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Group 6 element

Group 6 elements, also known as the chromium group, comprise the transition metals (Cr, atomic number 24), (Mo, atomic number 42), (W, atomic number 74), and the synthetic superheavy element (Sg, atomic number 106). These d-block elements share group valence electrons in their respective nd orbitals (n=3 for Cr, 4 for Mo, 5 for W, 6 for Sg) and characteristically exhibit a maximum of +6, forming stable oxo compounds, though lower states like +3 for Cr and +4 for Mo and W are also common. Physical properties of the Group 6 elements trend toward increasing and down the group due to stronger and effects, with having a of 1907°C and of 7.15 g/cm³, molybdenum at 2622°C and 10.2 g/cm³, and at 3414°C and 19.3 g/cm³—the highest melting point among metals—while seaborgium's properties remain largely unmeasured owing to its short of about 2.4 minutes for its most stable , ^{271}Sg. Chemically, these elements are less electropositive than those in Groups 4 and 5, showing decreasing reactivity with and down the group; forms a passive layer for resistance, molybdenum and are inert under standard conditions but form volatile halides, and demonstrates +6 behavior akin to its homologues in limited gas-phase experiments. Chromium occurs naturally at 0.0122% abundance in , primarily in ore (FeCr₂O₄), and is extracted via aluminothermy; it is renowned for its lustrous, corrosion-resistant plating and role in stainless steels, while its +3 and +6 states yield colorful compounds like chromates used in pigments and tanning. , discovered in 1778 and named from the Greek for "lead-like," is found in (MoS₂) and serves as an essential in nitrogenase enzymes for in plants; its primary use is in high-strength alloys for tools and missiles, enhancing in steels. , isolated in 1783 from (FeWO₄) and (CaWO₄), derives its name from Swedish for "heavy stone" due to its extreme density; it dominates applications in incandescent lamp filaments, rocket nozzles, and cutting tools, with its +6 state forming yellow WO₃ for catalysts. , synthesized in 1974 at via bombardment of californium-249 with , has no natural occurrence or practical applications but confirms Group 6 trends through trace studies of its SgO₃-like volatility and +6 oxoanion formation. Overall, Group 6 elements play critical roles in modern technology and biology, with their compounds acting as catalysts in petroleum refining (e.g., and sulfides) and oxidation processes, though poses environmental toxicity concerns due to carcinogenicity. The stability of the +6 state increases down the group, correlating with stronger bonds in species, as evidenced by trends.

Introduction

Group members and electronic structure

Group 6 of the periodic table consists of the transition metals (Cr, atomic number 24), (Mo, atomic number 42), (W, atomic number 74), and (Sg, atomic number 106). These elements occupy the sixth column in the d-block, reflecting their position among the transition metals where the d subshell is progressively filled. The name derives from the Greek word , meaning "color," due to the vivid hues of its compounds. 's name comes from the Greek molybdos, referring to "lead," as its ores were historically confused with lead minerals. originates from the Swedish tung sten, translating to "heavy stone," alluding to the density of its ores, while honors American nuclear chemist for his contributions to synthesis. The electronic configurations of these elements highlight their nature, with valence electrons in both s and d orbitals. has the configuration [ \ce{Ar} ] 3d^5 4s^1, [ \ce{Kr} ] 4d^5 5s^1, and [ \ce{Xe} ] 4f^{14} 5d^4 6s^2. 's configuration is predicted to be [ \ce{Rn} ] 5f^{14} 6d^4 7s^2, influenced by relativistic effects, though direct experimental confirmation is limited due to its short . These configurations deviate from the expected for and , where half-filled d subshells provide greater stability by promoting one s to the d orbital. As d-block elements, Group 6 metals exhibit characteristic properties, including partially filled d orbitals that enable variable oxidation states from +2 to +6, arising from the availability of both ns and (n-1)d s for ionization. This flexibility in electron loss facilitates diverse coordination chemistries and formations, with higher oxidation states becoming more stable down the group due to increasing and effects in heavier homologs. For , relativistic effects further influence its predicted properties.
ElementSymbolAtomic NumberElectron Configuration
ChromiumCr24[ \ce{Ar} ] 3d^5 4s^1
MolybdenumMo42[ \ce{Kr} ] 4d^5 5s^1
TungstenW74[ \ce{Xe} ] 4f^{14} 5d^4 6s^2
SeaborgiumSg106[ \ce{Rn} ] 5f^{14} 6d^4 7s^2 (predicted)

