Ncert Exercises & Solutions

Electronic configuration of four elements A, B, C and D are given below :

A. $1 s^{2} 2 s^{2} 2 p^{6}$

B. $1 s^{2} 2 s^{2} 2 p^{4}$

C. $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{1}$

D. $1 s^{2} 2 s^{2} 2 p^{5}$

The correct order of increasing tendency to gain electron is :

1.	A < C < B < D	2.	A < B < C < D
3.	D < B < C < A	4.	D < A < B < C

HINT: Left to right across the period electron gain enthalpy increases.

Explanation:

Electronic configuration of elements indicates 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.

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