The transition metals are hard and have high melting temperatures as a result of high lattice energies. They a characterised by a progressive filling of the "d" subshell. Below are the electronic configurations of the the metals form scandium to zinc.
Sc -----1s2, 2s2,
2p6, 3s2, 3p6, 3d1, 4s2
Notice how Cr and Cu have only 1 electron in the 4s subshell. It seems that a small level of stability is gained by the atom if the 3d subshell is occupied by 5 or 10 electrons.
Unlike the trends across period 2 and 3 the transition metals show no trend across the period. They have very similar properties. As we move across the transition metals of period 4, the atomic radii of these metals does not decrease significantly. The reason for this apparent lack of reduction in atomic size, eventhough the nucleus is increasing in protons as we go accross the period, lies in the repulsion between the 4s and the 3d electrons. We encountered earlier that electrons move into the outer 4s subshell before they occupy the higher energy, inner 3d subshell. Repulsion of the 3d and 4s electrons increases and therefore repels the outer 4s electrons further from the nucleus thus maintaining a somewhat constant atomic radius.