Group 9 in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
group 8  group 10
IUPAC group number 9
Name by element cobalt group
CAS group number
(US, pattern A-B-A)
part of VIIIB
old IUPAC number
(Europe, pattern A-B)
part of VIII

↓ Period
Image: Cobalt, electrolytic made, 99,9%
Cobalt (Co)
27 Transition metal
Image: Rhodium, powder, pressed, remelted 99,99%
Rhodium (Rh)
45 Transition metal
Image: Pieces of pure iridium
Iridium (Ir)
77 Transition metal
7 Meitnerium (Mt)
109 unknown chemical properties


primordial element
synthetic element
Atomic number color:

Group 9, by modern IUPAC numbering,[1] is a group (column) of chemical elements in the d-block of the periodic table. Members of Group 9 include cobalt (Co), rhodium (Rh), iridium (Ir) and meitnerium (Mt).[2][page needed] These elements are among the rarest of the transition metals.[3]

Like other groups, the members of this family show patterns in electron configuration, especially in the outermost shells, resulting in trends in chemical behavior; however, rhodium deviates from the pattern.


"Group 9" is the modern standard designation for this group, adopted by the IUPAC in 1990.[2] In the older group naming systems, this group was combined with group 8 (iron, ruthenium, osmium, and hassium) and group 10 (nickel, palladium, platinum, and darmstadtium) and called group "VIIIB" in the Chemical Abstracts Service (CAS) "U.S. system", or "VIII" in the old IUPAC (pre-1990) "European system" (and in Mendeleev's original table).


Cobalt compounds have been used for centuries to impart a rich blue color to glass, glazes, and ceramics. Cobalt has been detected in Egyptian sculpture, Persian jewelry from the third millennium BC, in the ruins of Pompeii, destroyed in 79 AD, and in China, dating from the Tang dynasty (618–907 AD) and the Ming dynasty (1368–1644 AD).[4]

Swedish chemist Georg Brandt (1694–1768) is credited with discovering cobalt c. 1735, showing it to be a previously unknown element, distinct from bismuth and other traditional metals. Brandt called it a new "semi-metal".[5][6] He showed that compounds of cobalt metal were the source of the blue color in glass, which previously had been attributed to the bismuth found with cobalt. Cobalt became the first metal to be discovered since the pre-historical period. All other known metals (iron, copper, silver, gold, zinc, mercury, tin, lead and bismuth) had no recorded discoverers.


William Hyde Wollaston

Rhodium was discovered in 1803 by William Hyde Wollaston,[7] soon after he discovered palladium.[8][9][10] He used crude platinum ore presumably obtained from South America.[11] His procedure dissolved the ore in aqua regia and neutralized the acid with sodium hydroxide (NaOH). He then precipitated the platinum as ammonium chloroplatinate by adding ammonium chloride (NH
). Most other metals like copper, lead, palladium, and rhodium were precipitated with zinc. Diluted nitric acid dissolved all but palladium and rhodium. Of these, palladium dissolved in aqua regia but rhodium did not,[12] and the rhodium was precipitated by the addition of sodium chloride as Na
. After being washed with ethanol, the rose-red precipitate was reacted with zinc, which displaced the rhodium in the ionic compound and thereby released the rhodium as free metal.[13]


Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue.[14] In 1803, British scientist Smithson Tennant (1761–1815) analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids[15] and obtained a volatile new oxide, which he believed to be of this new metal—which he named ptene, from the Greek word πτηνός ptēnós, "winged".[16][13] Tennant, who had the advantage of a much greater amount of residue, continued his research and identified the two previously undiscovered elements in the black residue, iridium and osmium.[14][15] He obtained dark red crystals (probably of Na
) by a sequence of reactions with sodium hydroxide and hydrochloric acid.[13] He named iridium after Iris (Ἶρις), the Greek winged goddess of the rainbow and the messenger of the Olympian gods, because many of the salts he obtained were strongly colored.[a][17] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.[14][18]


Meitnerium was first synthesized on August 29, 1982, by a German research team led by Peter Armbruster and Gottfried Münzenberg at the Institute for Heavy Ion Research (Gesellschaft für Schwerionenforschung) in Darmstadt.[19] The team bombarded a target of bismuth-209 with accelerated nuclei of iron-58 and detected a single atom of the isotope meitnerium-266:[20]

+ 58

This work was confirmed three years later at the Joint Institute for Nuclear Research at Dubna (then in the Soviet Union).[20]


Z Element No. of electrons
per shell
M.P. B.P. Year of
27 cobalt 2, 8, 15, 2 1768 K
1495 °C
3200 K
2927 °C
~1735 Georg Brandt
45 rhodium 2, 8, 18, 16, 1 2237 K
1964 °C
3968 K
3695 °C
1803 W. H. Wollaston
77 iridium 2, 8, 18, 32, 15, 2 2719 K
2446 °C
4403 K
4130 °C
1803 S. Tennant
109 meitnerium 2, 8, 18, 32, 32, 15, 2[*] 1982 P. Armbruster and
G. Münzenberg

[*] Predicted.

