Transition Metals Defined by Partially Filled d Orbitals
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A transition metal has, or readily forms ions with, a partially filled d-orbital set, which among the d-block elements holds for groups 3–11.
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Iron's [Ar]3d64s2 configuration leaves its d orbitals partially filled, qualifying it as a transition metal.
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Group 12 fills d orbitals completely and loses only s electrons on ionizing, so strict definition places it with representative metals despite informal grouping.