CBSE Class 10 Answered

Any of the metallic elements within Groups 3 to 12 in the Periodic Table that have an incomplete inner electron shell (d orbital) are called transition elements. Due to incomplete penultimate shell (d orbital), and completely filled outermost shell (s orbital), the incoming electron enters the penultimate shell to acquire stable configuration (either completely filled or half filled d orbital). Hence in case of transition elements, the added electrons fill the penultimate shell, and not the ultimate shell.