View source for Group 4 element

{{Short description|Group of chemical elements}}
{{Infobox periodic table group
| title       = Group&nbsp;4 {{nowrap|in the periodic table}}
| group number= 4
| trivial name=
| by element  = titanium group
| CAS         = IVB
| old IUPAC   = IVA
| mark        = Ti,Zr,Hf,Rf
| left        = [[Group 3 element|group 3]]
| right       = [[Group 5 element|group 5]]}}
{| class="floatright"
! colspan=2 style="text-align:left;" | ↓&nbsp;<small>[[Period (periodic table)|Period]]</small>
|-
! [[Period 4 element|4]]
| {{element cell image|22|Titanium|Ti| |Solid|Transition metal|Primordial|image=Titan-crystal bar.JPG|image caption=Titanium crystal bar}}
|-
! [[Period 5 element|5]]
| {{element cell image|40|Zirconium|Zr| |Solid|Transition metal|Primordial|image=Zirconium crystal bar and 1cm3 cube.jpg|image caption=Zirconium crystal bar}}
|-
! [[Period 6 element|6]]
| {{element cell image|72|Hafnium|Hf| |Solid|Transition metal|Primordial|image=Hf-crystal bar.jpg|image caption=Hafnium crystal bar}}
|-
! [[Period 7 element|7]]
| {{element cell image|104|Rutherfordium|Rf| |Unknown phase|Transition metal|Synthetic}}
|-
| colspan="2"|{{hr}}''Legend''<div class="center">
<div style="border:{{element color|Primordial}}; background:{{Element color|table mark}}; padding:0 2px;">[[primordial element]]</div>
<div style="border:{{element color|Synthetic}}; background:{{Element color|table mark}}; padding:0 2px;">[[synthetic element]]</div>
</div>
|}

'''Group 4''' is the second group of [[transition metal]]s in the periodic table. It contains only the four elements [[titanium]] (Ti), [[zirconium]] (Zr), [[hafnium]] (Hf), and [[rutherfordium]] (Rf). The group is also called the '''titanium group''' or '''titanium family''' after its lightest member.

As is typical for early transition metals, zirconium and hafnium have only the group [[oxidation state]] of +4 as a major one, and are quite electropositive and have a less rich coordination chemistry. Due to the effects of the [[lanthanide contraction]], they are very similar in properties. Titanium is somewhat distinct due to its smaller size: it has a well-defined +3 state as well (although +4 is more stable).

All the group 4 elements are hard. Their inherent reactivity is completely masked due to the formation of a dense oxide layer that protects them from corrosion, as well as attack by many acids and alkalis. The first three of them occur naturally. Rutherfordium is strongly [[radioactive]]: it does not occur naturally and must be produced by artificial synthesis, but its observed and theoretically predicted properties are consistent with it being a heavier homologue of hafnium. None of them have any biological role.

==History==
[[Zircon]] was known as a gemstone from ancient times,<ref name="CRC2008">{{cite book|contribution=Zirconium|date=2007–2008|title=CRC Handbook of Chemistry and Physics|editor-last=Lide|editor-first=David R.|volume=4|page=42|place=New York|publisher=CRC Press|isbn=978-0-8493-0488-0}}</ref> but it was not known to contain a new element until the work of German chemist [[Martin Heinrich Klaproth]] in 1789. He analysed the zircon-containing mineral [[jargoon]] and found a new earth (oxide), but was unable to isolate the element from its oxide. Cornish chemist [[Humphry Davy]] also attempted to isolate this new element in 1808 through [[electrolysis]], but failed: he gave it the name zirconium.<ref name="nbb">{{harvnb|Emsley|2001|pp=506–510}}</ref> In 1824, Swedish chemist [[Jöns Jakob Berzelius]] isolated an impure form of zirconium, obtained by heating a mixture of potassium and potassium zirconium fluoride in an iron tube.<ref name="CRC2008"/>

Cornish mineralogist [[William Gregor]] first identified titanium in ilmenite sand beside a stream in [[Cornwall]], Great Britain in the year 1791.<ref name="Emsley2001p452">{{harvnb|Emsley|2001|p=452}}</ref> After analyzing the sand, he determined the weakly magnetic sand to contain [[iron oxide]] and a metal oxide that he could not identify.<ref name="Barksdale1968p732">{{harvnb|Barksdale|1968|p=732}}</ref> During that same year, mineralogist [[Franz Joseph Muller]] produced the same metal oxide and could not identify it. In 1795, chemist [[Martin Heinrich Klaproth]] independently rediscovered the metal oxide in [[rutile]] from the Hungarian village Boinik.<ref name="Emsley2001p452" /> He identified the oxide containing a new element and named it for the [[titan (mythology)|Titans]] of [[Greek mythology]].<ref name="weeksIII">{{cite journal|last = Weeks|first = Mary Elvira|author-link=Mary Elvira Weeks|year = 1932|title = III. Some Eighteenth-Century Metals|journal = Journal of Chemical Education|pages = 1231–1243|doi = 10.1021/ed009p1231|volume = 9|issue = 7|bibcode = 1932JChEd...9.1231W }}</ref> Berzelius was also the first to prepare titanium metal (albeit impurely), doing so in 1825.<ref name=Greenwood954>Greenwood and Earnshaw, p. 954</ref>

