Question

There is a recurrence of properties after 2, 8, 8, $x$ and 32 elements in the periodic table. The value of $x$ is:

• A. 16
• B. 18
• C. 20
• D. 8

Answer 1

The correct option is B 18

Electrons in various sub-orbits are filled in the increasing order of their energies as decided by Aufbau rule. $3d-$ sub-orbit is filled after $4s$; we can say that $3d$ is part of $4^{th}$ period. similarly $4d$ is part of $5^{th}$ period and so on.

	Period	Orbitals	Elements
1	${1s}^2$	1	2
2	${2s}^2{2p}^6$	4	8
3	${3s}^2{3p}^6$	4	8
4	${4s}^2{3d}^{10}{4p}^6$	9	18
5	${5s}^2{4d}^{10}{5p}^6$	9	18
6	${6s}^2{4f}^{14}{6d}^{10}{6p}^6$	16	32

Thus there are respectively 2, 8, 8, 18, 18 and 32 elements in first , second, third, fourth, fifth and sixth period. Hence, recurrence of properties takes place after theses number of elements.

Hence, the correct option is B.