45
  Rh  
102.905500
Rhodium

Name: Rhodium
Symbol: Rh
Atomic Number: 45
AtomicWeight: 102.905500
Family: Transition Metals
CAS RN: 7440-16-6
Description: Hard silver white metal that is unaffected by air and water up to 875K.
State (25 C): Solid
Oxidation states: +2, +3, +4

Molar Volume: 8.3 cm3/mole
Valence Electrons: 4d85s
Boiling Point:  4000K, 3727C, 6741F
Melting Point:
2239K, 1966C, 3571F
Electrons Energy Level: 2, 8, 18, 16, 1
Isotopes: 33 + 1 Stable
Heat of Vaporization: 493 kJ/mol
Heat of Fusion: 21.5 kJ/mol
Density: 12.41 g/cm3 @ 300K
Specific Heat: 0.242 J/gK
Atomic Radius: 1.83
Ionic Radius: 0.68
Electronegativity: 2.28 (Pauling); 1.45 (Allrod Rochow)
Vapor Pressure: 0.633 Pa @ 1966C

1s2 2s2p6 3s2p6d10 4s2p6d8 5s1

History

Rhodium (Greek rhodon meaning "Rose") was discovered in 1803 by William Hyde Wollaston, soon after his discovery of palladium They made this discovery in England using crude platinum ore that they presumably obtained from South America.

Their procedure involved dissolving the ore in aqua regia, neutralizing the acid with sodium hydroxide, NaOH.  They then precipitated the platinum metal by adding ammonium chloride, NH4Cl, as ammonium chloroplatinate.  The element palladium was removed as palladium cyanide after treating the solution with mercuric cyanide.  The material that remained was a red  sodium rhodium (III) chloride, Na3RhCl612H2O: rhodium metal was isolated via reduction with hydrogen gas, H2.

Characteristics

Rhodium is a hard silvery white and durable metal that has a high reflectance.  Rhodium metal does not normally form an oxide, even when heated.   Oxygen is absorbed from the atmosphere at the melting point of rhodium, but on solidification the oxygen is released.  Rhodium has both a higher melting point and lower density than platinum.  It is not attacked by acids: it is completely insoluble in nitric acid and slightly dissolves in aqua regia.  A complete dissolution of rhodium in powder form is only obtained in sulfuric acid.

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d8
5s1

Occurrence

The industrial extraction of rhodium is complex as the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold.  It is found in platinum ores and obtained free as a white inert metal which it is very difficult to fuse. Principal sources of this element are located in South Africa, in river sands of the Ural Mountains, in North and South America and also in the copper-nickel sulfide mining area of the Sudbury, Ontario, Canada region.  Although the quantity at Sudbury is very small, the large amount of nickel ore processed makes rhodium recovery cost effective.  The main exporter of rhodium is South Africa (>80%) followed by Russia.  However, the annual world production of this element is only about 20 tons and there are very few rhodium minerals.  However, it is generally difficult to determine if a rock sample does or does not contain platinum group elements.  As of 2006, rhodium cost approximately six times as much as gold, by weight.

Applications

The primary use of this element is as an alloying agent for hardening platinum and palladium.  These alloys are used in furnace windings, bushings for glass fiber production, thermocouple elements, electrodes for aircraft spark plugs, and laboratory crucibles. Other uses include:

Symbolic Uses

Rhodium has been used for honors, or to symbolize wealth, when more commonly used metals such as silver, gold, or platinum are deemed insufficient.  In 1979 the Guinness Book of World Records gave Paul McCartney a rhodium-plated disc for being history's all-time best-selling songwriter and recording artist.  Guinness has also noted items such as the world's "Most Expensive Pen" or "Most Expensive Board Game" as containing rhodium.

