Question

Three elements 'X', 'Y' and 'Z' have atomic numbers 18, 19 and 20 respectively. How many electrons are present in the M shell of these elements?

• A. 8, 9, 10
• B. 8, 10, 13
• C. 8, 8, 8
• D. 8, 9, 12

Answer 1

Answer: (C)

8, 8, 8

M shell means it is third shell $\Rightarrow n=3$

For element X: atomic number = 18

thus its electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^6$

Electrons in M shell=2+6=8

For element Y: atomic number = 19

thus its electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$

Electrons in M shell=2+6=8

For element Z: atomic number = 20

thus its electronic configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$

Electrons in M shell=2+6=8

thus in element X,Y,Z the number of electrons in M shell is 8,8,8 respectively.