View source for Group 9 element

{{Short description|Group of chemical elements}}
{{Infobox periodic table group
| title       = Group&nbsp;9 {{nowrap|in the periodic table}}
| group number= 9
| trivial name= 
| by element  = cobalt group
| CAS         = part of VIIIB
| old IUPAC   = part of VIII
| mark        = Co,Rh,Ir,Mt
| left        = [[Group 8 element|group 8]]
| right       = [[Group 10 element|group 10]]
}}
{| class="floatright"
! colspan=2 style="text-align:left;" | ↓&nbsp;<small>[[Period (periodic table)|Period]]</small>
|-
! [[Period 4 element|4]]
| {{element cell image|27|Cobalt|Co| |Solid|Transition metal|Primordial|image=Kobalt electrolytic and 1cm3 cube.jpg|image caption=Cobalt, electrolytic made, 99,9%}}
|-
! [[Period 5 element|5]]
| {{element cell image|45|Rhodium|Rh| |Solid|Transition metal|Primordial|image=Rhodium powder pressed melted.jpg|image caption=Rhodium, powder, pressed, remelted 99,99%}}
|-
! [[Period 6 element|6]]
| {{element cell image|77|Iridium|Ir| |Solid|Transition metal|Primordial|image=Iridium-2.jpg|image caption=Pieces of pure iridium}}
|-
! [[Period 7 element|7]]
| {{element cell image|109|Meitnerium|Mt| |Unknown phase|Unknown chemical properties|Synthetic|legend=unknown chemical properties}}
|-
| colspan="2"|
----
''Legend''
{| style="margin-left: auto; margin-right: auto; text-align:center; border:0;"
|-
| style="border:{{element color|Primordial}}; background:{{Element color|table mark}}; padding:0 2px;" | [[primordial element]]
|-
| style="border:{{element color|Synthetic}}; background:{{Element color|table mark}}; padding:0 2px;" | [[synthetic element]]
|}
|}

'''Group 9''', by modern [[Group (periodic table)#CAS and old IUPAC numbering (A/B)|IUPAC]] numbering,<ref>{{cite journal |last1=Fluck |first1=E. |year=1988 |title=New Notations in the Periodic Table |url=http://www.iupac.org/publications/pac/1988/pdf/6003x0431.pdf |journal=[[Pure and Applied Chemistry|Pure Appl. Chem.]] |volume=60 |issue=3 |pages=431–436 |doi=10.1351/pac198860030431 |access-date=24 March 2012 |s2cid=96704008}}</ref> is a group (column) of [[chemical element]]s in the [[d-block]] of the [[periodic table]]. Members of Group 9 include [[cobalt]] (Co), [[rhodium]] (Rh), [[iridium]] (Ir) and [[meitnerium]] (Mt).<ref name=leigh>Leigh, G. J. ''Nomenclature of Inorganic Chemistry: Recommendations 1990''. Blackwell Science, '''1990'''. p. 283. {{ISBN|0-632-02494-1}}.</ref> These elements are among the rarest of the [[transition metal]]s,<ref>{{Cite web |date=2020-08-15 |title=Group 9: Transition Metals |url=https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/3_d-Block_Elements/Group_09%3A_Transition_Metals |access-date=2022-03-24 |website=Chemistry LibreTexts |language=en}}</ref> and as of 2025 rhodium and iridium are the only non-radioactive metals with higher prices per weight than gold.<ref>{{Cite web |date=2025-03-20 |title=Spot metal prices in USD |url=https://www.dailymetalprice.com/metalpricescurr.php?x=USD |access-date=2025-03-20 |website=Daily Metal Prices |language=en}}</ref>

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.

==History==
"Group 9" is the modern standard designation for this group, adopted by the [[IUPAC]] in 1990.<ref name=leigh/> In the [[Group (periodic table)|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===
Cobalt compounds have been used for centuries to impart a rich blue color to [[glass]], [[ceramic glaze|glazes]], and [[Ceramics (art)|ceramics]]. Cobalt has been detected in Egyptian sculpture, Persian jewelry from the third millennium BC, in the ruins of [[Pompeii]], destroyed in 79&nbsp;AD, and in China, dating from the [[Tang dynasty]] (618–907&nbsp;AD) and the [[Ming dynasty]] (1368–1644&nbsp;AD).<ref>[https://www.britannica.com/EBchecked/topic/123235/cobalt-Co Cobalt], Encyclopædia Britannica Online.</ref>

