Niobium and tantalum, molybdenum and tungsten (belonging to the sub-group d area elements) atomic radius difference is not the reason: the electronic filling caused by the different: the main element of the electronic elements in order to fill the outermost orbit, the nuclear charge shielding effect (Σ = 0.35), resulting in significant increase in the effective nuclear charge; and the sub-family of the d element of the elemental electrons are followed by filling the sub-outer d-track, the nuclear charge shielding effect (σ = 0.85), it is effective The increase in nuclear charge is not significant.

So niobium and tantalum, molybdenum and tungsten atomic radius is not very different