Question

Why are transition metals called transition metals?

Answer 1

"The transition metals are called such because an English chemist , Charles Bury described them as the bunch of elements that fill inner shells and bridge the transition between stable states".
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Answer 2

 They are called that because while the outermost level contains at most two electrons, they are next to outermost main levels have incompletely filled d sub-orbitals, which are filled-up progressively on going across the periodic table from 8 to 18 electrons. The filling of the d sub-orbitals of the transition elements across a row of the periodic table is not always regular.