Periodic trends in properties

The atomic radii of Group 6 elements exhibit an overall increase down the group due to the addition of principal shells, but with a notable at relative to . Chromium has an empirical of 140 pm, 145 pm, and 135 pm, where the smaller size of arises from the —the poor shielding effect of the 4f s leading to a stronger pull from the on the outer s. Seaborgium's is predicted to be approximately 128 pm, influenced by both and contractions as well as relativistic effects stabilizing the 7s and 6d orbitals. Ionization energies in Group 6 show a general increasing trend down the group, consistent with rising nuclear charge and inadequate shielding by d electrons, which makes electron removal progressively harder. The first ionization energies are 652.9 kJ/mol for chromium, 684.3 kJ/mol for molybdenum, and 769.9 kJ/mol for tungsten. Chromium displays an anomaly in this pattern due to its electron configuration ([Ar] 3d⁵ 4s¹), where the half-filled 3d subshell confers extra stability; this results in a relatively low first ionization energy (as it involves loss of the single 4s electron) but a notably high second ionization energy (1590.6 kJ/mol) to avoid disrupting the stable d⁵ configuration. Seaborgium's first ionization energy is predicted to be approximately 757 kJ/mol. Electronegativities on the Pauling scale increase from chromium (1.66) to molybdenum (2.16) and tungsten (2.36), reflecting a greater effective nuclear charge that enhances the attraction for bonding electrons in heavier members. This trend contrasts with the general decrease down other groups but aligns with the contracted radii and higher ionization energies in this d-block series. Seaborgium's electronegativity is unknown, though relativistic effects are expected to influence its bonding affinity. The metallic character of Group 6 elements strengthens down the group, transitioning from the hard, brittle nature of to the more ductile forms of and . is notably brittle at due to its body-centered cubic and high Peierls barrier for motion, limiting its workability without alloying. and , while also exhibiting a ductile-brittle transition (around 200–400°C for and higher for ), are more malleable and can be drawn into wires or forged when pure. Densities rise markedly from 7.15 g/cm³ for to 10.2 g/cm³ for and 19.3 g/cm³ for , driven by increasing and the radial contraction at ; is forecasted to have an exceptionally high of 23–35 g/cm³.
PropertyChromium (Cr)Molybdenum (Mo)Tungsten (W)Seaborgium (Sg, predicted)
Empirical Atomic Radius (pm)140145135~128
First Ionization Energy (kJ/mol)652.9684.3769.9~757
(Pauling)1.662.162.36Unknown
Density (g/cm³)7.1510.219.323–35
(°C)190726233422Unknown
Melting and boiling points increase down the group, reaching maxima at (melting point 3422°C, boiling point 5555°C), attributable to enhanced from the greater involvement of d electrons in delocalized bonding and the higher atomic packing efficiency in the heavier elements. Chromium's lower melting point (1907°C) stems from weaker interatomic forces in its less contracted structure, while (2623°C) shows intermediate strength. Seaborgium's melting point is unknown but predicted to be high due to robust bonding, despite relativistic influences.

History

Discoveries of individual elements

The discovery of began with early confusions, as its primary ore, (MoS₂), was long mistaken for lead ore or due to superficial similarities in appearance and marking properties. In 1778, Swedish chemist identified as a of a previously unknown element by decomposing it with hot , producing molybdic acid. Three years later, in 1781, fellow Swedish chemist Peter Jacob Hjelm isolated the metal in impure form by reducing the acid with carbon, naming it after the ore. Tungsten's identification followed closely, with Swedish chemist extracting an "unknown earth"—later identified as tungsten oxide or —from the mineral (then called "tungsten" or heavy stone) sourced from the Bispberg iron mine in 1781; Scheele collaborated with in proposing it as a new element akin to . In 1783, Spanish brothers and chemists and Fausto d'Elhuyar independently confirmed the element by reducing the oxide with charcoal to obtain metallic tungsten from , marking the first isolation of the pure metal and dubbing it "wolfram" in reference to the mineral. Chromium was discovered in 1797 by French chemist Louis Nicolas Vauquelin, who obtained chromium oxide by treating crocoite ore (PbCrO₄) with hydrochloric acid, revealing a new element through its colorful compounds. Vauquelin isolated the metal itself in 1798 via carbon reduction of the oxide, naming it chromium from the Greek word for color due to the vivid hues of its salts. Seaborgium, the synthetic Group 6 element, was first produced in 1974 by a team led by American physicist at through the of californium-249 with ions in the SuperHILAC accelerator, yielding the seaborgium-263. This claim faced disputes with Soviet researchers, but an independent confirmation via the same reaction occurred in 1993 at Berkeley, leading to IUPAC recognition of the discovery and official naming as in 1997.

