72
  Hf  
178.490000
Hafnium

Name: Hafnium
Symbol: Hf
Atomic Number: 72
AtomicWeight: 178.490000
Family: Transition metals
CAS RN: 7440-58-6
Description: Silver colored ductile transition metal that resists corrosion.
State (25 C): Solid
Oxidation states: +4

Molar Volume: 13.41 cm3/mole
Valence Electrons: 5d26s2

Boiling Point:  4876K, 4603C, 8317F
Melting Point:
2500K, 2227C, 4041F
Electrons Energy Level: 2, 8, 18, 32, 10, 2
Isotopes: 28 + 5 Stable
Heat of Vaporization: 575 kJ/mol
Heat of Fusion: 24.06 kJ/mol
Density: 13.31 g/cm3 @ 300K
Specific Heat: 0.14 J/gK
Atomic Radius: 2.16
Ionic Radius: 0.71
Electronegativity: 1.3 (Pauling); 1.23 (Allrod Rochow)
Vapor Pressure: 0.00112 Pa @ 2227C

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d2 6s2

History

The 1869 periodic table by Mendeleev had implicitly predicted the existence of a heavier analog of titanium and zirconium, but in 1871 Mendeleev placed lanthanum in that spot.

The existance of a gap in the periodic table for an as-yet undiscovered element 72 was predicted by Henry Moseley in 1914.  Hafnium (Latin Hafnia for "Copenhagen", the home town of Niels Bohr)  was discovered by Dirk Coster and Georg Charles von Hevesy in 1923 in Copenhagen, Denmark, validating the original 1869 prediction of Mendeleev.  Soon after, the new element was predicted to be associated with zirconium by using the Bohr theory and was finally found in zircon through X-ray spectroscopy analysis in Norway.

It was separated from zirconium through repeated recrystallization of double ammonium or potassium fluorides by Jantzen and von Hevesey. Metallic hafnium was first prepared by Anton Eduard van Arkel and Jan Hendrik de Boer by passing hafnium tetraiodide vapor over a heated tungsten filament.  This process for differential purification of Zr and Hf is still in use today.

The Faculty of Science of the University of Copenhagen uses in its seal a stylized image of hafnium.

Characteristics

Hafnium is a lustrous, shiny silvery-gray,  tetravalent, ductile transition metal that is corrosion resistant and chemically similar to zirconium.  The properties of hafnium are markedly affected by zirconium impurities and these two elements are amongst the most difficult to separate.  A notable physical difference between them is their density (zirconium is about half as dense as hafnium), but chemically the elements are extremely similar.  Nevertheless, separation of them becomes important in the nuclear power industry, as Zr is common fuel-rod cladding alloy material, with the desirable properties of low neutron cross section and high chemical stability at high temperatures.   However, because of Hf's neutron absorbing properties, hafnium impurities in zirconium cause it to be far less useful for nuclear reactor materials applications.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d2
6s2

Hafnium carbide is the most refractory binary compound known and hafnium nitride is the most refractory of all known metal nitrides with a melting point of 3310oC.   This has led to proposals that hafnium or Hf carbides might be useful as construction materials subjected to very high temperatures.

The metal is resistant to concentrated alkalis, but halogens react with it to form hafnium tetrahalides.  At higher temperatures hafnium reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. 

The nuclear isomer Hf-178-m2 is also a source of cascades of gamma rays whose energies total to 2.45MeV per decay.  It is notable because it has the highest excitation energy of any comparably long-lived isomer of any element.  One gram of pure Hf-178-m2 would contain approximately 1330 megajoules of energy, the equivalent of exploding about 317 kilograms (700 pounds) of TNT.  Possible applications requiring such highly concentrated energy storage are of interest. For example, it has been studied as a possible power source for gamma ray lasers.

Occurrence

Hafnium is estimated to make up about 0.00058% of the Earth's upper crust by weight.   It is found combined in natural zirconium compounds but it does not exist as a free element in nature.  Minerals that contain zirconium, such as alvite [(Hf, Th, Zr) SiO4H2O, thortveitite and zircon (ZrSiO4), usually contain between 1 and 5% hafnium. Hafnium and zirconium have nearly identical chemistry, which makes the two difficult to separate.  About half of all hafnium metal manufactured is produced by a by-product of zirconium refinement.  This is done through reducing hafnium (IV) chloride with magnesium or sodium in the Kroll process.