The first three elements are hard silvery-white metals:

All known isotopes of meitnerium are radioactive with short half-lives. Only minute quantities have been synthesized in laboratories. It has not been isolated in pure form, and its physical and chemical properties have not been determined yet.[citation needed] Based on what is known, meitnerium is considered a homologue to iridium.

Biological role

Main article: Cobalt § Biological role

Of the group 9 elements, only cobalt has a biological role. It is a key constituent of cobalamin, also known as vitamin B12, the primary biological reservoir of cobalt as an ultratrace element.[21][22] Bacteria in the stomachs of ruminant animals convert cobalt salts into vitamin B12, a compound which can only be produced by bacteria or archaea. A minimal presence of cobalt in soils therefore markedly improves the health of grazing animals, and an uptake of 0.20 mg/kg a day is recommended, because they have no other source of vitamin B12.[23]

Proteins based on cobalamin use corrin to hold the cobalt. Coenzyme B12 features a reactive C-Co bond that participates in the reactions.[24] In humans, B12 has two types of alkyl ligand: methyl and adenosyl. MeB12 promotes methyl (−CH3) group transfers. The adenosyl version of B12 catalyzes rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine. Methylmalonyl coenzyme A mutase (MUT) converts MMl-CoA to Su-CoA, an important step in the extraction of energy from proteins and fats.[25]

See also


  1. ^ Iridium literally means "of rainbows".


