|Atomic mass||178.49 amu|
Hafnium's existence was suspected in the latter part of the 1800s. Through these years, scientists who were studying zirconium thought that there was another element that existed hiding in the background of even the purest known samples of zirconium. By the early 1900s researchers used X-ray instruments to study the patterns of outer electrons in zirconium ores and confirmed the faint, but real lines that proved Neils Bohr's prediction of element 72 was correct. In 1923, the discovery of Hafnia was announced. Acknowledging that Bohr played a vital role in the discovery of this element, it was named after the ancient name of Bohr's hometown of Copenhagen, Hafnia.
Hafnium resembles stainless steel. It is protected from corrosion by an oxide film that forms when the metal is exposed to the atmosphere. The chemical and physical properties of hafnium and zirconium are very alike and the two elements are impossible to separate entirely. The highest grade zirconium metal has up to 1% hafnium. Likewise, hafnium that contains up to 2% zirconium is still considered a high-grade product.
Hafnium is an excellent material for absorbing neutrons. Together with its high resistance to corrosion, hafnium is used in neutron-absorbing control rods which slows down nuclear chain reactions and quenches chain reactions in nuclear reactors.