The [[X-ray spectroscopy]] done by [[Henry Moseley]] in 1914 showed a direct dependency between [[spectral line]] and [[effective nuclear charge]]. This led to the nuclear charge, or [[atomic number]] of an element, being used to ascertain its place within the periodic table. With this method, Moseley determined the number of [[lanthanides]] and showed that there was a missing element with atomic number 72.<ref>{{cite journal|last = Heilbron|title=The Work of H. G. J. Moseley|first = John L.|volume=57|page=336|date=1966|journal=Isis|doi=10.1086/350143|issue = 3|s2cid=144765815}}</ref> This spurred chemists to look for it.<ref>{{cite journal|last = Heimann|first = P. M.|date = 1967|title = Moseley and celtium: The search for a missing element|journal = [[Annals of Science]]|volume = 23|pages=249–260|doi = 10.1080/00033796700203306|issue = 4}}</ref> [[Georges Urbain]] asserted that he found element 72 in the [[rare earth element]]s in 1907 and published his results on ''celtium'' in 1911.<ref>{{cite journal|last = Urbain|first = M. G.|title = Sur un nouvel élément qui accompagne le lutécium et le scandium dans les terres de la gadolinite: le celtium (On a new element that accompanies lutetium and scandium in gadolinite: celtium)|journal = Comptes Rendus|page=141|url = http://gallica.bnf.fr/ark:/12148/bpt6k3105c/f141.table|access-date=2008-09-10|date = 1911|language = fr}}</ref> Neither the spectra nor the chemical behavior he claimed matched with the element found later, and therefore his claim was turned down after a long-standing controversy.<ref name="Mel">{{cite journal|journal = Centaurus|volume = 26|pages =317–322|title = Some Details in the Prehistory of the Discovery of Element 72|first = V. P.|last = Mel'nikov|doi = 10.1111/j.1600-0498.1982.tb00667.x|date = 1982|bibcode = 1982Cent...26..317M|issue = 3 }}</ref>

By early 1923, several physicists and chemists such as [[Niels Bohr]]<ref>{{cite book|title = The Theory of Spectra and Atomic Constitution: Three Essays|url = https://archive.org/details/TheTheoryOfSpectraAndAtomicConstitution|first = Niels|last = Bohr| date=June 2008 |page=[https://archive.org/details/TheTheoryOfSpectraAndAtomicConstitution/page/n123 114]| publisher=Kessinger |isbn = 978-1-4365-0368-6}}</ref> and [[Charles Rugeley Bury]]<ref>{{cite journal|journal = J. Am. Chem. Soc.|title = Langmuir's Theory of the Arrangement of Electrons in Atoms and Molecules|first = Charles R.|last = Bury|volume = 43|date = 1921|pages=1602–1609|doi = 10.1021/ja01440a023|issue = 7|url = https://zenodo.org/record/1428812}}</ref> suggested that element 72 should resemble zirconium and therefore was not part of the rare earth elements group. These suggestions were based on Bohr's theories of the atom, the X-ray spectroscopy of Moseley, and the chemical arguments of [[Friedrich Paneth]].<ref>{{cite book|first = F. A.|last = Paneth|chapter = Das periodische System (The periodic system)|title = Ergebnisse der Exakten Naturwissenschaften 1|date =1922|page=362|language = de}}</ref><ref>{{cite journal|title = Hafnium|url = http://www.jce.divched.org/Journal/Issues/1982/Mar/jceSubscriber/JCE1982p0242.pdf|journal = Journal of Chemical Education|last = Fernelius|first = W. C.|date = 1982|page = 242|doi = 10.1021/ed059p242|volume = 59|issue = 3|bibcode = 1982JChEd..59..242F|access-date = 2021-02-03|archive-date = 2020-03-15|archive-url = https://web.archive.org/web/20200315031648/http://www.jce.divched.org/Journal/Issues/1982/Mar/jceSubscriber/JCE1982p0242.pdf|url-status = dead}}</ref> Encouraged by this, and by the reappearance in 1922 of Urbain's claims that element 72 was a rare earth element discovered in 1911, [[Dirk Coster]] and [[Georg von Hevesy]] were motivated to search for the new element in zirconium ores.<ref>{{cite journal|volume = 174|date = 1922|last = Urbain|first = M. G.|title = Sur les séries L du lutécium et de l'ytterbium et sur l'identification d'un celtium avec l'élément de nombre atomique 72 |trans-title=The L series from lutetium to ytterbium and the identification of element 72 celtium|journal = Comptes Rendus|page=1347|url = http://gallica.bnf.fr/ark:/12148/bpt6k3127j/f1348.table|access-date=2008-10-30|language = fr}}</ref> [[Hafnium]] was discovered by the two in 1923 in Copenhagen, Denmark.<ref>{{cite journal|journal = Nature|volume = 111|page=79|date=1923|doi = 10.1038/111079a0|title = On the Missing Element of Atomic Number 72|first = D.|last = Coster|author2=Hevesy, G.|issue=2777|bibcode=1923Natur.111...79C|doi-access = free}}</ref><!--follow up publications of Urbain's claim that celtium and hafnium are identical {{doi|10.1038/111218a0}}{{doi|10.1038/111252a0}}--><ref>{{cite journal|title = The Discovery and Properties of Hafnium|first = G.|last = Hevesy|journal = Chemical Reviews|date = 1925|volume = 2|pages=1–41|doi = 10.1021/cr60005a001}}</ref> The place where the discovery took place led to the element being named for the Latin name for "Copenhagen", ''Hafnia'', the home town of [[Niels Bohr]].<ref name ="Scerri">{{cite journal|title = Prediction of the nature of hafnium from chemistry, Bohr's theory and quantum theory|first = Eric R.|last = Scerri|journal = Annals of Science|date = 1994|volume = 51|pages= 137–150|doi =10.1080/00033799400200161|issue = 2}}</ref>