Early isolation and characterization

The isolation of pure chromium marked a significant advancement in the late 19th century, achieved by French chemist Henri Moissan in 1893 through aluminothermic reduction of chromium(III) oxide using aluminum powder in a controlled high-temperature reaction. This method overcame the impurities inherent in earlier carbon reduction attempts, yielding a ductile metallic form suitable for further study. Early characterizations emphasized the vivid colors of chromium compounds, such as the red of chromates derived from crocoite, which Vauquelin had noted upon the element's discovery in 1797, highlighting their potential in pigments and dyes. Molybdenum's initial isolation occurred in 1781 when Swedish chemist Peter Jacob Hjelm reduced with carbon in under an inert atmosphere, producing an impure gray powder that confirmed its metallic character. Throughout the , refinements using carbon reduction in furnaces improved purity, allowing observers to note its exceptionally high of approximately 2623°C, which positioned it among the most known at the time. This property was systematically documented in chemical analyses, underscoring molybdenum's resistance to heat and its distinction from more fusible metals. For , the d'Elhuyar brothers in achieved the first isolation of the metal in 1783 by reducing tungstic oxide—extracted from —with charcoal at high temperatures, resulting in a brittle, high-melting solid. Early processing of ores involved fusing the with ash around 1785 to form water-soluble , facilitating separation from impurities like tin; this alkaline roast-leach approach addressed the ore's nature and technological limitations of the era. By the early 1900s, demand for tungsten filaments in incandescent lamps spurred purification innovations, including repeated recrystallization and , enabling production of ductile wire from previously intractable powders./Descriptive_Chemistry/Elements_Organized_by_Block/3_d-Block_Elements/Group_06%3A_Transition_Metals/Chemistry_of_Tungsten) Seaborgium, the heaviest group 6 element, presented unique challenges due to its synthetic and highly radioactive nature; it was first produced in 1974 via bombardment of californium-249 with , yielding isotope ²⁶³Sg with a of 0.8 seconds, characterized through chains linking to known and daughters. Subsequent experiments in the identified isotope ²⁶⁴Sg, which undergoes with a of 37 milliseconds, confirmed by detecting correlated events and alpha particles in gas-filled separators. These short-lived isotopes limited physical isolation, relying on on-line detection systems for property determination. Advancements in spectroscopy during the 19th and 20th centuries played a crucial role in confirming the metallic nature of group 6 elements, with techniques like flame emission spectroscopy revealing characteristic line spectra for chromium, molybdenum, and tungsten vapors. By the mid-1800s, Bunsen and Kirchhoff's spectroscope enabled precise identification of atomic emissions, distinguishing these transition metals from non-metals and verifying their electronic configurations amid impure samples. In the 20th century, atomic absorption and X-ray spectroscopy further refined characterizations, quantifying metallic bonding and oxidation states despite high melting points that complicated traditional analyses.

Occurrence and allotropy

Natural abundance and sources

Group 6 elements, particularly , , and , are primarily synthesized through processes in massive and supernovae, where explosive silicon burning contributes to iron-peak elements like , while and arise from a combination of rapid (r-process) neutron capture in supernovae and slow () capture in . In the solar system, their abundances relative to (normalized to 10^6 atoms) reflect these origins: at 470 atoms, at 10.3 atoms, and at 0.13 atoms. These values, derived from meteoritic compositions representative of solar system material, indicate 's relatively higher cosmic prevalence compared to the rarer and . , being a synthetic , has no natural cosmic abundance. On , the distribution of Group 6 elements is uneven due to geochemical during . is the most abundant in the continental crust at approximately 100 , ranking as the seventh most abundant after iron, , , , , and . occurs at about 1.2 , while is slightly more abundant at 1.25 ; is absent in nature as it is artificially produced. These crustal concentrations result from magmatic and hydrothermal processes that concentrate the elements into specific phases, with overall levels influenced by the siderophile and lithophile behaviors of and the chalcophile tendencies of and . The primary natural sources of Group 6 elements are specific minerals formed in igneous, metamorphic, and sedimentary environments. Chromium is predominantly found in , FeCr₂O₄, a associated with ultramafic rocks like layered intrusions and ophiolites. Molybdenum's main is , MoS₂, a typically occurring in deposits and systems. Tungsten is sourced chiefly from , CaWO₄, and , (Fe,Mn)WO₄, which form in granitic pegmatites, skarns, and deposits linked to late-stage magmatic fluids. In aqueous environments, Group 6 elements exhibit varying solubilities and concentrations. Seawater contains at around 10 ppb, reflecting its conservative behavior and enrichment relative to crustal averages through riverine input and oceanic cycling; this concentration supports its role in biological systems, where it is bioaccumulated in enzymes like . Chromium and levels in seawater are lower, typically below 1 ppb and 0.1 ppb respectively, due to their rapid scavenging by particles and sediments. These marine distributions highlight molybdenum's higher mobility and biological relevance among the group.

Allotropic forms

Group 6 elements display varying degrees of , with lighter members exhibiting temperature-dependent changes while heavier ones are more structurally stable. , the lightest stable member, has a single body-centered cubic (BCC) crystal structure stable from up to its of 1907 °C. No allotropic transformations occur under ambient or moderate pressure conditions. The BCC structure of promotes directional bonding and limited slip systems, contributing to its characteristic at lower temperatures. This is critical for understanding 's behavior in high-temperature processing and applications requiring thermal stability. Molybdenum, in contrast, exhibits no allotropic transformations and maintains a single body-centered cubic (BCC) crystal structure across its entire solid temperature range, from room temperature to its melting point of 2623 °C. This structural invariance arises from the element's electronic configuration and bonding, which favor the BCC lattice without energetic incentives for phase changes under normal pressures. The stability of this form underscores molybdenum's reliability in refractory applications where consistent mechanical properties are essential. Tungsten similarly adopts a body-centered cubic (BCC) α-phase as its stable form at ambient conditions, persisting up to its exceptionally high of 3422 °C without temperature-induced allotropic shifts. Under extreme high pressures exceeding 100 GPa, a β-phase with a tetragonal distortion emerges, representing a compression-driven transformation that alters the lattice parameters for higher density. While an A15-type superstructure (cubic Cr₃Si-type) appears in certain -based compounds, it does not occur in pure elemental . These pressure-induced behaviors highlight tungsten's resilience, with the β-phase potentially relevant to geophysical modeling of Earth's conditions. Seaborgium, the synthetic Group 6 element with 106, has not had its allotropic forms experimentally confirmed due to its extremely short ; the longest-lived , ²⁷¹Sg, has a of approximately 2.4 minutes. Relativistic calculations predict that would favor a body-centered cubic (BCC) structure at ambient conditions, analogous to its lighter homologues, driven by similar d-electron configurations and relativistic effects stabilizing the BCC lattice over alternatives like hexagonal close-packed. This prediction aligns with trends in the 6d transition series, though experimental verification remains elusive given the element's radioactivity and scarcity.

