Question

Which of the following elements shown as pairs with their atomic numbers belong to the same period?

• A. Z=19 and Z = 38
• B. Z= 12 and Z= 17
• C. Z=11 and Z = 21
• D. Z=16 and Z = 35

Answer 1

The correct option is B Z= 12 and Z= 17

Elements with Z= 12 and Z= 17 belong to the same (third) period.

The electronic configuration for $Z=12$ is $1s^{2}2s^{2}2p^{6}3s^2$ (n = 3)

The electronic configuration for $Z=17$ is $1s^{2}2s^{2}2p^{6}3s^{2}3p^5$ (n = 3)