Hafnium was separated from zirconium through repeated recrystallization of the double [[ammonium]] or [[potassium]] fluorides by [[Valdemar Thal Jantzen]] and von Hevesy.<ref name="Ark1924a" >{{cite journal|title = Die Trennung von Zirkonium und Hafnium durch Kristallisation ihrer Ammoniumdoppelfluoride (The separation of zirconium and hafnium by crystallization of the double ammonium fluorides)|journal = Zeitschrift für Anorganische und Allgemeine Chemie|volume = 141|date = 1924|pages= 284–288|first = A. E.|last = van Arkel|author2 = de Boer, J. H.|doi = 10.1002/zaac.19241410117|language = de}}</ref> [[Anton Eduard van Arkel]] and [[Jan Hendrik de Boer]] were the first to prepare metallic hafnium by passing hafnium tetraiodide vapor over a heated [[tungsten]] filament in 1924.<ref name="Ark1924b">{{cite journal|title = Die Trennung des Zirkoniums von anderen Metallen, einschließlich Hafnium, durch fraktionierte Distillation (The separation of zirconium and hafnium by fractionated distillation)| journal = Zeitschrift für Anorganische und Allgemeine Chemie|volume = 141|date = 1924|pages= 289–296|first = A. E.|last = van Arkel|author2 = de Boer, J. H.|doi = 10.1002/zaac.19241410118|language = de}}</ref><ref name="Ark1925">{{cite journal|title = Darstellung von reinem Titanium-, Zirkonium-, Hafnium- und Thoriummetall (Production of pure titanium, zirconium, hafnium and Thorium metal)|journal = Zeitschrift für Anorganische und Allgemeine Chemie|volume = 148|date = 1925|pages= 345–350|first = A. E.|last = van Arkel|author2 = de Boer, J. H.|doi = 10.1002/zaac.19251480133|language = de}}</ref> The long delay between the discovery of the lightest two group 4 elements and that of hafnium was partly due to the rarity of hafnium, and partly due to the extreme similarity of zirconium and hafnium, so that all previous samples of zirconium had in reality been contaminated with hafnium without anyone knowing.<ref name="EncyChem">{{cite book|editor-last=Hampel|editor-first=Clifford A.|title=The Encyclopedia of the Chemical Elements|year=1968|last=Barksdale|first=Jelks|publisher=Reinhold Book Corporation|location=Skokie, Illinois|pages=732–738|chapter=Titanium|lccn=68-29938}}</ref>