Swedish chemist [[Georg Brandt]] (1694–1768) is credited with discovering cobalt <abbr>c.</abbr> 1735, showing it to be a previously unknown element, distinct from bismuth and other traditional metals. Brandt called it a new "semi-metal".<ref>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.<br />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) [https://web.archive.org/web/20100703175508/http://www.chem.unt.edu/Rediscovery/Riddarhyttan.pdf "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.</ref><ref name="Wang">{{cite journal|journal =Journal of the Minerals, Metals and Materials Society|volume = 58|issue = 10|date = 2006|doi = 10.1007/s11837-006-0201-y|pages = 47–50|title = Cobalt—Its recovery, recycling, and application|first = Shijie|last = Wang|bibcode = 2006JOM....58j..47W|s2cid = 137613322}}</ref> 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.

===Rhodium===
[[File:Wollaston William Hyde Jackson color.jpg|thumb|upright|left|[[William Hyde Wollaston]]]]

Rhodium was discovered in 1803 by [[William Hyde Wollaston]],<ref>{{cite journal |title= On a New Metal, Found in Crude Platina |first=W. H. |last=Wollaston|author-link=William Hyde Wollaston |journal=[[Philosophical Transactions of the Royal Society of London]] |volume=94 |year=1804 |pages=419–430 |doi=10.1098/rstl.1804.0019| url = https://books.google.com/books?id=7AZGAAAAMAAJ&pg=PA419|doi-access=free }}</ref> soon after he discovered [[palladium]].<ref>{{cite journal|journal = Platinum Metals Review|url = http://www.platinummetalsreview.com/dynamic/article/view/47-4-175-183|title = Rhodium and Palladium – Events Surrounding Its Discovery|first = W. P. |last = Griffith|volume = 47|issue = 4|year = 2003|pages = 175–183| doi=10.1595/003214003X474175183 |doi-access = free}}</ref><ref>{{cite journal|title = On the Discovery of Palladium; With Observations on Other Substances Found with Platina|first = W. H.|last = Wollaston|author-link = William Hyde Wollaston|journal = [[Philosophical Transactions of the Royal Society of London]]|volume = 95|year = 1805|pages = 316–330|doi = 10.1098/rstl.1805.0024|doi-access = free}}</ref><ref name="contr">{{cite journal | doi = 10.1080/00033797800200431 | title = The Wollaston/Chenevix controversy over the elemental nature of palladium: A curious episode in the history of chemistry |year = 1978 | last1 = Usselman | first1 = Melvyn | journal = Annals of Science | volume = 35 | issue = 6 | pages = 551–579}}</ref> He used crude [[platinum]] ore presumably obtained from [[South America]].<ref>{{Cite book|author=Lide, David R.|title=CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data|year=2004|publisher=CRC Press|location=Boca Raton|isbn=978-0-8493-0485-9|pages=[https://archive.org/details/crchandbookofche81lide/page/4 4–26]|url-access=registration|url=https://archive.org/details/crchandbookofche81lide/page/4}}</ref> 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]] ({{chem|NH|4|Cl}}). 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,<ref>{{Greenwood&Earnshaw2nd|page=1113}}</ref> and the rhodium was precipitated by the addition of [[sodium chloride]] as {{chem| Na|3|[RhCl|6|]·''n''H|2|O}}. After being washed with ethanol, the rose-red precipitate was reacted with zinc, which [[Displacement reaction|displaced]] the rhodium in the ionic compound and thereby released the rhodium as free metal.<ref name="griffith">{{cite journal |title=Bicentenary of Four Platinum Group Metals: Osmium and iridium – events surrounding their discoveries |author=Griffith, W. P. |journal=Platinum Metals Review |volume=47 |issue=4 |year=2003 |pages=175–183|doi=10.1595/003214003X474175183 |doi-access=free }}</ref>

