Hafnium

Introduction

Hafnium is a chemical element with an atomic number of 72 in the periodic table of elements. It makes up about  3.3 parts per million (ppm) of Earth’s crust. However, it does not exist in its free metallic form in nature. This tetravalent transition metal is characterized as the most refractory of all nitrides of metals. 

Fact Box

Chemical and Physical Properties of Hafnium

The symbol in the periodic table of elements: Hf

Atomic number: 72

Atomic weight (mass): 178.49 g.mol-1

Group number: 4 (Transition elements)

Period: 6

Color: A lustrous and bright silvery metallic substance

Physical state: Solid metal at room temperature

Half-life: From 400# ms [>200 ns] to 2.0(4)×1015 years

Electronegativity according to Pauling: 1.3

Density: 13.07 g.cm-3 at 20°C

Melting point: 2200°C

Boiling point: 5200°C

Van der Waals radius: 

Ionic radius: 0.075 nm

Isotopes: 32 (5 stable isotopes)

Most characteristic isotopes: 177Hf, 178Hf, 180Hf 

Electronic shell: [Xe] 4f145d26s2 

The energy of the first ionization: 530 kJ.mol-1

The energy of the second ionization: 1425.5 kJ.mol-1

The energy of the third ionization: 2244.3 kJ.mol-1

Discovery date: In 1923 by Georg von Hevesy and Dirk Coster

The chemical structure of this element is almost identical to the one of zirconium. With the periodic table symbol Hf, atomic number 72, atomic mass of 178.49 g.mol-1, and electron configuration X, actinium is a corrosion-resistant and ductile metal that reaches its boiling point at 5200°C, while the melting point is achieved at 2200°C.

This member of the transition metals family of elements in the periodic table has an electronegativity of 1.3 according to Pauling, whereas the atomic radius according to van der Waals is 0.161 nm. 

As a result of the oxide film formation on its exposed surfaces, hafnium possesses strong anti-corrosive qualities. This transition metal is not reactive with water molecules, air, alkalis and acids, but readily reacts with hydrogen fluoride. The powder form of hafnium spontaneously ignites, i.e. it has a pyrophoric property.       

How Was Hafnium Discovered?

Hafnium is one of the chemical elements predicted by Dmitri Mendeleev in 1869. Several decades before hafnium was introduced to the world of chemistry, Mendeleev postulated the existence of element 72. This chemical was supposed to be heavier than zirconium and titanium, and fall between element 71 (lutetium) and element 73 (tantalum). 

Georges Urbain’s Breakthrough Analysis

In 1911, the French chemist and discoverer of the rare earth element lutetium Georges Urbain (1872 – 1938) thought that he was on the way to discovery of the missing element as predicted by Mendeleev. Upon conducting his spectral analysis of rare-earth elements, Urbain recognized the properties of the missing element in the result of his experiment. On that occasion, he labeled the newly discovered substance as celtium.

However, several years after what he believed was a monumental discovery, additional analysis of the chemical proved that it was merely a compound of already discovered lanthanides. While attempting a more thorough research on the subject,  the English chemist Henry Moseley (1887 – 1915) performed an X-ray spectroscopy. This led him to scientific evidence that proved Georges Urbain’s element ‘celtium’ was not element 72.

Niels Bohr’s Postulate 

Since it was not clear whether this elusive element  could be classified as a transition metal or a rare-earth metal, Danish physicist Niels Bohr (1885 – 1962) proposed the missing element to be included among the transition metals. Bohr, who is also one of the founders of quantum theory, based his postulate upon the basis of the electron configuration of the element that we know today as hafnium. 

Georg von Hevesy and Dirk Coster’s Contribution to the Discovery 

In order to support his postulate, he assigned the Hungarian chemist Georg von Hevesy and the Dutch physicist Dirk Costerto to attempt to isolate the element as a confirmation to his theory. For this reason, the two scientists employed at Bohr’s institute performed a scrutinized research of the mineral zircon via an X-ray spectroscopic analysis. The result of their extensive scientific efforts proved that the zirconium ore contained the missing element 72, which was later named hafnium (Hf).

The High-Purity Hafnium Production Method

In 1925, the Dutch chemists Anton Eduard van Arkel and Jan Hendrik de Boer managed to produce high-purity hafnium by decomposing hafnium tetraiodide over a heated tungsten filament. This resulted in a crystal bar of pure hafnium. 

How Did Hafnium Get Its Name?

The Latin word for Copenhagen ‘Hafnia’ served as a label of the new element. Copenhagen was both the city where hafnium was first isolated, and the hometown of Niels Bohr – one of the one of the founders of quantum theory and the scientist who encouraged hafnium’s discoverers to conduct the revolutionary analysis of the zirconium ore. 

Where Can You Find Hafnium?