Isotopes

Naturally occurring rhodium is composed of only one isotope, 103Rh.   The most stable radioisotopes are 101Rh with a half-life of of 3.3 years, 102Rh with a half-life of 207 days, 102mRh with a half-life of 2.9 years, and 99Rh with a half-life of 16.1 days. Thirty-three radioisotopes have been characterized with atomic weights ranging from 88.948 u (93Rh) to 121.94 u (122Rh). Most of these have half-lifes that are less than an hour except 100Rh (half-life: 20.8 hours) and 105Rh (half-life: 35.36 hours). There are also numerous meta states with the most stable being 102mRh (0.141 MeV) with a half-life of about 207 days and 101mRh (0.157 MeV) with a half-life of 4.34 days.

The primary decay mode before the only stable isotope, 103Rh, is electron capture and the primary mode after is beta emission.  The primary decay product before 103Rh is ruthenium and the primary product after is palladium.

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
89Rh 88.94884 ~10 ms
90Rh 89.94287 15 ms
91Rh 90.93655 1.74 seconds
92Rh 91.93198 4.3 seconds
93Rh 92.92574 11.9 seconds
94Rh 93.92170 70.6 seconds
95Rh 94.91590 5.02 minutes
96Rh 95.914461 9.90 minutes
97Rh 96.91134 30.7 minutes
98Rh 97.910708 8.72 minutes
99Rh 98.908132 16.1 days
100Rh 99.908122 20.8 hours
101Rh 100.906164 3.3 years
102Rh 101.906843 207.0 days
103Rh 102.905504 Stable
104Rh 103.906656 42.3 seconds
105Rh 104.905694 35.36 hours
106Rh 105.907287 29.80 seconds
107Rh 106.906748 21.7 minutes
108Rh 107.90873 16.8 seconds
109Rh 108.908737 80 seconds
110Rh 109.91114 28.5 seconds
111Rh 110.91159 11 seconds
112Rh 111.91439 3.45 seconds
113Rh 112.91553 2.80 seconds
114Rh 113.91881 1.85 seconds
115Rh 114.92033 0.99 seconds
116Rh 115.92406 0.68 seconds
117Rh 116.92598 0.44 seconds
118Rh 117.93007 310 ms
119Rh 118.93211 ~300 ms
120Rh 119.93641 ~200 ms
121Rh 120.93872 ~100 ms
122Rh 121.94321 ~50 ms

Precautions

Rhodium metal is, as a noble metal, inert.

40px-Skull_and_crossbones.svg.jpg (1420 bytes) However, when rhodium is chemically bound, it is reactive.  Rhodium compounds are not often encountered by most people and should be considered to be highly toxic and carcinogenic.  Lethal intake (LD50) for rats is 12.6 mg/kg of rhodium chloride (RhCl3).  Rhodium compounds can stain human skin very strongly.   This element plays no biological role in humans.

It is also possible to extract rhodium from spent nuclear fuel, which contains an average of 400 g of rhodium per metric ton. Rhodium produced in such a way contains radioactive isotopes with half-lives of up to 2.9 years and is therefore stored at least 20 years in a secured area to allow it to become stable.  Every 2.9 years of isolation reduces the radioactivity by 50%. Fissioned rhodium has a specific activity of 8.1 curies of radioactivity per gram after 5 years isolation.  Under health physics safety rules, any isotope that emits more than 1 curie of activity is a hazard; however after 8 years the activity falls to 4.1 Ci, after 11 years it is 2.2, after 14 years 1.1 Ci, after 17 years .55 Ci, and after 20 years only .27 Ci.  After 30 years the activity falls to 2.702E-4 Ci, which is under the threshold for low level risk by even the most stringent health physics rules.  The radioactivity of rhodium falls off so quickly because the percentage of Rh-102 in the recovered rhodium is only a trace amount and the remainder of the material absorbs the energy released.  Rhodium averaged $26.43 per gram or $820 per troy ounce over the last 30 years and $6200 per troy ounce in May 2006, adding significantly to the resource value of reprocessed fission waste.

atom.gif (700 bytes)