===Iridium===
Chemists who studied platinum dissolved it in [[aqua regia]] (a mixture of [[hydrochloric acid|hydrochloric]] and [[nitric acid]]s) to create soluble salts. They always observed a small amount of a dark, insoluble residue.<ref name="hunt">{{cite journal |title=A History of Iridium |first=L. B. |last=Hunt |journal=Platinum Metals Review |volume=31 |issue=1 |date=1987 |pages=32–41 |doi=10.1595/003214087X3113241 |url=https://technology.matthey.com/documents/496120/626258/pmr-v31-i1-032-041.pdf/ |access-date=2023-10-12 |archive-date=2022-09-29 |archive-url=https://web.archive.org/web/20220929092320/https://technology.matthey.com/documents/496120/626258/pmr-v31-i1-032-041.pdf/ |url-status=dead |url-access=subscription }}</ref> 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<ref name="Emsley">{{cite book| title=Nature's Building Blocks: An A–Z Guide to the Elements| last=Emsley| first=J.| publisher=[[Oxford University Press]]| date=2003| location=Oxford, England, UK| isbn=978-0-19-850340-8| chapter=Iridium| pages=[https://archive.org/details/naturesbuildingb0000emsl/page/201 201–204]| chapter-url=https://archive.org/details/naturesbuildingb0000emsl/page/201}}</ref> and obtained a volatile new oxide, which he believed to be of this new metal—which he named ''ptene'', from the Greek word {{lang|el|πτηνός}} ''ptēnós'', "winged".<ref>{{cite book |title=A System of Chemistry of Inorganic Bodies |url=https://archive.org/details/asystemchemistr07thomgoog |author=Thomson, T. |author-link=Thomas Thomson (chemist) |publisher=Baldwin & Cradock, London; and William Blackwood, Edinburgh |date=1831 |volume=1 |page=[https://archive.org/details/in.ernet.dli.2015.32266/page/n721/mode/2up 693]}}</ref><ref name="griffith" /> 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.<ref name="hunt" /><ref name="Emsley" /> He obtained dark red crystals (probably of {{chem|Na|2|[IrCl|6}}]·''n''{{chem|H|2|O}}) by a sequence of reactions with [[sodium hydroxide]] and [[hydrochloric acid]].<ref name="griffith" /> He named iridium after [[Iris (mythology)|Iris]] ({{lang|el|Ἶρις}}), the Greek winged goddess of the rainbow and the messenger of the Olympian gods, because many of the [[Salt (chemistry)|salts]] he obtained were strongly colored.{{efn|''Iridium'' literally means "of rainbows".}}<ref>{{cite book |title=Discovery of the Elements |url=https://archive.org/details/discoveryofeleme0000week |url-access=registration |pages=[https://archive.org/details/discoveryofeleme0000week/page/414 414–418] |author=Weeks, M. E. |date=1968 |edition=7th |publisher=Journal of Chemical Education |isbn=978-0-8486-8579-9 |oclc=23991202}}</ref> Discovery of the new elements was documented in a letter to the [[Royal Society]] on June 21, 1804.<ref name="hunt"/><ref>{{cite journal |title=On Two Metals, Found in the Black Powder Remaining after the Solution of Platina |first=S. |last=Tennant |journal=Philosophical Transactions of the Royal Society of London |volume=94 |date=1804 |pages=411–418 |jstor=107152 |doi=10.1098/rstl.1804.0018 |url=https://zenodo.org/record/1432312 |doi-access=}}</ref>

===Meitnerium===
Meitnerium was [[discovery of the chemical elements|first synthesized]] on August 29, 1982, by a German research team led by [[Peter Armbruster]] and [[Gottfried Münzenberg]] at the [[Gesellschaft für Schwerionenforschung|Institute for Heavy Ion Research]] (Gesellschaft für Schwerionenforschung) in [[Darmstadt]].<ref name="82Mu01">{{cite journal|title=Observation of one correlated α-decay in the reaction <sup>58</sup>Fe on <sup>209</sup>Bi→<sup>267</sup>109|doi=10.1007/BF01420157|year=1982|journal=Zeitschrift für Physik A|volume=309|issue=1|pages=89|last1=Münzenberg|first1=G.|last2=Armbruster|first2=P.|last3=Heßberger|first3=F. P.|last4=Hofmann|first4=S.|last5=Poppensieker|first5=K.|last6=Reisdorf|first6=W.|last7=Schneider|first7=J. H. R.|last8=Schneider|first8=W. F. W.|last9=Schmidt|first9=K.-H.|first10=C.-C.|last10=Sahm|first11=D.|last11=Vermeulen|bibcode = 1982ZPhyA.309...89M|s2cid=120062541}}</ref> The team bombarded a target of [[bismuth-209]] with accelerated nuclei of [[iron]]-58 and detected a single atom of the [[isotope]] meitnerium-266:<ref name="93TWG">{{Cite journal|doi=10.1351/pac199365081757|title=Discovery of the transfermium elements. Part II: Introduction to discovery profiles. Part III: Discovery profiles of the transfermium elements|year=1993|author=Barber, R. C.|journal=Pure and Applied Chemistry|volume=65|pages=1757|last2=Greenwood|first2=N. N.|last3=Hrynkiewicz|first3=A. Z.|last4=Jeannin|first4=Y. P.|last5=Lefort|first5=M.|last6=Sakai|first6=M.|last7=Ulehla|first7=I.|last8=Wapstra|first8=A. P.|last9=Wilkinson|first9=D. H.
|issue=8|s2cid=195819585|doi-access=free}} (Note: for Part I see Pure Appl. Chem., Vol. 63, No. 6, pp. 879–886, 1991)</ref>

