What oxidation states does mercury have?

Mercury has three oxidation states: 0 (elemental liquid mercury), +1 (mercurous, always as the Hg₂²⁺ dimer — never as a simple Hg⁺ monomer), and +2 (mercuric, the most versatile state). A rare +4 state in HgF₄ has been detected at very low temperatures but has no practical chemistry.

Why does mercury form Hg₂²⁺ instead of simple Hg⁺?

A simple Hg⁺ monomer would have the configuration [Xe] 4f¹⁴ 5d¹⁰ 6s¹ — a single unpaired 6s electron. Two Hg⁺ atoms spontaneously pair their 6s¹ electrons into a Hg–Hg covalent bond, forming the diamagnetic [Hg–Hg]²⁺ dimer. Relativistic contraction of the 6s orbital in heavy mercury strengthens this bond and stabilizes the dimer. Isolated Hg⁺ monomers are not stable under ordinary conditions.

Which mercury oxidation state is more toxic?

All mercury species are toxic, but the form matters. Elemental Hg(0) vapor is readily absorbed through the lungs. Hg²⁺ (mercuric) salts are acutely toxic, damaging kidneys and the nervous system on ingestion. Methylmercury (CH₃Hg⁺, formed by microbial methylation of Hg²⁺) is the most dangerous environmental form because it bioaccumulates in the food chain and crosses the blood–brain barrier. Hg₂²⁺ mercurous salts are less readily absorbed but still toxic.

What is calomel and what oxidation state does mercury have in it?

Calomel is mercury(I) chloride, Hg₂Cl₂. Mercury carries the +1 oxidation state (as part of the Hg₂²⁺ dimer). It is a white powder, sparingly soluble in water, historically used as a purgative and in skin-lightening creams. Today its main use is in the calomel electrode (Hg/Hg₂Cl₂/KCl), a common reference electrode in electrochemistry.

What is the difference between mercurous and mercuric compounds?

"Mercurous" refers to mercury in the +1 oxidation state (in the Hg₂²⁺ dimer), and "mercuric" refers to mercury in the +2 state. For example: Hg₂Cl₂ is mercurous chloride (calomel) while HgCl₂ is mercuric chloride (highly toxic corrosive sublimate). Mercuric compounds are generally more soluble and more acutely toxic than their mercurous counterparts.

Does mercury have a +4 oxidation state?

HgF₄ — mercury(IV) fluoride — has been synthesized and detected at very low temperatures in noble gas matrices. It is the only confirmed +4 mercury compound. It is extremely reactive and has no practical chemistry. Mercury +4 requires removing electrons from the filled 5d¹⁰ shell and is energetically very unfavorable at room temperature.

Mercury Oxidation States

Reference for all mercury oxidation states: 0 (liquid metal), +1 (the unique Hg₂²⁺ dimer — mercurous), and +2 (mercuric). Includes electron configurations, common compounds, toxicity context, and why mercury forms a stable +1 dimer unlike most metals.

Atomic # 80 Hg Mercury

Atomic Mass

200.59 u

Group

12 (IIB)

Period

Block

d-block

Electronegativity

2.00 (Pauling)

Oxidation States

0, +1 (Hg₂²⁺), +2

Mercury's defining chemistry: Unlike zinc and cadmium, mercury forms a stable +1 oxidation state as the Hg₂²⁺ dimer — two mercury atoms joined by a Hg–Hg covalent bond. Relativistic contraction of the 6s orbital strengthens this bond and explains why mercury is both a liquid metal at room temperature and the only metal with a well-characterized diatomic +1 ion in ordinary chemistry.

Mercury (Z=80) ground-state configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s². Three accessible oxidation states exist under practical conditions: 0 (liquid metal), +1 (as the Hg₂²⁺ dimer), and +2 (mercuric). A +4 state in HgF₄ has been observed only at cryogenic temperatures.

State	Ion / Form	Config (after [Xe] 4f¹⁴)	Stability	Notes
0	Hg (metal)	5d¹⁰ 6s²	Elemental	Liquid metal at room temperature (mp −38.8 °C). Used in thermometers, barometers, switches, and fluorescent lamps. Hg vapor is toxic.
+1	Hg₂²⁺ (dimer)	5d¹⁰ 6s¹ (per Hg)	Stable — dimer only	Exists exclusively as the [Hg–Hg]²⁺ dimer. Each mercury formally carries +1 but the two 6s¹ electrons form a covalent Hg–Hg bond. No stable Hg⁺ monomer exists. Example: Hg₂Cl₂ (calomel).
+2	Hg²⁺	5d¹⁰	Most versatile	Most common state in solution and coordination chemistry. Forms ionic and covalent compounds. HgCl₂, HgO, HgS (cinnabar/vermilion), Hg(NO₃)₂, organomercury species.
+4	Hg⁴⁺	5d⁸	Cryogenic only	Observed only in HgF₄ at cryogenic temperatures in noble gas matrices. Requires removing 5d electrons. No practical chemistry.

