Question

there is a close similarity in physical and chemical properties of the 4d and 5d series of the transition elements much more than expected on the basis of usual family relationship

Answer 1

Due to poor shielding effect of 4f orbitals , the size of 5d elements is less than the expected value. The size of 4d and 5d elements is nearly same. This is due to their similar size that 4d and 5d elements show similar properties.