The last element of the group, [[rutherfordium]], does not occur naturally and had to be made by synthesis. The first reported detection was by a team at the [[Joint Institute for Nuclear Research]] (JINR), which in 1964 claimed to have produced the new element by bombarding a [[plutonium]]-242 target with [[neon]]-22 ions, although this was later put into question.<ref name=93TWG>{{cite journal |title =Discovery of the transfermium elements. Part II: Introduction to discovery profiles. Part III: Discovery profiles of the transfermium elements |date = 1993 |author= Barber, R. C. |author2=Greenwood, N. N. |author3=Hrynkiewicz, A. Z. |author4=Jeannin, Y. P. |author5=Lefort, M. |author6=Sakai, M. |author7=Ulehla, I. |author8=Wapstra, A. P. |author9= Wilkinson, D. H. |journal = Pure and Applied Chemistry| volume = 65 |issue = 8 |pages = 1757–1814 |doi = 10.1351/pac199365081757|s2cid = 195819585 |doi-access= free }}</ref> More conclusive evidence was obtained by researchers at the [[University of California, Berkeley]], who synthesised element 104 in 1969 by bombarding a [[californium]]-249 target with [[carbon-12]] ions.<ref name=69Gh01>{{cite journal |doi = 10.1103/PhysRevLett.22.1317 |title = Positive Identification of Two Alpha-Particle-Emitting Isotopes of Element 104 |date = 1969 |last1 = Ghiorso |first1 = A. |last2 = Nurmia |first2=M. |journal = Physical Review Letters |volume = 22 |issue = 24 |pages = 1317–1320 |bibcode=1969PhRvL..22.1317G|last3 = Harris |first3 = J. |last4 = Eskola |first4 = K. |last5 = Eskola |first5 = P. |url = https://cloudfront.escholarship.org/dist/prd/content/qt3fm666nq/qt3fm666nq.pdf }}</ref> A [[Transfermium Wars|controversy]] erupted on who had discovered the element, which each group suggesting its own name: the Dubna group named the element ''kurchatovium'' after [[Igor Kurchatov]], while the Berkeley group named it ''rutherfordium'' after [[Ernest Rutherford]].<ref>{{cite web |url=http://www.rsc.org/chemistryworld/podcast/Interactive_Periodic_Table_Transcripts/Rutherfordium.asp |title=Rutherfordium |publisher=Rsc.org |access-date=2010-09-04}}</ref> Eventually a joint working party of [[IUPAC]] and [[IUPAP]], the Transfermium Working Group, decided that credit for the discovery should be shared. After various compromises were attempted, in 1997, IUPAC officially named the element rutherfordium following the American proposal.<ref name=97IUPAC>{{cite journal |doi =10.1351/pac199769122471 |title =Names and symbols of transfermium elements (IUPAC Recommendations 1997) |date =1997 |journal =Pure and Applied Chemistry |volume =69 |issue = 12 |pages =2471–2474|doi-access =free }}</ref>

==Characteristics==

===Chemical===

{| class="wikitable" style="float:right; font-size:95%;white-space:nowrap;"
|+
! colspan=4 | [[Electron configuration]]s of the group 4 elements
|-
! {{abbr|1=''Z''|2=Atomic number}} !! Element !! Electrons per [[Electron shell|shell]] !! Electron configuration
|-
| style="text-align:right" | 22 || Ti, titanium || {{mono|2, 8, 10, &nbsp;2}}    || {{mono|1=[Ar] &nbsp;&nbsp;<sup>&nbsp;&nbsp;</sup> 3d<sup>2</sup> 4s<sup>2</sup>}}
|-
| style="text-align:right" | 40 || Zr, zirconium   || {{mono|2, 8, 18, 10, &nbsp;2}}|| {{mono|1=[Kr] &nbsp;&nbsp;<sup>&nbsp;&nbsp;</sup> 4d<sup>2</sup> 5s<sup>2</sup>}}
|-
| style="text-align:right" | 72 || Hf, hafnium || {{mono|2, 8, 18, 32, 10, &nbsp;2}}  || {{mono|1=[Xe] 4f<sup>14</sup> 5d<sup>2</sup> 6s<sup>2</sup>}}
|-
| style="text-align:right" | 104 || Rf, rutherfordium || {{mono|2, 8, 18, 32, 32, 10, 2}} || {{mono|1=[Rn] 5f<sup>14</sup> 6d<sup>2</sup> 7s<sup>2</sup>}}
|}
Like other groups, the members of this family show patterns in their electron configurations, especially the outermost shells, resulting in trends in chemical behavior. Most of the chemistry has been observed only for the first three members of the group; chemical properties of rutherfordium are not well-characterized, but what is known and predicted matches its position as a heavier homolog of hafnium.<ref>{{cite journal | doi=10.1524/ract.2005.93.9-10.519 | title=Chemical studies on rutherfordium (Rf) at JAERI | date=2005 | last1=Nagame | first1=Y. | journal=Radiochimica Acta | volume=93 | issue=9–10_2005 | page=519 | url=http://wwwsoc.nii.ac.jp/jnrs/paper/JN62/jn6202.pdf | last2=Tsukada | first2=K. | last3=Asai | first3=M. | last4=Toyoshima | first4=A. | last5=Akiyama | first5=K. | last6=Ishii | first6=Y. | last7=Kaneko-Sato | first7=T. | last8=Hirata | first8=M. | last9=Nishinaka | first9=I. | last10=Ichikawa | first10=S. | last11=Haba | first11=H. | last12=Enomoto | first12=Shuichi | s2cid=96299943 | display-authors=1 | url-status=dead | archive-url=https://web.archive.org/web/20080528125634/http://wwwsoc.nii.ac.jp/jnrs/paper/JN62/jn6202.pdf | archive-date=2008-05-28 }}</ref>