Element 72 is relatively scarce in nature and difficult to be produced. Hafnium metal can be rarely found in nature in its pure form. It mostly occurs in compounded form with thortveitite, and alvite [(Hf, Th, Zr) SiO4 H2O]. However, zircon (ZrSiO4) is the primary source of hafnium.  For the use in the nuclear, chemical, and aerospace industries, this chemical is obtained as a byproduct during the refinement of zirconium. 

Not only was hafnium found in many zirconium minerals, but it also resembled the chemical structure of zirconium. This made element 72 difficult to isolate it in its pure elemental form and the process would take a long time.

Hafnium in Everyday Life

Despite being not so evident in our everyday life, hafnium is commonly used in the electronics and nuclear power industries:

• The isotopes 174Hf, 176Hf, and 177Hf find application in the nuclear physics researches and gamma ray spectrometry;
• Hafnium-180 isotope is used in the pharmaceuticals industries and in medicine, as well as in the production of radionuclides and the tantalum-179 isotope;
• Hafnium displays not only excellent absorption cross section for thermal neutrons, but also  great mechanical properties which makes hafnium almost perfect for use in the nuclear reactor control rods;
• As a neutron absorber, this transition metal is also used in nuclear submarines;
• The super-alloys made with hafnium (in combination with niobium, titanium, or tungsten) are resistant to extremely high temperatures. For this reason, they are often used in the manufacturing of space vehicles parts;
• Hafnium is also a part of the tungsten alloys. It can also be found in filaments and light bulbs;
• Cathodes and capacitors are commonly made with this transition metal as one of the main substances used in the production process; 
• Since some hafnium compounds are very refractory, this chemical is used in the manufacturing of ceramics. In this way, the ceramic items become resistant to very high temperatures. In this regard, hafnium nitride is the most refractory of all known metal nitrides;
• Hafnium is used in vacuum tubes as a getter; it first combines with the gases and then removes them from the tubes;
• This transition metal is also applied in the dating of Earth’s layers. 

How Dangerous Is Hafnium?

The free elemental form of hafnium is not considered toxic. However, the hafnium compounds do impose a greater risk of toxicity that may trigger adverse health effects. Since this chemical element is water-soluble, the exposure to its hazardous properties can occur via ingestion, skin or eye contact, or inhalation of the hafnium dust particles. 

A prolonged exposure to high levels of hafnium and hafnium compounds may result in skin and eye irritation, as well as irritation of the mucous membranes. 

Environmental Effects of Hafnium

The compounds of hafnium may impose a fire hazard due to their explosive properties. The pure form of hafnium, however, is not considered to be an environmental hazard. 

Isotopes of Hafnium

176Hf, 177Hf, 178Hf, 179Hf, and 180Hf are the five stable isotopes of hafnium that make up the pure elemental form of the element 72, ie. natural hafnium (72Hf). Among the 32 isolated isotopes of hafnium, the 174Hf isotope is the one with the longest half-life with 2×1015 years. The other isotopes have an average half-life of less than one minute. 

There are also 30 radioactive isotopes of this chemical element. WIth a half-life of 8.9×106 years, the 182Hf radioisotope of hafnium is the most stable one of this group. In addition, 26nuclear isomers of hafnium have also been isolated. 

The following is a tabular representation of some of the hafnium isomers:

Nuclide [n 1]	Z	N	Isotopic mass (Da) [n 2][n 3]	Half-life [n 4][n 5]	Decay mode [n 6]	Daughter isotope [n 7]	Spin and parity [n 8][n 5]	Natural abundance (mole fraction)
	Excitation energy[n 5]							Normal proportion	Range of variation
153Hf	72	81	152.97069(54)#	400# ms [>200 ns]			1/2+#
153mHf	750(100)# keV			500# ms			11/2−#
154Hf	72	82	153.96486(54)#	2(1) s	β+	154Lu	0+
					α (rare)	150Yb
155Hf	72	83	154.96339(43)#	890(120) ms	β+	155Lu	7/2−#
					α (rare)	151Yb
156Hf	72	84	155.95936(22)	23(1) ms	α (97%)	152Yb	0+
					β+ (3%)	156Lu
156mHf	1959.0(10) keV			480(40) µs			8+
157Hf	72	85	156.95840(21)#	115(1) ms	α (86%)	153Yb	7/2−
					β+ (14%)	157Lu
158Hf	72	86	157.954799(19)	2.84(7) s	β+ (55%)	158Lu	0+
					α (45%)	154Yb
159Hf	72	87	158.953995(18)	5.20(10) s	β+ (59%)	159Lu	7/2−#
					α (41%)	155Yb
160Hf	72	88	159.950684(12)	13.6(2) s	β+ (99.3%)	160Lu	0+
					α (.7%)	156Yb
161Hf	72	89	160.950275(24)	18.2(5) s	β+ (99.7%)	161Lu	3/2−#
					α (.3%)	157Yb
162Hf	72	90	161.94721(1)	39.4(9) s	β+ (99.99%)	162Lu	0+
					α (.008%)	158Yb
163Hf	72	91	162.94709(3)	40.0(6) s	β+	163Lu	3/2−#
					α (10−4%)	159Yb
164Hf	72	92	163.944367(22)	111(8) s	β+	164Lu	0+
165Hf	72	93	164.94457(3)	76(4) s	β+	165Lu	(5/2−)
166Hf	72	94	165.94218(3)	6.77(30) min	β+	166Lu	0+
167Hf	72	95	166.94260(3)	2.05(5) min	β+	167Lu	(5/2)−
168Hf	72	96	167.94057(3)	25.95(20) min	β+	168Lu	0+
169Hf	72	97	168.94126(3)	3.24(4) min	β+	169Lu	(5/2)−
170Hf	72	98	169.93961(3)	16.01(13) h	EC	170Lu	0+
171Hf	72	99	170.94049(3)	12.1(4) h	β+	171Lu	7/2(+)
171mHf	21.93(9) keV			29.5(9) s	IT	171Hf	1/2(−)
172Hf	72	100	171.939448(26)	1.87(3) y	EC	172Lu	0+
172mHf	2005.58(11) keV			163(3) ns			(8−)
173Hf	72	101	172.94051(3)	23.6(1) h	β+	173Lu	1/2−
174Hf[n 9]	72	102	173.940046(3)	2.0(4)×1015 y	α	170Yb	0+	0.0016(1)	0.001619–0.001621
175Hf	72	103	174.941509(3)	70(2) d	β+	175Lu	5/2−
176Hf[n 10]	72	104	175.9414086(24)	Observationally Stable[n 11]			0+	0.0526(7)	0.05206–0.05271
177Hf	72	105	176.9432207(23)	Observationally Stable[n 12]			7/2−	0.1860(9)	0.18593–0.18606
178Hf	72	106	177.9436988(23)	Observationally Stable[n 13]			0+	0.2728(7)	0.27278–0.27297
179Hf	72	107	178.9458161(23)	Observationally Stable[n 14]			9/2+	0.1362(2)	0.13619–0.1363
179m1Hf	375.0367(25) keV			18.67(4) s			1/2−
179m2Hf	1105.84(19) keV			25.05(25) d			25/2−
180Hf	72	108	179.9465500(23)	Observationally Stable[n 15]			0+	0.3508(16)	0.35076–0.351
181Hf	72	109	180.9491012(23)	42.39(6) d	β−	181Ta	1/2−
182Hf	72	110	181.950554(7)	8.90(9)×106 y	β−	182Ta	0+

Source: Wikipedia

List of Hafnium Compounds 

Hafnium mainly occurs in oxidation state +4. However, when compounded with other elements (especially with halogens) it may adopt the oxidation states from 0 to +3. This chemical element mainly forms hydrides, fluorides, oxides, bromides, and iodides. Hafnium fluoride (HfF4), hafnium chloride (HfCl4), and hafnium oxide (HfO2) are among the most commonly occurring compounds of hafnium:

 

• Hafnium acetylacetonate
• Hafnium oxide
• Hafnium(IV) oxychloride octahydrate
• Hafnium(IV) hypochlorite
• Hafnium diboride
• Hafnium dioxide
• Hafnium disulfide
• Hafnium iodide
• Hafnium(IV) fluoride
• Hafnium tetrabromide
• Hafnium tetrafluoride
• Hafnium(IV) sulfate
• Hafnium sulfide
• Hafnium tetraiodide
• Hafnium(IV) carbide
• Hafnium(IV) bromide
• Hafnium nitrate
• Hafnium tetrachloride
• Hafnium(IV) silicate
• Hafnium selenide
• Hafnium cyanide
• Tantalum hafnium carbide
  

5 Interesting Facts and Explanations

1. Hafnium was the penultimate chemical element with stable nuclei that found its place in the Mendeleev’s periodic table in 1923 before the element rhenium was discovered in 1925 and classified according to the Periodic Law. 
2. Having a high melting point at nearly 7,034 degrees Fahrenheit (3,890 degrees Celsius), hafnium carbide (HfC) has the highest melting point of any known chemical compound composed of two elements. 
3. Since both hafnium and zirconium have same-sized atoms and extremely similar chemical properties, it’s very hard to tell them apart.  
4. The radioactive isomers of hafnium may be considered as a nuclear weapon source. However, their rapid release of energy potential is still being analyzed. 
5. When hafnium oxidizes, it creates a beautiful rainbow colored surface effect.