Production

Extraction from ores

The extraction of Group 6 elements chromium, molybdenum, and tungsten from their primary ores involves multi-stage processes that typically include ore concentration, chemical conversion to intermediate oxides, and high-temperature reduction to the metal. These methods are designed for industrial-scale production, emphasizing efficiency and recovery of the metals from low-grade sources. Chromium is primarily derived from chromite ore (FeCr₂O₄), molybdenum from molybdenite (MoS₂), and tungsten from scheelite (CaWO₄) or wolframite ((Fe,Mn)WO₄). The processes are energy-intensive due to the refractory nature of these metals, requiring significant thermal and electrical inputs for reduction steps. For chromium, the dominant industrial product is ferrochrome, produced by carbothermic reduction of chromite ore in a submerged electric arc furnace. The ore is pelletized with coke and flux (such as quartz), then smelted at temperatures around 1,600–1,700°C, where carbon reduces the chromite to chromium metal alloyed with iron: the reaction yields ferrochrome containing 50–70% chromium. This process accounts for over 95% of global chromium production, primarily for stainless steel manufacturing. For high-purity chromium metal (>99.4% Cr), chromite is first roasted with soda ash to form sodium chromate (Na₂CrO₄), which is leached, acidified to chromic acid, and reduced to chromium(III) oxide (Cr₂O₃); this oxide is then subjected to aluminothermic reduction using aluminum powder in a vacuum or inert atmosphere: Cr₂O₃ + 2Al → 2Cr + Al₂O₃. The resulting metal achieves purities of 99.4–99.9%, with further vacuum arc remelting for ultra-high purity grades. These extraction methods consume approximately 4,000 kWh per ton of ferrochrome, highlighting their energy demands. Molybdenum extraction begins with flotation of concentrate from ore, often as a of , where it constitutes up to 60% of global supply. The concentrate is in air at 500–700°C to convert MoS₂ to (MoO₃), releasing : 2MoS₂ + 7O₂ → 2MoO₃ + 4SO₂. This MoO₃ is then purified by or chemical to remove and other impurities. Final occurs in a two-stage atmosphere at 600–1,000°C: first to MoO₂, then to via MoO₃ + 3H₂ → Mo + 3H₂O. The process yields molybdenum powder with purity exceeding 99.9%, suitable for further consolidation into ingots. recovery from deposits enhances economic viability, as is separated during flotation and processing. Energy use in and stages is substantial, contributing to the overall high intensity of extraction. Tungsten extraction varies by ore type but commonly involves alkali digestion for scheelite. The is roasted with (Na₂CO₃) at 800–1,000°C to form soluble sodium (Na₂WO₄): CaWO₄ + Na₂CO₃ → Na₂WO₄ + CaCO₃. The is purified by solvent extraction or to remove impurities like and , followed by acidification with to precipitate (H₂WO₄·H₂O), which is calcined at 500–700°C to (WO₃). Reduction to metal powder uses gas at 800–1,200°C: WO₃ + 3H₂ → W + 3H₂O. For wolframite ores, or is employed similarly. The resulting powder achieves purities of 99.9–99.95%, with pressing and for final products. These steps are highly energy-intensive, with beneficiation and reduction requiring up to 6,000–7,000 kWh per ton equivalent of WO₃ concentrate in some operations.