  1. ^ Fluck, E. (1988). "New Notations in the Periodic Table" (PDF). Pure Appl. Chem. 60 (3): 431–436. doi:10.1351/pac198860030431. S2CID 96704008. Retrieved 24 March 2012.
  2. ^ a b Leigh, G. J. Nomenclature of Inorganic Chemistry: Recommendations 1990. Blackwell Science, 1990. ISBN 0-632-02494-1.
  3. ^ "Group 9: Transition Metals". Chemistry LibreTexts. 2020-08-15. Retrieved 2022-03-24.
  4. ^ Cobalt, Encyclopædia Britannica Online.
  5. ^ Georg Brandt first showed cobalt to be a new metal in: G. Brandt (1735) "Dissertatio de semimetallis" (Dissertation on semi-metals), Acta Literaria et Scientiarum Sveciae (Journal of Swedish literature and sciences), vol. 4, pages 1–10.
    See also: (1) G. Brandt (1746) "Rön och anmärkningar angäende en synnerlig färg—cobolt" (Observations and remarks concerning an extraordinary pigment—cobalt), Kongliga Svenska vetenskapsakademiens handlingar (Transactions of the Royal Swedish Academy of Science), vol. 7, pp. 119–130; (2) G. Brandt (1748) "Cobalti nova species examinata et descripta" (Cobalt, a new element examined and described), Acta Regiae Societatis Scientiarum Upsaliensis (Journal of the Royal Scientific Society of Uppsala), 1st series, vol. 3, pp. 33–41; (3) James L. Marshall and Virginia R. Marshall (Spring 2003) "Rediscovery of the Elements: Riddarhyttan, Sweden". The Hexagon (official journal of the Alpha Chi Sigma fraternity of chemists), vol. 94, no. 1, pages 3–8.
  6. ^ Wang, Shijie (2006). "Cobalt—Its recovery, recycling, and application". Journal of the Minerals, Metals and Materials Society. 58 (10): 47–50. Bibcode:2006JOM....58j..47W. doi:10.1007/s11837-006-0201-y. S2CID 137613322.
  7. ^ Wollaston, W. H. (1804). "On a New Metal, Found in Crude Platina". Philosophical Transactions of the Royal Society of London. 94: 419–430. doi:10.1098/rstl.1804.0019.
  8. ^ Griffith, W. P. (2003). "Rhodium and Palladium – Events Surrounding Its Discovery". Platinum Metals Review. 47 (4): 175–183.
  9. ^ Wollaston, W. H. (1805). "On the Discovery of Palladium; With Observations on Other Substances Found with Platina". Philosophical Transactions of the Royal Society of London. 95: 316–330. doi:10.1098/rstl.1805.0024.
  10. ^ Usselman, Melvyn (1978). "The Wollaston/Chenevix controversy over the elemental nature of palladium: A curious episode in the history of chemistry". Annals of Science. 35 (6): 551–579. doi:10.1080/00033797800200431.
  11. ^ Lide, David R. (2004). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton: CRC Press. pp. 4–26. ISBN 978-0-8493-0485-9.
  12. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 1113. ISBN 978-0-08-037941-8.
  13. ^ a b c Griffith, W. P. (2003). "Bicentenary of Four Platinum Group Metals: Osmium and iridium – events surrounding their discoveries". Platinum Metals Review. 47 (4): 175–183.
  14. ^ a b c Hunt, L. B. (1987). "A History of Iridium". Platinum Metals Review. 31 (1): 32–41.
  15. ^ a b Emsley, J. (2003). "Iridium". Nature's Building Blocks: An A–Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 201–204. ISBN 978-0-19-850340-8.
  16. ^ Thomson, T. (1831). A System of Chemistry of Inorganic Bodies. Vol. 1. Baldwin & Cradock, London; and William Blackwood, Edinburgh. p. 693.
  17. ^ Weeks, M. E. (1968). Discovery of the Elements (7th ed.). Journal of Chemical Education. pp. 414–418. ISBN 978-0-8486-8579-9. OCLC 23991202.
  18. ^ Tennant, S. (1804). "On Two Metals, Found in the Black Powder Remaining after the Solution of Platina". Philosophical Transactions of the Royal Society of London. 94: 411–418. doi:10.1098/rstl.1804.0018. JSTOR 107152.
  19. ^ Münzenberg, G.; Armbruster, P.; Heßberger, F. P.; Hofmann, S.; Poppensieker, K.; Reisdorf, W.; Schneider, J. H. R.; Schneider, W. F. W.; Schmidt, K.-H.; Sahm, C.-C.; Vermeulen, D. (1982). "Observation of one correlated α-decay in the reaction 58Fe on 209Bi→267109". Zeitschrift für Physik A. 309 (1): 89. Bibcode:1982ZPhyA.309...89M. doi:10.1007/BF01420157. S2CID 120062541.
  20. ^ a b Barber, R. C.; Greenwood, N. N.; Hrynkiewicz, A. Z.; Jeannin, Y. P.; Lefort, M.; Sakai, M.; Ulehla, I.; Wapstra, A. P.; Wilkinson, D. H. (1993). "Discovery of the transfermium elements. Part II: Introduction to discovery profiles. Part III: Discovery profiles of the transfermium elements". Pure and Applied Chemistry. 65 (8): 1757. doi:10.1351/pac199365081757. S2CID 195819585. (Note: for Part I see Pure Appl. Chem., Vol. 63, No. 6, pp. 879–886, 1991)
  21. ^ Yamada, Kazuhiro (2013). "Chapter 9. Cobalt: Its Role in Health and Disease". In Astrid Sigel; Helmut Sigel; Roland K. O. Sigel (eds.). Interrelations between Essential Metal Ions and Human Diseases. Metal Ions in Life Sciences. Vol. 13. Springer. pp. 295–320. doi:10.1007/978-94-007-7500-8_9. PMID 24470095.
  22. ^ Cracan, Valentin; Banerjee, Ruma (2013). "Chapter 10 Cobalt and Corrinoid Transport and Biochemistry". In Banci, Lucia (ed.). Metallomics and the Cell. Metal Ions in Life Sciences. Vol. 12. Springer. pp. 333–374. doi:10.1007/978-94-007-5561-1_10. ISBN 978-94-007-5560-4. PMID 23595677. electronic-book ISBN 978-94-007-5561-1 ISSN 1559-0836 electronic-ISSN 1868-0402.
  23. ^ Schwarz, F. J.; Kirchgessner, M.; Stangl, G. I. (2000). "Cobalt requirement of beef cattle – feed intake and growth at different levels of cobalt supply". Journal of Animal Physiology and Animal Nutrition. 83 (3): 121–131. doi:10.1046/j.1439-0396.2000.00258.x.
  24. ^ Voet, Judith G.; Voet, Donald (1995). Biochemistry. New York: J. Wiley & Sons. p. 675. ISBN 0-471-58651-X. OCLC 31819701.
  25. ^ Smith, David M.; Golding, Bernard T.; Radom, Leo (1999). "Understanding the Mechanism of B12-Dependent Methylmalonyl-CoA Mutase: Partial Proton Transfer in Action". Journal of the American Chemical Society. 121 (40): 9388–9399. doi:10.1021/ja991649a.