Applications

Hafnium is used to make control rods for nuclear reactors because of its ability to absorb neutrons (its thermal neutron absorption cross section is nearly 600 times that of zirconium), excellent mechanical properties and exceptional corrosion-resistance properties.

Other Uses

Compounds

Melting near 3890C, hafnium carbide, HfC, has the highest melting point of any known two-element compound.  Hafnium nitride, HfN, also has a high melting point, around 3305C. Other hafnium compounds include: hafnium chloride, HfCl4, hafnium fluoride, HfF4, and hafnium oxide, HfO2.

Hafnium Chloride, HfCl4 Hafnium Fluoride, HfF4
Hafnium Oxide, HfO2 Hafnium Nitride, HfN
Hafnium Carbide, HfC

Isotopes

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
Hf154 153.964 2 seconds
Hf155 154.963 0.89 seconds
Hf156 155.959 25 ms
Hf157 156.958 110 ms
Hf158 157.955 2.85 seconds
Hf159 158.954 5.6 seconds
Hf160 159.951 13.6 seconds
Hf161 160.9503 16.8 seconds
Hf162 161.9472 37.6 seconds
Hf163 162.947 40 seconds
Hf164 163.944 111 seconds
Hf165 164.945 76 seconds
Hf166 165.942 6.77 minutes
Hf167 166.943 2.05 minutes
Hf168 167.941 25.95 minutes
Hf169 168.9412 3.24 minutes
Hf170 169.94 16.01 hours
Hf171 170.94 12.1 hours
Hf172 171.9395 1.87 years
Hf173 172.941 23.6 hours
Hf174 173.94 2.0E 15 years
Hf175 174.9415 70 days
Hf176 175.9414 Stable
Hf177 176.9432 Stable
Hf178 177.9437 Stable
Hf179 178.9458 Stable
Hf180 179.9465 Stable
Hf181 180.9491 42.39 days
Hf182 181.9506 9E 6 years
Hf183 182.9535 1.067 hours
Hf184 183.9554 4.12 hours
Hf185   3.5 minutes
Hf186   2.6 minutes

Precautions

80px-Flammable.jpg (2186 bytes) Care needs to be taken when machining hafnium because, like its sister metal zirconium, when hafnium is divided into fine particles, it is pyrophoric and can ignite spontaneously in air.

Compounds that contain this metal are rarely encountered by most people and the pure metal is not normally toxic but all its compounds should be handled as if they are toxic (although there appears to be limited danger to exposed individuals).

atom.gif (700 bytes)

Hafnium Data
 

Atomic Structure

  • Atomic Radius: 2.16
  • Atomic Volume: 13.6cm3/mol
  • Covalent Radius: 1.44
  • Cross Section (Thermal Neutron Capture) Barns: 104
  • Crystal Structure: Hexagonal
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d2 6s2
  • Electrons per Energy Level: 2, 8, 18, 32, 10, 2
  • Ionic Radius: 0.71
  • Filling Orbital: 5d2
  • Number of Electrons (with no charge): 72
  • Number of Neutrons (most common/stable nuclide): 106
  • Number of Protons: 72
  • Oxidation States: 4
  • Valence Electrons: 5d6 6s2

Chemical Properties

  • Electrochemical Equivalent: 1.6649 g/amp-hr
  • Electron Work Function: 3.9eV
  • Electronegativity: 1.3 (Pauling); 1.23 (Allrod Rochow)
  • Heat of Fusion: 24.06 kJ/mol
  • Incompatibilities:
    Strong oxidizers, chlorine
  • Ionization Potential
    • First: 6.65
    • Second: 14.925
    • Third: 23.32
  • Valence Electron Potential (-eV): 81

