THE hafnium, Hf, is a transition metal of atomic number 72, located in group 4 of the Periodic table. It occurs naturally with the element that is just above it, zirconium, but they are difficult to separate given the great chemical similarity between them. Lanthanic contraction causes hafnium to have a atomic radius nearly equal to that of zirconium, facilitating the exchange between the two in the composition of minerals.
Hafnium is scarcely present in the earth's crust, but it has important applications. One of them is in the manufacture of neutron control rods in nuclear reactors, which control the fission reactions. It can also be used in the production of metallic superalloys and high temperature ceramics.
Read too: Yttrium — metal widely used in electronics
hafnium summary
It occurs naturally with zirconium.
It is not very present in the Earth's crust.
Lanthanic contraction makes the separation of hafnium and zirconium difficult.
It is basically found in zirconite.
It is used in the manufacture of neutron control rods in nuclear reactors.
It was discovered by Georg von Hevesey and Dirk Coster.
Hafnium Properties
Symbol: Hf
atomic number: 72
atomic mass: 178.49 c.u.s.
electronic configuration: [Xe] 6s2 4f14 5d2
Fusion point: 2233 °C
Boiling point: 4600 °C
Density: 13.3 g.cm-3
chemical series: transition metal, group 4
hafnium characteristics
Hafnium is a grayish metal naturally occurring in the earth's crust, with about 5.3 mg for every kilogram of crust. When finely divided, it is a pyrophoric material, that is, it is prone to combustion spontaneous in contact with the air, however, in its raw form, it is not.
Hafnium is one of the first elements of the Periodic Table to have the effect of the so-called lanthanide contraction, in which a contraction of the atomic radius during the lanthanide series. As a consequence, the hafnium ray is similar to the element just above him in the periodic table, the zirconium, whose difference is only 1 pm (picometer, 10-12 m). As a consequence, some properties are very similar to each other, making them occur together in nature and being difficult to separate.
It is a metal which can undergo acid attack at high temperatures, but does not suffer any action from alkaline solutions, even at higher temperatures. The chemistry of hafnium is poorly understood compared to zirconium. However, much of the chemical behavior of hafnium resembles that of zirconium, such as the predominance of the +4 oxidation state in solution and the reaction with most of the nonmetals at high temperature.
Hf + O2 → HfO2
Hf + 2 Cl2 → HfCl4
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Occurrence of hafnium
hafnium is little present in the earth's crust, occurring primarily associated with zirconium in minerals such as zirconite, a mixed silicate of zirconium and hafnium, which may also contain other elements. The chemical formula can be represented by (Zr, Hf) SiO4 and the hafnium content usually varies from 1% to 4% by mass. The ratio of zirconium to hafnium is 50:1 in zirconite, and as said, they are quite difficult to separate.
THE extraction of zirconium-hafnium mixture from zircon can occur with the conversion of the oxide of these metals into their tetrachloride, at high temperature. In a second step, the tetrachloride of the metals will be reduced by magnesium in atmosphere of argon, at very high temperature. The following reactions demonstrate the process, where M can be either Hf or Zr.
MO2 → MCl4 (using CCl4 at 770 K temperature)
MCl4 → M (using Mg in an atmosphere of air at 1420 K temperature)
THE separation between the two may involve some techniques, such as the fractional crystallization of K salts2ZrF6 and K2HfF6, which have different solubilities in water. It is also possible to do a solvent extraction, in which compounds of Zr and Hf are dissolved in water and then selectively extracted with organic solvents. It is worth noting that these are not the only techniques for separating hafnium and zirconium. The industry has already developed hydrometallurgical (ie, which occur in aqueous solution) and pyrometallurgical (without the presence of water) routes.
hafnium applications
When mixed with zirconium, hafnium can be a important improver of the physical properties of steel. When pure, metallic hafnium can be incorporated into alloys of iron, titanium and niobium. The similarities to zirconium make it possible for hafnium to be a good substitute for this metal, although it is a bit unlikely given the higher naturally occurring zirconium.
However, a major use of hafnium is in production of sticks(also knownThes like sticks or rods) of control in nuclear power plants. As it is a metal with good absorption capacity of neutrons, hafnium can be used to prevent chain reactions from occurring in the plant, allowing control of the energy generated and minimizing the probability of accidents. It is worth remembering that uranium fission, for example, always generates neutrons, which could collide with new uranium nuclei, in an effect that would generate energy in a geometric progression.
Finally, hafnium can also be used in high temperature ceramics, as it is capable of producing highly refractory materials such as borides and carbides that exceed 3000 °C melting point.
hafnium history
Hafnium followed the trend of elements discovered throughout the 20th century. Were discovered in small amounts and he also had his discovery erroneously pointed out. This occurred with Georges Urbain, who believed that element 72 was a rare earth and not a transition metal. That's why, Urbain began to look for it in mixtures of the mineral ytterbium, in which he co-discovered the element lutetium, atomic number 71. So, in 1911, he published a paper in which he presented what would be spectroscopic data of a new element, which he called celtium.
To determine its atomic number and confirm his discovery, Urbain went to England in 1914 to carry out X-ray emission experiments developed by Henry Moseley. However, experiments failed to prove that the supposed element Celtium was, in fact, element 72. So convinced of his efforts, Georges Urbain went so far as to say to Rutherford, later, that the failure to verify his discovery was due to flaws in Moseley's methods.
In the opposite direction and in the face of new ideas about the atomic structure, Georg von Hevesy assumed that element 72 must be a transition metal and thus began further studies with his colleague Dirk Coster. X-ray analysis of small samples of zirconium silicate revealed the existence of a substance unknown, with spectroscopic characteristics similar to those predicted by Moseley for such an element.
Thus, after sample purification,Von Hevesy and Coster published their findings, suggesting the name hafnium for the new element, alluding to the Latin name for the city of Copenhagen, Hafnia, the site of the discovery. Even so, Urbain continued to champion the discovery of celtium for many years, until experimental techniques proved that hafnium and celtium produced different responses. In response to this, what Moseley already suspected was confirmed: Celtium was, in fact, highly purified lutetium.
Read too: Discovery of oxygen — the feat that changed the course of combustion studies
Exercises solved on hafnium
question 1
Hafnium is an element very similar to zirconium, which is just above it in the Periodic Table. We can explain this great similarity because:
(A) Hafnium and zirconium have the same mass.
(B) Hafnium and zirconium have the same number of protons.
(C) Hafnium and zirconium are in the same group in the Periodic Table.
(D) Hafnium and zirconium have the same number of electrons.
(E) Hafnium and zirconium are both metallic elements.
Reply: letter C
The similarity between Hf and Zr stems from their belonging to the same group in the Periodic Table. The table places in the groups the elements that have similar chemical properties. Thus, the template is the letter C.
question 2
Like zirconium, hafnium appears in its most stable form with an oxidation number of +4. Commonly, hafnium can bind halogens.
The most suitable formula for hafnium fluoride IV would be:
(A) HfF
(B) HfF2
(C) HfF3
(D) HfF4
(E) Hf2F3
Reply: letter D
THE fluorine It has a fixed oxidation number, always equal to -1. Since Hf is an element with NOx equal to +4, four fluorine atoms are needed to neutralize the charge of Hf. Thus, the compound hafnium fluoride IV is HfF4, described in letter D.
By Stefano Araújo Novais
Chemistry teacher
