Question

what electron configuration matches an chromium atom? 1s²2s²2p⁶3s²3p⁶4s²3d⁴ 1s²2s²2p⁶3s² 3p⁶4s¹3d⁵ 1s²2s²2p⁴ 1s²2s²2p⁶

Explanation:

Step1: Determine the atomic number of chromium

Chromium (\(Cr\)) has an atomic number of \(24\). So, a chromium atom has \(24\) electrons.

Step2: Apply the electron - filling rules

• The Aufbau principle: electrons fill orbitals in order of increasing energy (\(1s<2s < 2p<3s<3p<4s<3d\)).
• Hund's rule: electrons occupy degenerate orbitals (orbitals of the same energy) singly before pairing up.
• For \(Cr\), due to the stability of half - filled \(d\) orbitals (\(d^{5}\) is half - filled), the electron configuration is \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^{5}\) instead of the "expected" \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{4}\) (where \(4s\) is filled before \(3d\) according to the Aufbau principle, but the half - filled \(d\) state is more stable for \(Cr\)).
• Option \(C\) (\(1s^{2}2s^{2}2p^{4}\)) has \(2 + 2+4=8\) electrons (for \(O\) atom). Option \(D\) (\(1s^{2}2s^{2}2p^{6}\)) has \(2 + 2+6 = 10\) electrons (for \(Ne\) atom). Option \(A\) (\(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{4}\)) is not the most stable configuration for \(Cr\) as explained above.

Answer:

B. \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}3d^{5}\)