Group 4 is the second group oftransition metals in the periodic table. It contains only the four elementstitanium (Ti),zirconium (Zr),hafnium (Hf), andrutherfordium (Rf). The group is also called thetitanium group ortitanium family after its lightest member.

↓ Period
4	Titanium (Ti) 22Transition metal
5	Zirconium (Zr) 40Transition metal
6	Hafnium (Hf) 72Transition metal
7	Rutherfordium (Rf) 104Transition metal
Legend primordial element synthetic element

As is typical for early transition metals, zirconium and hafnium have only the groupoxidation state of +4 as a major one, and are quite electropositive and have a less rich coordination chemistry. Due to the effects of thelanthanide 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 stronglyradioactive: 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.

Zircon was known as a gemstone from ancient times,^[1] but it was not known to contain a new element until the work of German chemistMartin Heinrich Klaproth in 1789. He analysed the zircon-containing mineraljargoon and found a new earth (oxide), but was unable to isolate the element from its oxide. Cornish chemistHumphry Davy also attempted to isolate this new element in 1808 throughelectrolysis, but failed: he gave it the name zirconium.^[2] In 1824, Swedish chemistJöns Jakob Berzelius isolated an impure form of zirconium, obtained by heating a mixture of potassium and potassium zirconium fluoride in an iron tube.^[1]

Cornish mineralogistWilliam Gregor first identified titanium in ilmenite sand beside a stream inCornwall, Great Britain in the year 1791.^[3] After analyzing the sand, he determined the weakly magnetic sand to containiron oxide and a metal oxide that he could not identify.^[4] During that same year, mineralogistFranz Joseph Muller produced the same metal oxide and could not identify it. In 1795, chemistMartin Heinrich Klaproth independently rediscovered the metal oxide inrutile from the Hungarian village Boinik.^[3] He identified the oxide containing a new element and named it for theTitans ofGreek mythology.^[5] Berzelius was also the first to prepare titanium metal (albeit impurely), doing so in 1825.^[6]

TheX-ray spectroscopy done byHenry Moseley in 1914 showed a direct dependency betweenspectral line andeffective nuclear charge. This led to the nuclear charge, oratomic number of an element, being used to ascertain its place within the periodic table. With this method, Moseley determined the number oflanthanides and showed that there was a missing element with atomic number 72.^[7] This spurred chemists to look for it.^[8]Georges Urbain asserted that he found element 72 in therare earth elements in 1907 and published his results onceltium in 1911.^[9] 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.^[10]

By early 1923, several physicists and chemists such asNiels Bohr^[11] andCharles Rugeley Bury^[12] 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 ofFriedrich Paneth.^[13][14] 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 andGeorg von Hevesy were motivated to search for the new element in zirconium ores.^[15]Hafnium was discovered by the two in 1923 in Copenhagen, Denmark.^[16][17] The place where the discovery took place led to the element being named for the Latin name for "Copenhagen",Hafnia, the home town ofNiels Bohr.^[18]

Hafnium was separated from zirconium through repeated recrystallization of the doubleammonium orpotassium fluorides byValdemar Thal Jantzen and von Hevesy.^[19]Anton Eduard van Arkel andJan Hendrik de Boer were the first to prepare metallic hafnium by passing hafnium tetraiodide vapor over a heatedtungsten filament in 1924.^[20][21] 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.^[22]

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 theJoint Institute for Nuclear Research (JINR), which in 1964 claimed to have produced the new element by bombarding aplutonium-242 target withneon-22 ions, although this was later put into question.^[23] More conclusive evidence was obtained by researchers at theUniversity of California, Berkeley, who synthesised element 104 in 1969 by bombarding acalifornium-249 target withcarbon-12 ions.^[24] Acontroversy erupted on who had discovered the element, which each group suggesting its own name: the Dubna group named the elementkurchatovium afterIgor Kurchatov, while the Berkeley group named itrutherfordium afterErnest Rutherford.^[25] Eventually a joint working party ofIUPAC andIUPAP, 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.^[26]

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.^[27]

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 asnitric acids indeed tend to reduce reactivity as they induce the formation of this oxide layer. The exception ishydrofluoric acid, as it forms soluble fluoro complexes of the metals. When finely divided, their reactivity shows as they becomepyrophoric, directly reacting withoxygen andhydrogen, and evennitrogen in the case of titanium. All three are fairly electropositive, although less so than their predecessors ingroup 3.^[28] The oxidesTiO₂,ZrO₂ andHfO₂ are white solids with high melting points and unreactive against most acids.^[29]

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).^[29] This is an effect of thelanthanide 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.^[28] Rutherfordium should have a still more basic oxide than zirconium and hafnium.^[30]

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^[28] (and should be even less well-represented for rutherfordium);^[30] the +3 oxidation state of zirconium and hafnium reduces water. For titanium, this oxidation state is merely easily oxidised, forming a violet Ti³⁺ 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.^[28]

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 silveryrefractory metals, though impurities ofcarbon,nitrogen, and oxygen make them brittle.^[31] They all crystallize in thehexagonal close-packed structure at room temperature,^[32] and rutherfordium is expected to do the same.^[33] At high temperatures, titanium, zirconium, and hafnium transform to abody-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.^[32]

