A transition metal is a metal that can form one or more stable ion with an incomplete d orbital.The first ionisation energies increase across a period due to the increase in nuclear charge without an increase in shielding electrons in inner orbitals. The effective nuclear charge across the d-block does not change as dramatically as in period 3 as although the actual nuclear charge increases, the number of shielding electrons in the 3d subshell increases as well. This means that the first ionisation energies of the elements in the d-block of period 4 are similar.There is a smaller difference between successive ionisation energies of transition metals.