Titanium, zirconium, and hafnium are reactive metals, but this is masked in the bulk form because they form a dense oxide layer that sticks to the metal and reforms even if removed. As such, the bulk metals are very resistant to chemical attack; most aqueous acids have no effect unless heated, and aqueous alkalis have no effect even when hot. Oxidizing acids such as [[nitric acid]]s indeed tend to reduce reactivity as they induce the formation of this oxide layer. The exception is [[hydrofluoric acid]], as it forms soluble fluoro complexes of the metals. When finely divided, their reactivity shows as they become [[pyrophoricity|pyrophoric]], directly reacting with [[oxygen]] and [[hydrogen]], and even [[nitrogen]] in the case of titanium. All three are fairly electropositive, although less so than their predecessors in [[group 3 element|group 3]].<ref name=Greenwood958>Greenwood and Earnshaw, pp. 958–61</ref> The oxides [[Titanium dioxide|TiO<sub>2</sub>]], [[Zirconium dioxide|ZrO<sub>2</sub>]] and [[Hafnium(IV) oxide|HfO<sub>2</sub>]] are white solids with high melting points and unreactive against most acids.<ref name="Holl">{{cite book|publisher = Walter de Gruyter|year = 1985|edition = 91–100|pages=1056–1057|isbn = 3-11-007511-3|title = Lehrbuch der Anorganischen Chemie|first1 = Arnold F.|last1 = Holleman|last2= Wiberg|first2=Egon|last3=Wiberg|first3=Nils|language = de}}</ref>

The chemistry of group 4 elements is dominated by the group oxidation state. Zirconium and hafnium are in particular extremely similar, with the most salient differences being physical rather than chemical (melting and boiling points of compounds and their solubility in solvents).<ref name=Holl/> This is an effect of the [[lanthanide contraction]]: the expected increase of atomic radius from the 4d to the 5d elements is wiped out by the insertion of the 4f elements before. Titanium, being smaller, is distinct from these two: its oxide is less basic than those of zirconium and hafnium, and its aqueous chemistry is more hydrolyzed.<ref name=Greenwood958/> Rutherfordium should have a still more basic oxide than zirconium and hafnium.<ref name=primefan>[http://www.primefan.ru/stuff/chem/front2019.png Periodic table poster] by A. V. Kulsha and T. A. Kolevich</ref>

The chemistry of all three is dominated by the +4 oxidation state, though this is too high to be well-described as totally ionic. Low oxidation states are not well-represented for zirconium and hafnium<ref name=Greenwood958/> (and should be even less well-represented for rutherfordium);<ref name=primefan/> the +3 oxidation state of zirconium and hafnium reduces water. For titanium, this oxidation state is merely easily oxidised, forming a violet Ti<sup>3+</sup> aqua cation in solution. The elements have a significant coordination chemistry: zirconium and hafnium are large enough to readily support the coordination number of 8. All three metals however form weak sigma bonds to carbon and because they have few d electrons, [[pi backbonding]] is not very effective either.<ref name=Greenwood958/>

===Physical===
The trends in group 4 follow those of the other early d-block groups and reflect the addition of a filled f-shell into the core in passing from the fifth to the sixth period. All the stable members of the group are silvery [[refractory metal]]s, though impurities of [[carbon]], [[nitrogen]], and oxygen make them brittle.<ref name=Greenwood956>Greenwood and Earnshaw, pp. 956–8</ref> They all crystallize in the [[hexagonal close-packed]] structure at room temperature,<ref name=Greenwood946>Greenwood and Earnshaw, pp. 946–8</ref> and rutherfordium is expected to do the same.<ref name=hcp>{{cite journal|doi=10.1103/PhysRevB.84.113104|title=First-principles calculation of the structural stability of 6d transition metals|year=2011|last1=Östlin|first1=A.|last2=Vitos|first2=L.|journal=Physical Review B|volume=84|issue=11|page=113104|bibcode=2011PhRvB..84k3104O }}</ref> At high temperatures, titanium, zirconium, and hafnium transform to a [[body-centered cubic]] structure. While they are better conductors of heat and electricity than their group 3 predecessors, they are still poor compared to most metals. This, along with the higher melting and boiling points, and enthalpies of fusion, vaporization, and atomization, reflects the extra d electron available for metallic bonding.<ref name=Greenwood946/>