The table below is a summary of the key physical properties of the group 4 elements. The four question-marked values are extrapolated.^[34]

As ametal, titanium is recognized for its highstrength-to-weight ratio.^[35] It is a strong metal with lowdensity that is quiteductile (especially in anoxygen-free environment),^[36] lustrous, and metallic-white incolor.^[37] Due to its relatively high melting point (1,668 °C or 3,034 °F) it has sometimes been described as arefractory metal, but this is not the case.^[38] It isparamagnetic and has fairly lowelectrical andthermal conductivity compared to other metals.^[36] Titanium issuperconducting when cooled below its critical temperature of 0.49 K.^[39][40]

Zirconium is alustrous, greyish-white, soft, ductile, malleable metal that is solid at room temperature, though it is hard andbrittle at lesser purities.^[2] In powder form, zirconium is highly flammable, but the solid form is much less prone to ignition. Zirconium is highly resistant to corrosion by alkalis, acids, salt water and other agents.^[1] However, it will dissolve inhydrochloric andsulfuric acid, especially whenfluorine is present.^[41]Alloys withzinc aremagnetic at less than 35 K.^[1]

Hafnium is a shiny, silvery,ductilemetal that iscorrosion-resistant and chemically similar to zirconium^[42] in that they have the same number ofvalence electrons and are in the same group. Also, theirrelativistic effects are similar: The expected expansion of atomic radii from period 5 to 6 is almost exactly canceled out by thelanthanide contraction. Hafnium changes from its alpha form, a hexagonal close-packed lattice, to its beta form, a body-centered cubic lattice, at 2,388 K (2,115 °C; 3,839 °F).^[43] The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity.^[42]

Rutherfordium is expected to be a solid under normal conditions and have ahexagonal close-packed crystal structure (^c/_a = 1.61), similar to its lightercongener hafnium.^[33] It should be a metal withdensity ~17 g/cm³.^[44][45] The atomic radius of rutherfordium is expected to be ~150 pm. Due to relativistic stabilization of the 7s orbital and destabilization of the 6d orbital, Rf⁺ and Rf²⁺ ions are predicted to give up 6d electrons instead of 7s electrons, which is the opposite of the behavior of its lighter homologs.^[34] When under high pressure (variously calculated as 72 or ~50GPa), rutherfordium is expected to transition tobody-centered cubic crystal structure; hafnium transforms to this structure at 71±1 GPa, but has an intermediate ω structure that it transforms to at 38±8 GPa that should be lacking for rutherfordium.^[46]

The production of the metals itself is difficult due to their reactivity. The formation ofoxides,nitrides, andcarbides must be avoided to yield workable metals; this is normally achieved by theKroll process. The oxides (MO₂) are reacted withcoal andchlorine to form the chlorides (MCl₄). The chlorides of the metals are then reacted with magnesium, yieldingmagnesium chloride and the metals.

Further purification is done by achemical transport reaction developed byAnton Eduard van Arkel andJan Hendrik de Boer. In a closed vessel, the metal reacts withiodine at temperatures above 500 °C forming metal(IV) iodide; at a tungsten filament of nearly 2000 °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.^[29][21]

M + 2 I₂ (low temp.) → MI₄

MI₄ (high temp.) → M + 2 I₂

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 ppm, while zirconium has an abundance of 162 ppm and hafnium has only an abundance of 3 ppm.^[47]

All three stable elements occur inheavy mineral sands ore deposits, which areplacer deposits formed, most usually inbeach environments, by concentration due to thespecific gravity of the mineral grains of erosion material frommafic andultramafic rock. The titanium minerals are mostlyanatase andrutile, and zirconium occurs in the mineralzircon. 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 areAustralia,South Africa andCanada.^{[48][49][50][51][52]}

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 highthermal neutron-capture cross-section. Therefore, zirconium (mostly aszircaloy) is used ascladding offuel rods innuclear reactors,^[42] while hafnium is used incontrol rods fornuclear reactors, because each hafnium atom can absorb multiple neutrons.^[53][54]

Smaller amounts of hafnium^[55] and zirconium are used in super alloys to improve the properties of those alloys.^[56]

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.^[57]

Zirconium plays no known role in any biological system,^[58] but is common in biological systems. Certain antiperspirant products useAluminium zirconium tetrachlorohydrex gly to block sweat pores in the skin.^[59]

Hafnium plays no known role in any biological system, and has low toxicity.^[60]

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.

Titanium is non-toxic even in large doses and does not play any natural role inside thehuman body.^[61] 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.^[61] It does, however, sometimesbio-accumulate in tissues that containsilica. One study indicates a possible connection between titanium andyellow nail syndrome.^[62]

Zirconium powder can cause irritation, but only contact with the eyes requires medical attention.^[63] OSHA recommendations for zirconium are 5 mg/m³time weighted average limit and a 10 mg/m³ short-term exposure limit.^[64]

Only limited data exists on the toxicology of hafnium.^[65] Care needs to be taken whenmachining hafnium because it ispyrophoric—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.^[65]

• Greenwood, Norman N.; Earnshaw, Alan (1997).Chemistry of the Elements (2nd ed.).Butterworth-Heinemann.ISBN 978-0-08-037941-8.