Group 5: the vanadium group

Five electrons beyond the core

Vanadium is [Ar] 3d³ 4s² and tantalum [Xe] 4f¹⁴ 5d³ 6s², with three d and two s electrons. Niobium rearranges them as [Kr] 4d⁴ 5s¹, one of several elements that break the simple filling order, but it still has five electrons to use. Tantalum appears almost only as +5, as in Ta₂O₅, while niobium forms +5 and +3 compounds.

Vanadium is the most flexible member, with common oxidation states of +2, +3, +4 and +5. Each has its own color in solution: yellow for +5, blue for +4, green for +3 and violet for +2, so a vanadium solution that is reduced step by step runs through a little rainbow.

Trends down the group

Melting point and density rise steadily. Vanadium melts at 2183 K, niobium at 2750 K and tantalum at 3290 K, while density goes from 6 g/cm³ to 8.57 g/cm³ and then to 16.69 g/cm³. Electronegativity changes little: 1.63, 1.6 and 1.5.

As in group 4, the lanthanide series makes the period 6 atom almost the same size as the one above it, so niobium and tantalum occur together in the same ores and are hard to separate. Fittingly, tantalum is named after Tantalus from Greek myth, and niobium after his daughter Niobe.

Where you meet group 5

A little vanadium makes steel tougher, and vanadium steel went into tools, springs and the Ford Model T. Large vanadium flow batteries store electricity from wind and solar farms. Niobium–titanium wire, chilled with liquid helium, becomes a superconductor in the magnets of MRI scanners and particle accelerators. Tantalum makes tiny, reliable capacitors for phones and laptops, and because the body tolerates it, it is also used in implants.

Dubnium

Dubnium (105), named after the Russian research town of Dubna, has been made only a few atoms at a time. Single-atom experiments suggest it behaves like a heavier niobium or tantalum, but its bulk properties have never been measured.