Electron Configuration of Chromium (Cr)

Orbital Filling Order

Following the aufbau principle, the 24 electrons in Chromium fill orbitals in order of increasing energy: 1s (2 electrons), then 2s (2 electrons), then 2p (6 electrons), then 3s (2 electrons), then 3p (6 electrons), then 3d (5 electrons), then 4s (1 electron).

Each orbital is filled according to the Pauli exclusion principle (at most 2 electrons per orbital) and Hund's rule (electrons occupy degenerate orbitals singly before pairing).

Noble Gas Shorthand

The inner electrons of Chromium match the configuration of Ar. This allows us to abbreviate the configuration as [Ar] 3d⁵ 4s¹, where [Ar] represents the filled core and the remaining entries show the valence electrons.

Electron Configuration Exception

Chromium is an exception to the standard aufbau filling order. Cr: [Ar] 3d5 4s1 - half-filled d subshell is more stable than [Ar] 3d4 4s2 Half-filled and fully filled d subshells provide extra stability due to symmetric electron distribution and favorable exchange energy, causing an electron to shift from the 4s orbital to the 3d orbital.

Valence Electrons

The valence shell of Chromium contains 5 electrons in the 3d subshell and 1 electron in the 4s subshell. These outermost electrons determine Chromium's chemical reactivity, bonding behavior, and position in the periodic table as a d-block element in period 4.

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