Titanium, zirconium and hafnium, which are situated in group 4 of the periodic table, are all silvery metals with high melting points. The layer of oxide they gain on exposure to air gives them considerable resistance to corrosion, and they are resistant to attack from most acids and alkalis. Unnilquadium, the fourth member of the group, differs from the other three in being radioactive. It is sometimes considered along with the actinide elements.
Despite the protective layer of oxide, the elements are not unreactive. When finely divided, the metals become spontaneously flammable, and at high temperatures they can be induced to combine with most of the non-metallic elements.
Zirconium and hafnium, although similar in many respects, differ markedly in their ability to absorb neutrons. Hafnium displays high neutron absorption, zirconium low. As a consequence, hafnium is used in control rods in nuclear reactors on board nuclear submarines.
The main oxidation state of the elements is +4: they usually form four bonds when combining with another element.
