Hafnium is a lustrous, silvery-gray transition metal closely related to zirconium. It is corrosion-resistant and used in nuclear reactors, alloys, and electronics.
The electron configuration of hafnium is [Xe] 4f14 5d2 6s2. In this configuration, the 4f shell is completely filled, and the 5d and 6s electrons contribute to its metallic bonding and chemical reactivity.
Hafnium and zirconium have nearly identical ionic radii and chemical properties because they belong to the same group (Group 4). As a result, they occur together in minerals such as zircon (ZrSiO4). Separation of Hf from Zr requires specialized techniques like solvent extraction or ion exchange.
The most stable and common oxidation state of hafnium is +4, forming compounds such as HfO2 (hafnium dioxide) and HfCl4 (hafnium tetrachloride). Lower oxidation states like +3 and +2 are rare and usually found only under reducing conditions.
Hafnium has an exceptionally high neutron absorption cross-section, which makes it ideal for use in control rods of nuclear reactors. These rods regulate the fission reaction by absorbing excess neutrons. In contrast, zirconium (its chemical twin) is used in fuel cladding because it has a low neutron absorption capacity.
\(\mathrm{n + ^{177}Hf \rightarrow ^{178}Hf} \)
Hafnium is used in:
Hafnium is a dense, ductile, and corrosion-resistant metal with a melting point of approximately 2233 °C and a boiling point near 4603 °C. It resists attack by acids and alkalis but reacts with halogens to form halides such as HfCl4 and HfF4.
When exposed to air, hafnium forms a protective oxide layer, hafnium dioxide (HfO2), which prevents further corrosion. It reacts readily with halogens at high temperatures:
\(\mathrm{Hf(s) + 2Cl_2(g) \rightarrow HfCl_4(s)}\)
Hafnium dioxide is a high-dielectric-constant (high-k) material used as a gate insulator in modern CMOS transistors. It replaces silicon dioxide (SiO₂) in advanced microchips, improving power efficiency and performance in electronics.
Hafnium and its compounds are generally considered to have low toxicity. However, fine hafnium metal powder can be flammable and should be handled carefully. Inhalation of hafnium dust may cause irritation to the respiratory system.
A common oxidation reaction of hafnium with oxygen forms hafnium dioxide:
\(\mathrm{Hf(s) + O_2(g) \rightarrow HfO_2(s)}\)
HfO₂ is a refractory material that is chemically stable and widely used in electronics and optics.