84. Define transition elements. Name different transition series.

The transition elements are those elements which represent a change of state from the highly metallic elements of s-block to the non-metals of p-block i.e. they lie between the elements of s-and p-blocks. There are four transition series each marking the filling of 3d, 4d, 5d and 6d orbitals respectively as:

(i) First transition series (3d). Scandium (₂₁Sc) to Zinc (₃₀Zn).
This series lies in the fourth period of the periodic table and consists of 10 elements.
General electronic configuration is 3d^1-104s^1-2.

(ii) Second transition series (4d). Ytterium (₃₉Y) to Cadmium (₄₈Cd).
This series lies in the fifth period of the periodic table and consists of 10 elements.
General electronic configuration is 4d^1-10 5s^0-2

(iii) Third transition series (5d). Lanthanum (₅₇La), Hafinium ₇₂Hf) to Mercury (₈₀Hg).
This series lies in the sixth period of the periodic table and consists of 10 elements.
General electronic configuration is 5d^1-10 6s^1-2.

(iv) Fourth transition series (6d). Actinium (₈₉Ac), Kurchatovium (₁₀₄Ku) to atoic number 112 have been synthesised. This series lies in the seventh period of the periodic table and consits of 10 elements.

Hence d-block elements consist of 40 elements.