earth science

There are four series of transition metal in our periodic table . The 3d, 4d,5d(lathanoids)and 6d(actinoids).

In the 5d series the 4f orbital needs to be filled before the 5s and d orbitals and as 4f orbitals have weaker shielding effect than the d orbitals there is a regular decrease in the size of lanthanoids with increasing atomic number. Due to this contraction the atoms of 4d and 5d series have similar atomic radii and have very much alike physical and chemical properties .