4d–series consists of elements from Y (atomic number 39) to Cd (atomic number 48). These elements lie in the 5^th period of the periodic table.In this series, the differentiating electron occupies 4d orbitals, i.e., the elements of this series involve the progressive filling of 4d orbitals as we proceed from Y₃₉ to Cd₄₈.

In this series there are more elements which have anomalous configurations. The elements having anomalous configuration are Nb₄₁, Mo₄₂, Ru₄₄, Rh₄₅, Pd₄₆ and Ag₄₇ (six elements). These anomalous configurations are explained on the basis of nuclear–electron and electron–electron forces existing in these atoms.

Complete and valence–shell electronic configuration of the atoms of 4d–series elements can be written as follows. Here [Kr]₃₆ = 2, 8, 18, 8 (four shells).

Complete configuration = [Kr]₃₆ 4d^1–10 5s ^0–2 = 2, 8, 18, (8 + [1 to 10] ), 0 to 2(five shells); which is otherwise written as 2, 8, 18, (9 to 18), 0 to 2 ; i.e., 2, 8, 18, 4s² p⁶ d^{1 – 10}, 5s^{0 – 2}. Valence–shell configuration = 4d^1–10 5s^0–2

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