Synthesis of seaborgium

Seaborgium, element 106, is produced exclusively through reactions in particle accelerators, as it does not occur naturally. The first synthesis occurred in 1974 at (LBNL) using the heavy-ion linear accelerator known as the Super Heavy Ion Linear Accelerator (SuperHILAC). In this experiment, a team led by bombarded a target of ^{249}\mathrm{Cf} with ^{18}\mathrm{O} ions accelerated to energies around 103 MeV, resulting in the reaction ^{249}\mathrm{Cf}(^{18}\mathrm{O},4n)^{263}\mathrm{Sg}. This produced the ^{263}\mathrm{Sg}, with a measured production cross-section of approximately 1 picobarn (pb), allowing for the detection of only a few atoms through their characteristic chains. Subsequent confirmations and productions of isotopes have utilized alternative fusion-evaporation reactions at facilities like the (JINR) in . In the , researchers at confirmed the synthesis using the reaction ^{206}\mathrm{Pb}(^{54}\mathrm{Cr},1n)^{259}\mathrm{Sg}, employing the U-400 to accelerate ions onto lead targets, achieving cross-sections on the order of picobarns and detecting decay events consistent with . Modern experiments often favor the ^{248}\mathrm{Cm}(^{22}\mathrm{Ne},xn)^{266-x}\mathrm{Sg} reaction for producing isotopes like ^{265}\mathrm{Sg} and ^{266}\mathrm{Sg}, as it provides higher yields for chemical studies, with beam energies optimized around 120-130 MeV to maximize probability while minimizing losses. Known isotopes of seaborgium range from ^{257}\mathrm{Sg} to ^{271}\mathrm{Sg}, with the more commonly produced ones including ^{261}\mathrm{Sg} to ^{266}\mathrm{Sg}, all exhibiting short half-lives between approximately 0.1 seconds and 20 seconds. For instance, ^{261}\mathrm{Sg} has a half-life of 0.23 seconds, decaying primarily by alpha emission to ^{257}\mathrm{Rf}, while ^{265}\mathrm{Sg} shows a partial alpha half-life of about 11 seconds, also leading to rutherfordium daughters. ^{263}\mathrm{Sg}, the discovery isotope, decays by alpha emission with a half-life of 0.8 seconds to ^{259}\mathrm{Rf}, and ^{266}\mathrm{Sg} undergoes spontaneous fission with a half-life of 0.36 seconds. In June 2025, the isotope ^{257}\mathrm{Sg} was discovered at GSI/FAIR using the reaction ^{206}\mathrm{Pb}(^{52}\mathrm{Cr},1n)^{257}\mathrm{Sg}, with a half-life of 12.6 ms by spontaneous fission and alpha emission. These decays are genetically linked through alpha chains to known isotopes of lighter elements, confirming the atomic number Z=106. All detections rely on position-sensitive silicon detectors implanted at gas-filled recoil separators, such as the Berkeley Gas-Filled Separator (BGS) at LBNL or the Dubna Gas-Filled Recoil Separator (DGFRS) at JINR, which separate the heavy evaporation residues from the beam in milliseconds based on their charge-to-mass ratio in a helium-filled flight path. The naming of element 106 as (Sg) was the subject of international controversy in the , with competing claims from U.S. and teams leading to temporary designations like "unnilhexium." The International Union of Pure and Applied Chemistry (IUPAC) resolved the issue in 1997, officially adopting "" in honor of nuclear chemist , recognizing the LBNL team's priority in its discovery while acknowledging contributions from other groups.

Chemical properties

Oxidation states and reactivity

Group 6 elements—chromium (Cr), molybdenum (Mo), tungsten (W), and seaborgium (Sg)—exhibit a range of oxidation states, with +6 being the highest and most stable across the group, particularly in oxo compounds such as chromates, molybdates, and tungstates. Chromium commonly achieves +2, +3, and +6 states, with +3 being notably stable in aqueous solutions and +6 appearing in highly oxidizing, toxic species like chromate anions. Molybdenum and tungsten display states from +2 to +6, with +4 and +5 also prevalent, while +2 is rare for all members. Seaborgium, being synthetic and short-lived, has poorly characterized chemistry, but experimental evidence suggests +6 as the preferred state in aqueous media, forming oxo anions analogous to those of Mo and W. Reactivity patterns vary significantly due to these oxidation states. Chromium metal passivates rapidly in air at , forming a thin, protective Cr₂O₃ layer that inhibits further oxidation and confers resistance. In contrast, and are inert to and oxygen under ambient conditions but oxidize to their respective trioxides (MoO₃ and WO₃) upon heating in air above 400–500°C. behavior is illustrated by standard reduction potentials; for instance, the Cr³⁺/Cr²⁺ couple has E° = –0.424 V, indicating Cr²⁺ is a strong reductant, while the MoO₄²⁻/Mo(IV) couple in acidic media has E° ≈ +0.65 V, reflecting greater stability of the higher for . Down the group, the stability of the +6 state increases from to , attributed to progressively larger atomic sizes that better accommodate the high positive and facilitate d⁰ configurations in oxo species. This trend is evident in aqueous chemistry: chromium(III) exists primarily as the violet [Cr(H₂O)₆]³⁺ in acidic solutions, while chromate (CrO₄²⁻) predominates in basic media but dimerizes to dichromate in acid. (MoO₄²⁻) remains monomeric in neutral or basic conditions but polymerizes into complex polymolybdate anions, such as [Mo₇O₂₄]⁶⁻, under acidic conditions, showcasing greater tendency for cluster formation compared to chromate. behaves similarly to but with even higher +6 stability, forming polytungstates more readily.