:{{nuclide|bismuth|209}} + {{nuclide|iron|58}} → {{nuclide|meitnerium|266}} + {{SubatomicParticle|neutron}}

This work was confirmed three years later at the [[Joint Institute for Nuclear Research]] at [[Dubna]] (then in the [[Soviet Union]]).<ref name="93TWG" />

==Properties==
{| class="wikitable"
|-
![[Atomic number|Z]] !! [[Chemical element|Element]] !! Electrons<br/>per [[Electron shell|shell]] !! M.P. || B.P. || Year of<br/>Discovery !! Discoverer
|-
| 27 || [[cobalt]] || 2, 8, 15, 2 || 1768 K<br/>1495&nbsp;°C || 3200 K<br/>2927&nbsp;°C || ~1735 || [[Georg Brandt]]
|-
| 45 || [[rhodium]] || 2, 8, 18, 16, 1 || 2237 K<br/>1964&nbsp;°C || 3968 K<br/>3695&nbsp;°C || 1803 || [[William Hyde Wollaston|W. H. Wollaston]]
|-
| 77 || [[iridium]] || 2, 8, 18, 32, 15, 2 || 2719 K<br/>2446&nbsp;°C || 4403 K<br/>4130&nbsp;°C || 1803 || [[Smithson Tennant|S. Tennant]]
|-
| 109 || [[meitnerium]] || 2, 8, 18, 32, 32, 15, 2<sup>[*]</sup> || — || — || 1982 || [[Peter Armbruster|P. Armbruster]] and<br/>[[Gottfried Münzenberg|G. Münzenberg]]
|}
[*] Predicted.

The first three elements are hard silvery-white metals:

* Cobalt is a metallic element that can be used to turn glass a deep blue color. Cobalt is primarily used in [[lithium-ion batteries]], and in the manufacture of [[magnetic]], wear-resistant and high-strength [[alloy]]s. The compounds cobalt silicate and [[Cobalt blue|cobalt(II) aluminate]] (CoAl<sub>2</sub>O<sub>4</sub>, cobalt blue) give a distinctive deep blue color to [[glass]], [[ceramic]]s, [[ink]]s, [[paint]]s and [[varnish]]es. Cobalt occurs naturally as only one stable [[isotope]], cobalt-59. [[Cobalt-60]] is a commercially important radioisotope, used as a [[radioactive tracer]] and for the production of high-energy [[gamma ray]]s. Cobalt is also used in the petroleum industry as a catalyst when refining crude oil. This is to clean it of its sulfur content, which is very polluting when burned and causes acid rain.
* Rhodium can be used in jewelry as a shiny metal. Rhodium is a hard, silvery, durable metal that has a high [[reflectance]]. Rhodium metal does not normally form an [[oxide]], even when heated. [[Oxygen]] is absorbed from the [[atmosphere]] only at the [[melting point]] of rhodium but is released on solidification. Rhodium has both a higher melting point and lower [[density]] than [[platinum]]. It is not attacked by most [[acid]]s as it is completely insoluble in [[nitric acid]] and dissolves slightly in [[aqua regia]].
* Iridium is mainly used as a hardening agent for platinum alloys. Iridium is the most [[corrosion-resistant]] metal known as it is not attacked by [[acid]]s, including [[aqua regia]]. In the presence of oxygen, it reacts with [[cyanide]] salts. Traditional oxidants also react, including the [[halogen]]s and oxygen at higher temperatures. Iridium also reacts directly with [[sulfur]] at atmospheric pressure to yield [[iridium disulfide]].

