Determine the element of lowest atomic number that contains a half-filled d subshell in the ground state?

Answer is: chromium (Cr).

Chromium with atomic number 24, it means it has 24 protons and 24 electrons.

Electron configuration of chromim atom: ₂₄Cr 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹.

In chromium, half full d-sub level (3d) is more stable than a partially filled d sub-level (3d⁴), thai is why an electron from the 4s orbital goes to a 3d orbital.

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