Trends in group behavior

As atomic size increases down Group 6 from to , the +6 becomes more stable, with compounds exhibiting greater covalent character due to reduced on the central metal ion and enhanced orbital overlap. This trend is evident in the formation of stable hexavalent fluorocomplexes; for instance, (WF₆) is a stable, widely used gas, whereas chromium hexafluoride (CrF₆) decomposes readily above −100 °C. Relativistic effects play a crucial role in the heavier members, particularly tungsten and seaborgium, where the high nuclear charge accelerates inner electrons to speeds approaching 80% of the speed of light, causing contraction of the 6s orbital and expansion of the 5d orbitals. This results in higher electron density near the nucleus, increased ionization energies for 6s electrons (inert pair effect), and elevated densities (tungsten at 19.3 g/cm³, seaborgium predicted around 35 g/cm³), rendering seaborgium's chemistry highly analogous to tungsten's despite its position further down the group. Experimental studies confirm seaborgium forms volatile oxychlorides and carbonyl complexes similar to tungsten, validating periodic law predictions adjusted for these effects. Acid-base properties of the +6 oxoanions shift toward greater basicity down the group, reflecting decreasing acidity of the parent acids. (H₂CrO₄) is strongly acidic (pKₐ₁ ≈ 0.7), forming chromate (CrO₄²⁻) that hydrolyzes readily in neutral water, while molybdic (H₂MoO₄) and tungstic (H₂WO₄) acids are weaker (pKₐ₁ ≈ 3.7 for both), yielding (MoO₄²⁻) and (WO₄²⁻) anions that are more stable in basic conditions and form polymeric species like isopolyacids. This trend arises from increasing metal-oxygen bond lengths and reduced polarizing power. In catalytic applications such as (HDS) of feedstocks, and sulfides (often promoted with or ) outperform sulfides due to superior activity, sulfur tolerance, and resistance to under high-temperature conditions. Chromium-based catalysts show lower HDS rates and poorer selectivity for deep desulfurization of refractory compounds like dibenzothiophene, limiting their industrial use. A key anomaly in group trends is the pronounced toxicity of chromium in the +6 state; chromates are classified as Group 1 carcinogens by the International Agency for Research on Cancer due to their ability to cross cell membranes, generate reactive oxygen species, and induce DNA damage, whereas molybdates and tungstates in the +6 state exhibit much lower toxicity and carcinogenicity, with no such classification.

Compounds

Oxides and oxoacids

The trioxides of Group 6 elements in the +6 , MO₃ (M = , Mo, W), represent the highest oxides and exhibit increasing metallic character down the group. (CrO₃) is a dark red, deliquescent that behaves as a strong , dissolving in to form chromic acid solutions and acting as a powerful . (MoO₃) appears as a white to pale powder with a layered polymeric consisting of edge- and corner-sharing MoO₆ octahedra, showing limited in but reactivity with bases to form molybdates. (WO₃) is a featuring a defect-rich with oxygen vacancies that influence its electronic properties, rendering it nearly insoluble in yet amphoteric. Volatility decreases down the group, with CrO₃ subliming readily at around 197 °C, MoO₃ at 1155 °C, and WO₃ requiring high temperatures (melting at 1472 °C) before volatilization, reflecting stronger metal-oxygen bonding in heavier congeners. The corresponding oxoacids derive from these trioxides and display varying stability and condensation tendencies. (H₂CrO₄) is unstable, existing primarily in equilibrium with dichromate ions in solution and decomposing above 250 °C, which limits its isolation as a pure . (H₂MoO₄) and (H₂WO₄) are more stable but tend to form polymeric species in aqueous media, such as the heptamolybdate [Mo₇O₂₄]⁶⁻ or its protonated form H₂Mo₇O₂₄²⁻ under acidic conditions, and analogous isopolyoxotungstates for . These acids dehydrate upon heating to yield the respective trioxide anhydrides, a process facilitated by the labile oxygen atoms in the +6 state. In their deprotonated forms, the simple oxoanions chromate (CrO₄²⁻), (MoO₄²⁻), and (WO₄²⁻) adopt tetrahedral geometries with short M=O double bonds and longer M-O- single bonds, as confirmed by calculations and spectroscopic data. The amphoteric character of the oxides and oxoacids strengthens down the group: CrO₃ and chromate are predominantly acidic, reacting with bases to form salts but not appreciably with acids, whereas MoO₃ and WO₃, along with their derived acids, display dual reactivity, dissolving in both strong acids (to form lower valent species) and bases (to yield polyanions). For seaborgium, the superheavy homolog, experimental studies have confirmed the formation of volatile SgO₃, with adsorption and volatility behavior similar to its homologues, particularly tungsten. It is the most stable oxide, soluble in alkali to form oxyanions, consistent with relativistic predictions and group trends.

Halides and organometallic compounds

Group 6 elements form a variety of compounds, with stability and properties varying across the series due to increasing metal size and strength down the group. Chromium halides in the +3 , such as octahedral complexes like [CrF₆]³⁻ and [CrCl₆]³⁻, are common and exhibit significant tendencies in aqueous media, forming hydroxo . In contrast, the +6 halides are more prominent for heavier homologs; hexafluoride (MoF₆) is a volatile, hygroscopic colorless solid with a of 17.5°C and of 35°C, readily hydrolyzing to oxofluorides upon exposure to moisture. (WF₆) is a colorless gas at ( 17.1°C), notable for its use as a precursor in (CVD) processes to deposit thin films in manufacturing, owing to its high volatility and reactivity with reducing agents like or hydrogen. Lower halides of and include tetrachlorides such as MoCl₄ and WCl₄, which exist as polymeric chains or due to metal-metal bonding, displaying paramagnetic behavior consistent with d² electron configurations. compounds are particularly characteristic of in low s; for example, the octahedral Mo₆Cl₁₂ features Mo-Mo bonds and terminal chlorides, synthesized via high-temperature reduction of MoCl₅, and serves as a building block for extended structures with luminescent properties. Organometallic compounds of Group 6 elements highlight diverse coordination chemistries. Chromocene (Cp₂Cr), where Cp is cyclopentadienyl, is a sandwich complex with chromium in the +2 oxidation state, featuring a bent metallocene structure and high air sensitivity due to its 16-electron count. Molybdenum hexacarbonyl (Mo(CO)₆) is a prototypical 18-electron complex, with octahedral geometry stabilized by synergistic σ-donation from CO ligands and π-backbonding from the d⁶ metal center, commonly used as a source of Mo(0) in synthesis. Fischer-type carbene complexes, such as (CO)₅Cr=C(OMe)Ph, exemplify electrophilic carbenes with a metal-carbon double bond, where the chromium(0) center supports the carbene through donation to the empty p-orbital on carbon, enabling applications in organic synthesis like cyclopropanation. In Group 6 metal carbonyls, the CO stretching frequencies (ν_CO) are similar across the series at approximately 2000 cm⁻¹ (Cr(CO)₆ ~2000 cm⁻¹, Mo(CO)₆ ~2004 cm⁻¹, W(CO)₆ ~2000 cm⁻¹), indicating comparable π-backbonding strengths, with slight variations due to relativistic effects in leading to marginally stronger donation. For , relativistic effects and extrapolation from , supported by limited gas-phase experiments, confirm the formation of volatile fluorides like SgF₆, exhibiting high volatility suitable for separation in studies.

