Apentix/AP Chemistry/Unit 1: Atomic Structure and Properties/Question

AP Chemistrymediummcq1 pt

Why is the electron configuration of chromium written [Ar] 4s¹ 3d⁵ rather than [Ar] 4s² 3d⁴?

A.A half-filled d subshell is unusually stable

B.The 4s subshell can hold only one electron

C.Chromium has only five valence electrons

D.3d is always lower in energy than 4s

Explanation

Core Concept

Chromium promotes one 4s electron to 3d so each of the five 3d orbitals holds exactly one electron. Exchange-energy stabilization makes half-filled and filled d subshells especially favorable, overriding the simple aufbau prediction.

Correct Answer

AA half-filled d subshell is unusually stable

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