In these elements, the last electron normally enters the d orbital which is the penultimate shell, this is due to the aufbau principle of increasing order of energy level filling of orbitals i.e. the last electron goes to (n-1) d orbital. Hence these elements are named as d-block elements and they are called as transition metals because they form the bridge between the representative elements. These elements have partly filled d-subshells in their elementary form or in their simple ions. The electrons in the outer most orbit are sheilds by inner d-electrons. There fore the size of terminal elements is more than the preceding ones.