The table below is a summary of the key physical properties of the group 4 elements. The four question-marked values are extrapolated.<ref name=Haire>{{cite book| title=The Chemistry of the Actinide and Transactinide Elements| editor1-last=Morss|editor2-first=Norman M.| editor2-last=Edelstein| editor3-last=Fuger|editor3-first=Jean| last1=Hoffman|first1=Darleane C. |last2=Lee |first2=Diana M. |last3=Pershina |first3=Valeria |chapter=Transactinides and the future elements| publisher= [[Springer Science+Business Media]]| year=2006| isbn=1-4020-3555-1| location=Dordrecht, The Netherlands| edition=3rd| ref=CITEREFHaire2006}}</ref>
{| class="wikitable centered" style="text-align:center;"
|+Properties of the group 4 elements| Properties of the group 4 elements
! Name
! Ti, [[titanium]]
! Zr, [[zirconium]]
! Hf, [[hafnium]]
! Rf, [[rutherfordium]]
|-
| style="background:lightgrey; text-align:left;"|[[Melting point]]
| 1941 K (1668&nbsp;°C) || 2130 K (1857&nbsp;°C) || 2506 K (2233&nbsp;°C) || 2400 K (2100&nbsp;°C)?
|-
| style="background:lightgrey; text-align:left;"|[[Boiling point]]
| 3560 K (3287&nbsp;°C) || 4682 K (4409&nbsp;°C) || 4876 K (4603&nbsp;°C) || 5800 K (5500&nbsp;°C)?
|-
| style="background:lightgrey; text-align:left;"|[[Density]]
| 4.507&nbsp;g·cm<sup>−3</sup> || 6.511&nbsp;g·cm<sup>−3</sup> || 13.31&nbsp;g·cm<sup>−3</sup> || 17&nbsp;g·cm<sup>−3</sup>?
|-
| style="background:lightgrey; text-align:left;"|Appearance
| silver metallic || silver white || silver gray|| ?
|-
| style="background:lightgrey; text-align:left;"|[[Atomic radius]]
| 140 pm || 155 pm || 155 pm || 150 pm?
|}
====Titanium====
{{transcluded section|Titanium}}
{{#section:Titanium|properties}}
====Zirconium====
{{transcluded section|Zirconium}}
{{#section:Zirconium|properties}}

====Hafnium====
{{transcluded section|Hafnium}}
{{#section:Hafnium|properties}}
====Rutherfordium====
{{transcluded section|Rutherfordium}}
{{#section:Rutherfordium|properties}}

==Production==
The production of the metals itself is difficult due to their reactivity. The formation of [[oxide]]s, [[nitride]]s, and [[carbide]]s must be avoided to yield workable metals; this is normally achieved by the [[Kroll process]]. The oxides (MO<sub>2</sub>) are reacted with [[coal]] and [[chlorine]] to form the chlorides (MCl<sub>4</sub>). The chlorides of the metals are then reacted with magnesium, yielding [[magnesium chloride]] and the metals.

Further purification is done by a [[chemical transport reaction]] developed by [[Anton Eduard van Arkel]] and [[Jan Hendrik de Boer]]. In a closed vessel, the metal reacts with [[iodine]] at temperatures above 500&nbsp;°C forming metal(IV) iodide; at a tungsten filament of nearly 2000&nbsp;°C the reverse reaction happens and the iodine and metal are set free. The metal forms a solid coating on the tungsten filament and the iodine can react with additional metal resulting in a steady turnover.<ref name="Holl"/><ref name="Ark1925"/>
::M + 2 I<sub>2</sub> (low temp.) → MI<sub>4</sub>
::MI<sub>4</sub> (high temp.) → M + 2 I<sub>2</sub>

==Occurrence==
[[Image:HeavyMineralsBeachSand.jpg|right|thumb|Heavy minerals (dark) in a quartz beach sand ([[Chennai]], India).]]
The abundance of the group 4 metals decreases with increase of atomic mass. Titanium is the seventh most abundant metal in Earth's crust and has an abundance of 6320&nbsp;ppm, while zirconium has an abundance of 162&nbsp;ppm and hafnium has only an abundance of 3 ppm.<ref>{{cite web|url = http://www.webelements.com/periodicity/abundance_crust/|title = Abundance in Earth's Crust|publisher = WebElements.com|access-date = 2007-04-14|archive-url = https://web.archive.org/web/20080523082920/http://www.webelements.com/periodicity/abundance_crust/|archive-date = 2008-05-23|url-status = dead}}</ref>