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|date=March 2022}} Based on what is known, meitnerium is considered a homologue to iridium.

==Biological role==
{{Main|Cobalt#Biological role}}
Of the group 9 elements, only cobalt has a biological role. It is a key constituent of [[Vitamin B12|cobalamin]], also known as vitamin B{{sub|12}}, the primary biological reservoir of cobalt as an [[ultratrace element]].<ref>{{cite book|first1=Kazuhiro|last1=Yamada|editor=Astrid Sigel |editor2=Helmut Sigel |editor3=Roland K. O. Sigel|title=Interrelations between Essential Metal Ions and Human Diseases|series=Metal Ions in Life Sciences|volume=13|date=2013|publisher=Springer|pages=295–320|chapter=Chapter 9. Cobalt: Its Role in Health and Disease|doi=10.1007/978-94-007-7500-8_9|pmid=24470095
|isbn=978-94-007-7499-5 }}</ref><ref>{{cite book|last1=Cracan|first1=Valentin |last2=Banerjee|first2=Ruma |editor1-first=Lucia |editor1-last=Banci |series=Metal Ions in Life Sciences |volume=12|chapter= Chapter 10 Cobalt and Corrinoid Transport and Biochemistry|title=Metallomics and the Cell |date=2013 |pages=333–374 |publisher=Springer |isbn=978-94-007-5560-4|doi=10.1007/978-94-007-5561-1_10|pmid=23595677 }} electronic-book {{ISBN|978-94-007-5561-1}} {{issn|1559-0836}} electronic-{{issn|1868-0402}}.
</ref> [[Bacteria]] in the stomachs of [[ruminant]] animals convert cobalt salts into vitamin B{{sub|12}}, 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&nbsp;mg/kg a day is recommended, because they have no other source of vitamin B{{sub|12}}.<ref>{{cite journal |last1 = Schwarz |first1 = F. J. |last2 = Kirchgessner |first2 = M. |last3 = Stangl |first3 = G. I. |title = Cobalt requirement of beef cattle – feed intake and growth at different levels of cobalt supply |journal = Journal of Animal Physiology and Animal Nutrition |volume = 83 |pages = 121–131 |date = 2000 |doi = 10.1046/j.1439-0396.2000.00258.x |issue = 3}}</ref>

Proteins based on cobalamin use [[corrin]] to hold the cobalt. Coenzyme B<sub>12</sub> features a reactive C-Co bond that participates in the reactions.<ref>{{cite book |author=Voet, Judith G. |author2=Voet, Donald |title=Biochemistry |publisher=J. Wiley & Sons |location=New York |date=1995 |page=[https://archive.org/details/biochemistry00voet_0/page/675 675] |isbn=0-471-58651-X |oclc=31819701 |url-access=registration |url=https://archive.org/details/biochemistry00voet_0/page/675 }}</ref> In humans, B<sub>12</sub> has two types of [[Alkane|alkyl]] [[ligand]]: [[Methyl group|methyl]] and adenosyl. [[Methylcobalamin|MeB<sub>12</sub>]] promotes methyl (−CH<sub>3</sub>) group transfers. The adenosyl version of B<sub>12</sub> 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 [[L-methylmalonyl-CoA|MMl-CoA]] to [[succinyl-CoA|Su-CoA]], an important step in the extraction of energy from proteins and fats.<ref>{{cite journal |last1 = Smith |first1 = David M. |last2 = Golding |first2 = Bernard T. |last3 = Radom |first3 = Leo |title = Understanding the Mechanism of B12-Dependent Methylmalonyl-CoA Mutase: Partial Proton Transfer in Action |journal = Journal of the American Chemical Society |volume = 121 |pages = 9388–9399 |date = 1999 |doi = 10.1021/ja991649a |issue = 40}}</ref>

==See also==
*[[Iron group]]
*[[Platinum group]]

==Notes==
{{reflist|group=lower-alpha}}

==References==
{{reflist}}

{{Periodic table (navbox)}}
{{Navbox periodic table}}
{{Group 9 elements}}

{{DEFAULTSORT:Group 09}}
[[Category:Groups (periodic table)]]