Classifying Elements by Orbital Filling
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State
An element is representative, transition, or inner transition based on whether its highest-energy electrons fill s/p, d, or f orbitals.
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Sodium (3s) is representative, iron (3d) is transition, cerium (4f) is inner transition.
Watch for
Group 12 finishes filling d orbitals early, so its last electron enters s, placing it with representative metals despite position.