Rhodium Data
 

Atomic Structure

  • Atomic Radius: 1.83
  • Atomic Volume: 8.3cm3/mol
  • Covalent Radius: 1.25
  • Cross Section (Thermal Neutron Capture) Barns: 144.8
  • Crystal Structure: Cubic face centered
  • Electron Configuration:
    1s2 2s2p6 3s2p6d10 4s2p6d8 5s1
  • Electrons per Energy Level: 2, 8, 18, 16, 1
  • Ionic Radius: 0.68
  • Filling Orbital: 4d8
  • Number of Electrons(with no charge): 45
  • Number of Neutrons (most common/stable nuclide): 58
  • Number of Protons: 45
  • Oxidation States: 2, 3, 4
  • Valence Electrons: 4d8 5s1

Chemical Properties

  • Electrochemical Equivalent: 1.2798 g/amp-hr
  • Electron Work Function: 4.98eV
  • Electronegativity: 2.28 (Pauling); 1.45 (Allrod Rochow)
  • Heat of Fusion: 21.5 kJ/mol
  • Incompatibilities:
    Chlorine trifluoride, oxygen difluoride
  • Ionization Potential
    • First: 7.46
    • Second: 18.08
    • Third: 31.06
  • Valence Electron Potential (-eV): 64

Physical Properties

  • Atomic Mass Average: 102.9055
  • Boiling Point: 4000K, 3727C, 6741F
  • Coefficient of Lineal Thermal Expansion/K-1: 8.4E-6
  • Conductivity
    Electrical: 0.211 106/cm
    Thermal: 1.5 W/cmK
  • Density: 12.41 g/cm3 @ 300K
  • Description:
    Hard silver white metal that is unaffected by air and water up to 875K. It is also unaffected by acids but is attacked by molten alkalis.
  • Elastic Modulus:
    • Bulk: 276/GPa
    • Rigidity: 147/GPa
    • Youngs: 379/GPa
  • Enthalpy of Atomization: 556.5 kJ/mole @ 25C
  • Enthalpy of Fusion: 21.76 kJ/mole
  • Enthalpy of Vaporization: 495.4 kJ/mole
  • Flammablity Class: Non-combustible solid (except as dust)
  • Freezing Point: see melting point
  • Hardness Scale
    • Brinell: 1100 MN m-2
    • Mohs: 6
    • Vickers: 1246 MN m-2
  • Heat of Vaporization: 493 kJ/mol
  • Melting Point: 2239K, 1966C, 3571F
  • Molar Volume: 8.3 cm3/mole
  • Optical Reflectivity: 84%
  • Physical State (at 20C & 1atm): Solid
  • Specific Heat: 0.242 J/gK
  • Vapor Pressure: 0.633 Pa @ 1966C

Regulatory / Health

  • CAS Number
    • 7440-16-6
  • RTECS: VI9069000
  • OSHA Permissible Exposure Limit (PEL)
    • TWA: 0.1 mg/m3
  • OSHA PEL Vacated 1989
    • TWA: 0.1 mg/m3
  • NIOSH Recommended Exposure Limit (REL)
    • TWA: 0.1 mg/m3
    • IDLH: 100 mg/m3
  • Routes of Exposure: Inhalation
  • Target Organs: Respiratory system
  • 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: William H. Wollaston
  • Discovery Location: London England
  • Discovery Year: 1803
  • Name Origin:
    Greek: rhodon (rose). Its salts give a rosy solution.
  • Abundance:
    • Earth's Crust/p.p.m.: 0.0002
    • Seawater/p.p.m.: N/A
    • Atmosphere/p.p.m.: N/A
    • Sun (Relative to H=1E12): 25.1
  • Sources:
    Obtained as a by-product of nickel production. Rhodium is found in its native state in Montana USA. Around three tons of Rhodium are produced world wide on an annual basis.
  • Uses:
    Used as a coating to prevent wear on high quality science equipment and with platinum to make thermocouples. Also used in headlight reflectors, thelephone relays, fountain pen points and airplane spark plugs.

Ionization Energy (eV): 7.459 eV
Estimated Crustal Abundance: 110-3 milligrams per kilogram
Estimated Oceanic Abundance:
unknown

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
Uuu
272.0
112
Uub
277.0