Question

which electron configuration is that of an element in the fourth period?
a 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁴
b 1s²2s²2p⁶3s²
c 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p¹
d 1s²2s²

Explanation:

Step1: Recall period - electron configuration relation

Elements in the $n$th period have electrons in the $n$th energy level. For elements in the fourth period, the highest - occupied principal quantum number $n = 4$, and electrons can be in the $4s$, $3d$, and $4p$ sub - shells.

Step2: Analyze each option

• Option a: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{6}5s^{2}4d^{10}5p^{4}$ has electrons in the fifth energy level ($5s$ and $5p$), so it is an element in the fifth period.
• Option b: $1s^{2}2s^{2}2p^{6}3s^{2}$ has electrons only up to the third energy level, so it is an element in the third period.
• Option c: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{1}$ has electrons in the fourth energy level ($4s$ and $4p$) and is the electron configuration of an element in the fourth period.
• Option d: $1s^{2}2s^{2}$ has electrons only in the second energy level, so it is an element in the second period.

Answer:

C. $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{1}$