Niobium atomic radius

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

Identify the element with the larger atomic radius in each of the following pairs (a) scandium and titanium (b) copper and gold (c) vanadium and niobium. 

A similar phenomenon has been observed by the author and Sokolova, 189) for the catalysis of alcohols on pentoxides of vanadium, niobium and tantalum. The increase of dehydration as against dehydrogenation goes on simultaneously with the increase of the atomic radius in the series vanadium (1.36 A) niobium (1.47 A) tantalum (1.49 A.)... 

The radius of a niobium atom is 0.147 nm. Niobium adopts the A2 stmcture. Estimate the unit cell parameter of niobium crystals. 

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