Physical Properties

  • Atomic Mass Average: 178.49
  • Boiling Point: 4876K, 4603C, 8317F
  • Coefficient of Lineal Thermal Expansion/K-1: 5.9E-6
  • Conductivity
    Electrical: 0.0312 106/cm
    Thermal: 0.23 W/cmK
  • Density: 13.31 g/cm3 @ 300K
  • Description:
    Silver colored ductile transition metal that resists corrosion.
  • Elastic Modulus:
    • Bulk: 109/GPa
    • Rigidity: 56/GPa
    • Youngs: 141/GPa
  • Enthalpy of Atomization: 703 kJ/mole @ 25C
  • Enthalpy of Fusion: 25.1 kJ/mole
  • Enthalpy of Vaporization: 661.1 kJ/mole
  • Flammablity Class: Explosive in powder form
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 1700 MN m-2
    • Mohs: 5.5
    • Vickers: 1760 MN m-2
  • Heat of Vaporization: 575 kJ/mol
  • Melting Point: 2500K, 2227C, 4041F
  • Molar Volume: 13.41 cm3/mole
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.14 J/gK
  • Vapor Pressure: 0.00112 Pa @ 2227C

Regulatory / Health

  • CAS Number
    • 7440-58-6 powder, wet
  • UN/NA ID and ERG Guide Number
    • 1326  / 170 powder, wet
    • 2545  / 135 powder, dry
  • RTECS: MG4600000
  • OSHA Permissible Exposure Limit (PEL)
    • TWA: 0.5 mg/m3
  • OSHA PEL Vacated 1989
    • TWA: 0.5 mg/m3
  • NIOSH Recommended Exposure Limit (REL)
    • TWA: 0.5 mg/m3
    • IDLH: 50 mg/m3
  • Routes of Exposure: Inhalation; Ingestion; Skin and/or eye contact
  • Target Organs: Eyes, skin, mucous membrane , liver
  • Levels In Humans:
    Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
    • Blood/mg dm-3: n/a
    • Bone/p.p.m: n/a
    • Liver/p.p.m: n/a
    • Muscle/p.p.m: n/a
    • Daily Dietary Intake: n/a
    • Total Mass In Avg. 70kg human: n/a

Who / Where / When / How

  • Discoverer: Dirk Coster, Georg C. von Hevesy
  • Discovery Location: Copenhagen Denmark
  • Discovery Year: 1923
  • Name Origin:
    From Hafnia Latin name of Copenhagen.
  • Abundance:
    • Earth's Crust/p.p.m.: 5.3
    • Seawater/p.p.m.: 0.000007
    • Atmosphere/p.p.m.: N/A
    • Sun (Relative to H=1E12): 6
  • Sources:
    Obtained as a by-product zirconium refining. Around 50 tons are produced world wide annually.
  • Uses:
    Used in reactor control rods because of its ability to absorb neutrons also as a gas scavenger in vacuum tubes.
  • Additional Notes:
    Hafnium is poorly absorbed by the body. As a result poisoning by compounds of hafnium is rare.

Ionization Energy (eV): 6.825 eV
Estimated Crustal Abundance: 3.0 milligrams per kilogram
Estimated Oceanic Abundance:
710-6 milligrams per liter

Transition Metals
Group 3
(IIIB)
4
(IVB)
5
(VB)
6
(VIB)
7
(VIIB)
8
(VIIIB)
9
(VIIIB)
10 (VIIIB) 11
(IB)
12
(IIB)
Period 4 21
Sc
44.95
22
Ti
47.86
23
V
50.94
24
Cr
51.99
25
Mn
54.93
26
Fe
55.84
27
Co
58.93
28
Ni
58.69
29
Cu
63.54
30
Zn
65.39
Period 5 39
Y
88.90
40
Zr
91.22
41
Nb
92.90
42
Mo
95.94
43
Tc
98.00
44
Ru
101.0
45
Rh
102.9
46
Pd
106.4
47
Ag
107.8
48
Cd
112.4
Period 6 57
La
138.9
72
Hf
178.4
73
Ta
180.9
74
W
183.8
75
Re
186.2
76
Os
190.2
77
Ir
192.2
78
Pt
195.0
79
Au
196.9
80
Hg
200.5
Period 7 89
Ac
227.0
104
Rf
261.0
105
Db
262.0
106
Sg
266.0
107
Bh
264.0
108
Hs
269.0
109
Mt
268.0
110
Ds
269.0
111
Rg
272.0
112
Uub
277.0