Electronic configuration of four elements A, B, C, and D are given below: A. 1s22s22p6        B. 1s22s22p4          C. 1s22s22p63s1     D. 1s22s22p5Which of the following is the correct order of increasing tendency to gain electron?

Electronic configuration of elements indicate that A is a noble gas (i.e. Ne), B is oxygen (group 16), C is sodium metal (group 1) and D is fluorine (group 17).(i) Noble gases have no tendency to gain electrons since all their orbitals are completely filled. Thus, element A has the least electron gain enthalpy(ii) Since, element D has one electron less and element B has two electrons less than the corresponding noble gas configuration, hence, element D has the highest electron, gain enthalpy followed by element B(iii) Since element C has one electron in the s-orbital and hence, needs one more electron to complete it, therefore, electron gain enthalpy of C is less than that of element B. Combining all the facts given above, the electron gain enthalpies of the four elements increase in the order, A < C < B < D.