1402), O (1314), F (1681). Note that nitrogen is an exception to the simple increasing trend — its 2p subshell is exactly half-filled (2p3), which gives it extra stability (a symmetrical, minimally-repelling electron distribution), so it is harder to remove an electron from N than the simple trend would suggest, making IE1(N) greater than IE1(O). So the increasing order is: C < O < N < F.

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