All three stable elements occur in [[heavy mineral sands ore deposits]], which are [[placer deposit]]s formed, most usually in [[beach]] environments, by concentration due to the [[specific gravity]] of the mineral grains of erosion material from [[mafic]] and [[ultramafic rock]]. The titanium minerals are mostly [[anatase]] and [[rutile]], and zirconium occurs in the mineral [[zircon]]. Because of the chemical similarity, up to 5% of the zirconium in zircon is replaced by hafnium. The largest producers of the group 4 elements are [[Australia]], [[South Africa]] and [[Canada]].<ref>{{cite web|url = http://www.alkane.com.au/projects/nsw/dubbo/DZP%20Summary%20June07.pdf|title = Dubbo Zirconia Project Fact Sheet|date = June 2007|publisher = Alkane Resources Limited|access-date = 2008-09-10|archive-url = https://web.archive.org/web/20080228054038/http://www.alkane.com.au/projects/nsw/dubbo/DZP%20Summary%20June07.pdf|archive-date = 2008-02-28|url-status = dead}}</ref><ref name="usgs2008">{{cite journal| title = Zirconium and Hafnium| journal = Mineral Commodity Summaries| pages = 192–193| publisher = US Geological Survey|date=January 2008| url = http://minerals.usgs.gov/minerals/pubs/commodity/zirconium/mcs-2008-zirco.pdf| access-date = 2008-02-24}}</ref><ref>{{cite web|last=Callaghan|first=R.|title=Zirconium and Hafnium Statistics and Information|publisher=US Geological Survey|date=2008-02-21|url=http://minerals.usgs.gov/minerals/pubs/commodity/zirconium/|access-date=2008-02-24}}</ref><ref name="usgypTi2009">{{cite web|title = Minerals Yearbook Commodity Summaries 2009: Titanium |publisher = US Geological Survey|date=May 2009|url = http://minerals.usgs.gov/minerals/pubs/commodity/titanium/myb1-2007-titan.pdf| access-date = 2008-02-24}}</ref><ref name="usgcomTi2009">{{cite web|last = Gambogi|first= Joseph|title = Titanium and Titanium dioxide Statistics and Information|publisher=US Geological Survey|date=January 2009|url=http://minerals.usgs.gov/minerals/pubs/commodity/titanium/mcs-2009-titan.pdf|access-date = 2008-02-24}}</ref>

==Applications==
<!--Separation of hafnium and zirconium becomes very important in the nuclear power industry, since zirconium is a good fuel-rod cladding metal, with the desirable properties of a very low neutron capture cross-section and good chemical stability at high temperatures. However, because of hafnium's neutron-absorbing properties, hafnium impurities in zirconium would cause it to be far less useful for nuclear reactor applications. Thus a nearly complete separation of zirconium and hafnium is necessary for their use in nuclear power. The production of hafnium free zirconium is the main source for hafnium. The separation is difficult.<ref name="Stwertka">{{cite book|last=Stwertka|first=Albert|title=A Guide to the Elements|url=https://archive.org/details/guidetoelements00stwe|url-access=registration|publisher=Oxford University Press|year=1996|pages=[https://archive.org/details/guidetoelements00stwe/page/117 117–119]| isbn = 0-19-508083-1}}</ref>-->
Titanium metal and its alloys have a wide range of applications, where the corrosion resistance, the heat stability and the low density (light weight) are of benefit. The foremost use of corrosion-resistant hafnium and zirconium has been in nuclear reactors. Zirconium has a very low and hafnium has a high [[neutron capture|thermal neutron-capture cross-section]]. Therefore, zirconium (mostly as [[zircaloy]]) is used as [[Cladding (metalworking)|cladding]] of [[fuel rod]]s in [[nuclear reactor]]s,<ref name="ASTM">{{cite book|url = https://books.google.com/books?id=dI_LssydVeYC|title = ASTM Manual on Zirconium and Hafnium|first = J. H.|last = Schemel|publisher = ASTM International|year = 1977|isbn = 978-0-8031-0505-8|pages = 1–5}}</ref> while hafnium is used in [[control rod]]s for [[nuclear reactor]]s, because each hafnium atom can absorb multiple neutrons.<ref name="Hend" >{{cite web |title = Hafnium|first = James B. |last = Hedrick |url = http://minerals.er.usgs.gov/minerals/pubs/commodity/zirconium/731798.pdf|publisher = United States Geological Survey|access-date = 2008-09-10}}</ref><ref>{{cite journal|title = Reactive Metals. Zirconium, Hafnium, and Titanium|first = Donald|last = Spink|journal = Industrial and Engineering Chemistry|year = 1961|volume = 53|issue = 2|pages = 97–104|doi = 10.1021/ie50614a019}}</ref>

Smaller amounts of hafnium<ref name="hightemp">{{cite web|url = http://www.cbmm.com.br/portug/sources/techlib/science_techno/table_content/sub_3/images/pdfs/016.pdf|title = Niobium alloys and high Temperature Applications|first = John|last = Hebda|publisher = CBMM|year = 2001|access-date = 2008-09-04|url-status = dead|archive-url = https://web.archive.org/web/20081217080513/http://www.cbmm.com.br/portug/sources/techlib/science_techno/table_content/sub_3/images/pdfs/016.pdf|archive-date = 2008-12-17}}</ref> and zirconium are used in super alloys to improve the properties of those alloys.<ref name="Super">{{cite book|url = https://books.google.com/books?id=vjCJ5pI1QpkC&pg=PA235|title = Superalloys| first = Matthew J.|last = Donachie| publisher = ASTM International|year = 2002|isbn = 978-0-87170-749-9|pages = 235–236}}</ref>

==Biological occurrences==
The group 4 elements are hard refractory metals with low aqueous solubility and low availability to the biosphere. Titanium and zirconium are relatively abundant, whereas hafnium is rare in the environment, and rutherfordium non-existent.

