[tex]\boxed{{\text{Chromium}}\left( {{\text{Z}} = {\text{24}}} \right)}[/tex] is the element of lowest atomic number that contains a half-filled d subshell in the ground state.
Further Explanation:
The division of the electron shells that are separated by its orbitals is known as subshells. These are classified as s, p, d and f. The s, p, d and f subshells have 1, 3, 5 and 7 orbitals respectively. Each orbital can accommodate a maximum of two electrons. So s, p, d and f orbitals have 2, 6, 10 and 14 electrons respectively.
The elements in which the valence electrons are present in d-orbitals are called d-block elements or transition metals. These have partially filled d orbitals in their ground or in one of the stable oxidation states. Half-filled and fulfilled d-orbitals are more stable than the other partially filled d orbitals.
The electrons are filled in different energy levels (orbitals) according to the Aufbau principle. “Aufbau” is a German word that means “build-up”. So this principle states that the electrons are filled in various orbitals in the increasing order of their energies.
The expected electronic configuration of chromium is [tex]\left[ {{\text{Ar}}} \right]\;{\text{3}}{{\text{d}}^{\text{4}}}{\text{4}}{{\text{s}}^{\text{2}}}[/tex]. But if one of the electrons of 4s orbital is shifted to 3d orbital, it becomes half-filled. So its actual electronic configuration becomes [tex]\left[ {{\text{Ar}}} \right]\;{\text{3}}{{\text{d}}^{\text{5}}}{\text{4}}{{\text{s}}^{\text{1}}}[/tex].
Among all the d-block elements, the elements that have half-filled d subshells are chromium and manganese (Refer table drawn at the end). But chromium has the lower atomic number in between these two elements. So the lowest atomic number element with half-filled d -orbital is chromium.
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Answer details:
Grade: Senior School
Subject: Chemistry
Chapter: d and f block elements
Keywords: Half-filled, d subshells, ground state, lowest atomic number, expected configuration, actual configuration, chromium.