Applications

Industrial and technological uses

Group 6 elements, particularly , , and , play vital roles in various industrial and technological applications due to their unique chemical properties, such as resistance and catalytic efficiency. is extensively employed in surface treatments and chemical processes, while enhances refining and agricultural products. contributes to high-temperature and imaging technologies, whereas lacks practical applications owing to its instability. Chromium's primary industrial use is in , where it provides a hard, -resistant on metals like and aluminum, improving durability in automotive parts, tools, and household appliances. This process typically involves compounds to deposit a thin layer, enhancing aesthetic appeal and wear resistance. Additionally, compounds serve as pigments; for instance, lead chromate (PbCrO₄) produces vibrant yellow hues in paints and for protection and decorative purposes. In leather tanning, trivalent (Cr³⁺) salts fix hides by binding to proteins, yielding supple, durable for footwear and upholstery, accounting for a significant portion of global consumption in the industry. Global production of metal stands at approximately 15,000 metric tons annually, underscoring its economic importance. Molybdenum finds key applications as a catalyst in refining, where (MoS₂) facilitates to remove impurities from fuels, enabling compliance with environmental regulations and improving fuel quality. This compound's layered structure also makes it an effective solid lubricant, reducing friction in high-pressure environments like components and industrial machinery, where it outperforms traditional oils under extreme conditions. Furthermore, molybdates, such as , are added to fertilizers as a essential for in plants, boosting agricultural yields in molybdenum-deficient soils. Worldwide, molybdenum concentrate production reaches about 300,000 metric tons per year, reflecting its broad utility across energy and farming sectors. Tungsten is renowned for its use in filaments, where its high (3422°C) allows it to withstand operational temperatures, though largely phased out in favor of LEDs, it remains relevant in specialized . In , tungsten serves as the target material in tubes, generating high-energy X-rays through electron bombardment for diagnostic and computed scans. (WO₃) acts as a catalyst in chemical processes, including the epoxidation of olefins like to produce resins and glycols, offering selectivity and stability in heterogeneous systems. Seaborgium, being a synthetic with isotopes having half-lives under a second, has no known or technological uses and is confined to scientific research.

Alloys and catalysts

Group 6 elements, particularly , , and , play critical roles in advanced alloys due to their contributions to resistance, hardness, and high-temperature stability. is a key alloying element in stainless steels, where concentrations of 10-20% enable the formation of a passive layer that protects against in aggressive environments. This passivation mechanism enhances durability in structural applications, such as chemical processing equipment and medical implants. and are incorporated into high-speed tool steels at levels of 5-10% to improve red hardness and wear resistance, allowing tools to maintain sharpness during high-temperature machining operations. In nickel-based superalloys for blades, additions up to 10% by weight strengthen the material against and oxidation at elevated temperatures exceeding 1000°C, supporting efficient performance in engines. The exceptional resistance of these elements extends their use in extreme environments. Tungsten's high (3422°C) and thermal conductivity make it ideal for plasma-facing components in reactors, where it withstands intense heat fluxes and bombardment without significant . Similarly, 's ability to retain strength above 2000°C qualifies it for and components, such as nozzles and structural parts, where it resists thermal fatigue during . Recycling of these alloys is highly efficient, with recovery rates from scrap exceeding 90% through processes like melting, minimizing resource depletion and environmental impact. In catalysis, Group 6 elements enable key industrial processes through heterogeneous systems involving surface adsorption on sulfides or oxides. Molybdenum-based CoMo/Al₂O₃ catalysts are widely used in hydrotreating to remove sulfur from petroleum feedstocks, where cobalt-promoted molybdenum sulfide phases facilitate hydrogenolysis and hydrogenation via edge-site adsorption of organosulfur compounds. Tungsten and molybdenum imido alkylidene complexes, developed by Schrock, serve as highly active olefin metathesis catalysts, promoting carbon-carbon bond rearrangements in polymer synthesis; these operate through a metal carbene mechanism with turnover frequencies up to 0.6 s⁻¹ for propene metathesis on supported MoOₓ sites. Chromium-silica Phillips catalysts drive ethylene polymerization to high-density polyethylene (HDPE), accounting for over 40% of global production, by initiating chain growth on Cr²⁺ sites with adsorption of monomer onto coordinatively unsaturated surface species. These catalytic mechanisms highlight the elements' versatility in facilitating selective transformations under mild conditions.