Titanium has no known role in any organism's biology. However, many studies suggest that titanium could be biologically active. Most titanium on Earth is stored within insoluble minerals, so it is unlikely to be a part of any biological system in spite of being potentially biologically active.<ref>{{Cite web |title=Contemplating a role for titanium in organisms |url=https://academic.oup.com/metallomics/article/8/1/9/6000637 |access-date=2023-09-23 |website=academic.oup.com}}</ref>

Zirconium plays no known role in any biological system,<ref>{{Cite web |title=Zirconium - Element information, properties and uses {{!}} Periodic Table |url=https://www.rsc.org/periodic-table/element/40/zirconium#:~:text=Zirconium%20has%20no%20known%20biological,It%20has%20low%20toxicity.&text=Zirconium%20occurs%20in%20about%2030,ones%20being%20zircon%20and%20baddeleyite. |access-date=2023-09-23 |website=www.rsc.org}}</ref> but is common in biological systems. Certain antiperspirant products use [[Aluminium zirconium tetrachlorohydrex gly]] to block sweat pores in the skin.<ref>Karl Laden, 1999, Antiperspirants and Deodorants, CRC Press, {{ISBN|0-8247-1746-5}}</ref>

Hafnium plays no known role in any biological system, and has low toxicity.<ref>{{Cite web |title=Hafnium - Element information, properties and uses {{!}} Periodic Table |url=https://www.rsc.org/periodic-table/element/72/hafnium#:~:text=Hafnium%20has%20no%20known%20biological%20role,%20and%20it%20has%20low%20toxicity. |access-date=2023-09-23 |website=www.rsc.org}}</ref>

Rutherfordium is synthetic, expensive, and radioactive: the most stable isotopes have half-lives under an hour. Few chemical properties and no biological functions are known.

==Precautions==
Titanium is non-toxic even in large doses and does not play any natural role inside the [[human body]].<ref name="Emsley2001">{{cite book |last=Emsley |first=John |url=https://archive.org/details/naturesbuildingb0000emsl |title=Nature's Building Blocks: An A-Z Guide to the Elements |publisher=Oxford University Press |year=2001 |isbn=978-0-19-850341-5 |location=Oxford, England, UK |pages=[https://archive.org/details/naturesbuildingb0000emsl/page/457 457–458] |chapter=Titanium |url-access=registration}}{{vn|date=June 2020}}</ref> An estimated quantity of 0.8 milligrams of titanium is ingested by humans each day, but most passes through without being absorbed in the tissues.<ref name="Emsley2001" /> It does, however, sometimes [[bioaccumulation|bio-accumulate]] in tissues that contain [[silica]]. One study indicates a possible connection between titanium and [[yellow nail syndrome]].<ref>{{cite journal|last=Berglund|first=Fredrik|author2=Carlmark, Bjorn |title=Titanium, Sinusitis, and the Yellow Nail Syndrome|journal=Biological Trace Element Research|date=October 2011|pmc=3176400|volume=143|issue=1|pages=1–7|doi=10.1007/s12011-010-8828-5|pmid=20809268|bibcode=2011BTER..143....1B }}</ref> 

Zirconium powder can cause irritation, but only contact with the eyes requires medical attention.<ref>{{cite web |title=Zirconium |website=International Chemical Safety Card Database |date=October 2004 |publisher=International Labour Organization |url=http://www.ilo.org/legacy/english/protection/safework/cis/products/icsc/dtasht/_icsc14/icsc1405.htm |access-date=2008-03-30}}</ref> OSHA recommendations for zirconium are 5&nbsp;mg/m<sup>3</sup> [[permissible exposure limit|time weighted average]] limit and a 10&nbsp;mg/m<sup>3</sup> short-term exposure limit.<ref>{{cite web|title=Zirconium Compounds|publisher=National Institute for Occupational Health and Safety|date=2007-12-17|url=https://www.cdc.gov/niosh/pel88/7440-67.html|access-date=2008-02-17}}</ref> 

Only limited data exists on the toxicology of hafnium.<ref name = hafniumtox >{{cite web|url =https://www.osha.gov/SLTC/healthguidelines/hafnium/index.html|title= Occupational Safety & Health Administration: Hafnium|publisher=U.S. Department of Labor|access-date=2008-09-10|archive-url= https://web.archive.org/web/20080313003040/https://www.osha.gov/SLTC/healthguidelines/hafnium/index.html <!-- Bot retrieved archive --> |archive-date=2008-03-13}}</ref> Care needs to be taken when [[machining]] hafnium because it is [[pyrophoric]]—fine particles can spontaneously combust when exposed to air. Compounds that contain this metal are rarely encountered by most people. The pure metal is not considered toxic, but hafnium compounds should be handled as if they were toxic because the ionic forms of metals are normally at greatest risk for toxicity, and limited animal testing has been done for hafnium compounds.<ref name= hafniumtox />

==References==
{{Reflist|30em}}

==Bibliography==
*{{Greenwood&Earnshaw2nd}}

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