Question

Identify the correct ground state electron configuration for $Cr$.

• A. $\left[Ar\right]3s^{2}3d^4$
• B. $\left[Ar\right]3s^{2}3d^5$
• C. $\left[Ar\right]4s^{2}3d^5$
• D. $\left[Ar\right]4s^{1}3d^5$

Answer 1

The correct option is D $\left[Ar\right]4s^{1}3d^5$

According to the periodic table, $Cr$ has 24 electrons. We can eliminate choices (B) and (C) because those configurations have 25 electrons. Choice (A) is wrong because the 3s subshell is already accounted for in the $\left[Ar\right]$ core-i.e.,$\left[Ar\right]$ stands for 1s², 2s², 2p⁶, 3s², 3p⁶. Now, according to the Aufbau principle, we completely fill subshells before moving up to the next higher one. So the best answer would appear to be choice (D). However, remember that completely half-filled and filled d subshells bestow extra stabilization to an atom. Therefore, $Cr$ and $Cu$ actually violate the Aufbau principle and promote a 4s electron to become [Ar] 4s¹3d?⁵ and [Ar] 4s¹3d¹⁰, respectively. Choice (D) is the correct ground state configuration for$Cr.$