Biological and environmental role

Essentiality in organisms

Group 6 elements exhibit varying degrees of essentiality in biological systems, with and recognized as trace nutrients required for human health, while plays roles in certain microorganisms and has none due to its synthetic nature. Trivalent chromium (Cr³⁺) has been proposed as an essential trace element that may enhance insulin action, primarily through its role in chromodulin, a low-molecular-weight chromium-binding . This cofactor may facilitate insulin-mediated in cells, supporting , , and . However, as of 2025, the essentiality of chromium remains controversial, with some experts arguing it functions only pharmacologically rather than as a required . The adequate intake (AI) for adults is 25–35 µg per day, with higher amounts sometimes recommended in supplemental contexts up to 200 µg for metabolic support. Deficiency symptoms have been reported in rare cases of long-term without supplementation, including impaired glucose tolerance, though evidence is inconclusive and no clear dietary deficiency occurs in healthy populations. Chromium is influenced by and content, with whole grains such as and whole serving as primary sources, providing 1–2 µg per serving. Molybdenum functions as a cofactor in several enzymes via the molybdenum cofactor (Moco), a molybdopterin-based structure essential for redox reactions in , , and . In humans and animals, it is critical for , which catalyzes the oxidation of purines to , and sulfite oxidase, which detoxifies s from metabolism. In and nitrogen-fixing , is a key component of , enabling atmospheric for symbiotic nutrient cycles. The recommended dietary allowance (RDA) for adults is 45 µg per day, supporting these enzymatic activities without excess accumulation. Deficiency is uncommon but can manifest as neurological disorders, such as in molybdenum cofactor deficiency syndrome, leading to severe . homeostasis in humans is maintained by transporters like MOT2, which regulate uptake and intracellular distribution, while is high from such as lentils and black beans, which can provide up to 130 µg per half-cup serving. Tungsten, the heaviest element with known biological function, substitutes for in some and , forming tungstopterin cofactors in enzymes such as formate dehydrogenase and aldehyde ferredoxin oxidoreductase, which support anaerobic metabolism and in extreme environments. These roles are absent in higher organisms, and is not essential for humans, where it shows no incorporation into functional enzymes. , a synthetic with isotopes having half-lives of seconds to minutes, exhibits no biological role due to its instability and radioactivity, preventing any interaction with .

Toxicity and precautions

Group 6 elements exhibit varying degrees of toxicity depending on their oxidation states, forms, and exposure routes, with presenting the most significant health risks among the stable elements. (Cr(VI)) is a known , primarily associated with following of dusts, mists, or fumes in occupational settings such as and . In contrast, trivalent chromium (Cr(III)), which is less soluble and mobile, is far less toxic and even essential in trace amounts for metabolism. The U.S. (OSHA) has established a (PEL) of 5 µg/m³ for Cr(VI) as an 8-hour time-weighted average to mitigate these risks. Molybdenum toxicity in humans is generally low, with acute effects rare even at elevated exposures, though chronic overconsumption can lead to gout-like symptoms such as joint pain and elevated levels. In animals, particularly , chronic excess causes molybdenosis, characterized by , , and bone abnormalities due to interference with absorption. The tolerable upper intake level for in adults is 2 mg per day, beyond which adverse effects may occur, though some studies report symptoms at intakes of 10–15 mg daily over extended periods. Tungsten demonstrates low overall toxicity to humans, with soluble forms readily excreted via urine, but insoluble dust can act as a respiratory irritant, causing coughing, , and potential upon prolonged . Environmentally, tungsten persists in soils due to its low mobility and resistance to degradation, raising concerns about long-term accumulation near sites or discharges. Seaborgium, as a synthetic , poses extreme hazards primarily from its intense rather than chemical ; its isotopes rapidly, emitting alpha particles and gamma that can cause severe . Handling seaborgium requires specialized laboratory protocols, including remote manipulation, , and glove boxes to prevent . Precautions for Group 6 elements emphasize exposure control and monitoring, particularly in applications. In plating operations, local exhaust systems are essential to capture Cr(VI) mists and dusts, while wastewater from chromate processes must undergo —such as to Cr(III) followed by —to prevent release of toxic effluents. through urinary levels is recommended for workers, with levels above 15–30 µg/g indicating potential overexposure and necessitating further evaluation. For molybdenum and tungsten, standard respiratory protection and dust control suffice in and , with no specific upper limits beyond general practices. Environmental concerns arise from mining and military uses of these elements. Chromium and molybdenum extraction can contaminate soils and waterways with runoff, leading to in aquatic organisms and disruption. Tungsten-based munitions, often alloyed with and as alternatives, have sparked toxicity debates due to their fragmentation into respirable particles, potentially mimicking uranium's nephrotoxic and carcinogenic effects in conflict zones. Remediation strategies include for chromium-polluted sites and regulatory limits on discharges to protect integrity.