Relativistic effects and the liquid state: Mercury's 6s electrons undergo significant relativistic contraction — a quantum effect that grows with atomic number. This makes the 6s orbital smaller and lower in energy, reducing Hg–Hg metallic bonding in the solid and lowering the melting point below room temperature. The same contraction strengthens the Hg–Hg bond in Hg₂²⁺ and reduces mercury's reactivity compared to zinc and cadmium.

Toxicity summary: All mercury oxidation states are toxic. Hg(0) vapor causes neurological damage on inhalation. Hg²⁺ (mercuric) causes acute kidney and GI damage. Methylmercury (CH₃Hg⁺), formed by microbial methylation of Hg²⁺, is the most hazardous: it bioaccumulates through the food chain and crosses the blood–brain barrier. Hg₂²⁺ (mercurous) compounds are less soluble and less readily absorbed but still toxic.

Mercury forms compounds in the 0, +1 (always as Hg₂²⁺), and +2 states. Mercuric (+2) compounds are the most diverse and most soluble. Mercurous (+1) compounds are generally less soluble and less acutely toxic. All mercury compounds require careful handling due to toxicity.

Compound	Formula	Hg State	Notes
Mercury (liquid metal)	Hg	0	Only metallic element liquid at room temperature (bp 357 °C). Used historically in thermometers, barometers, electrical switches. Now largely replaced due to toxicity.
Mercury(I) chloride (calomel)	Hg₂Cl₂	+1	White insoluble powder. Historically used as a purgative and in skin creams. Still used as the calomel reference electrode in electrochemistry.
Mercury(I) nitrate	Hg₂(NO₃)₂	+1	White crystalline solid; soluble in dilute nitric acid. Used in synthesis to prepare other Hg(I) compounds.
Mercury(I) sulfate	Hg₂SO₄	+1	White to slightly yellow solid; sparingly soluble. Used in the Weston standard cell and as a reference electrode electrolyte.
Mercury(I) iodide	Hg₂I₂	+1	Pale yellow solid; sparingly soluble. Slowly decomposes to HgI₂ and Hg on exposure to light.
Mercury(II) chloride	HgCl₂	+2	"Corrosive sublimate" — highly toxic white crystalline solid; once used as an antiseptic. Important reagent in organic mercury chemistry.
Mercury(II) oxide	HgO	+2	Red or yellow solid. Decomposes on heating to Hg and O₂ (Priestley's 1774 oxygen discovery). Used in mercury-oxide batteries.
Mercury(II) sulfide (cinnabar)	HgS	+2	Bright red mineral (cinnabar) — the main mercury ore and the pigment vermilion. Black metacinnabar is the other polymorph. Very low solubility.
Mercury(II) iodide	HgI₂	+2	Brilliant red solid (yellow polymorph above 127 °C). Used in radiation detection, X-ray imaging sensors, and historically as a toxicology reagent.
Mercury(II) nitrate	Hg(NO₃)₂	+2	Colorless crystalline solid; highly soluble. Used in Millon's reagent for protein detection and in nitration reactions.
Mercury(II) sulfate	HgSO₄	+2	White crystalline solid. Catalyst for the industrial hydration of acetylene to acetaldehyde (Wacker process precursor).
Dimethylmercury	(CH₃)₂Hg	+2	Extremely toxic organomercury compound. Absorbs through latex gloves. Fatal in microgram quantities; caused the death of chemist Karen Wetterhahn (1997).
Methylmercury chloride	CH₃HgCl	+2	Environmental toxin; forms via microbial methylation of Hg²⁺ in aquatic sediments. Bioaccumulates in fish. Minamata disease agent.
Thiomersal (thimerosal)	C₉H₉HgNaO₂S	+2	Organomercury antiseptic and preservative used in multi-dose vaccine vials and contact lens solutions.

Ground State Configuration

[Xe]

…×54

Core (54 electrons from xenon)

…×14

Core — fully filled 4f¹⁴; lanthanide contraction compresses Hg

Valence — fully filled 5d¹⁰; retained in Hg⁺ (dimer) and Hg²⁺

Outer shell — relativistically contracted; both electrons lost → Hg²⁺

Notation

Full: [Xe] 4f¹⁴ 5d¹⁰ 6s²

Hg (neutral): [Xe] 4f¹⁴ 5d¹⁰ 6s²

Hg⁺ (in Hg₂²⁺): [Xe] 4f¹⁴ 5d¹⁰ 6s¹

Hg²⁺ (mercuric): [Xe] 4f¹⁴ 5d¹⁰

Ionization Steps

[Xe] 4f¹⁴ 5d¹⁰ 6s²

Neutral atom — liquid metal at 25 °C

Hg⁺

[Xe] 4f¹⁴ 5d¹⁰ 6s¹

IE₁ ≈ 1007 kJ/mol — dimerizes immediately to Hg₂²⁺ via Hg–Hg bond

Hg²⁺

[Xe] 4f¹⁴ 5d¹⁰

IE₂ ≈ 1810 kJ/mol — filled 5d¹⁰; most versatile state in solution

Hg³⁺

[Xe] 4f¹⁴ 5d⁹

IE₃ ~3300 kJ/mol est. — breaks 5d¹⁰; not observed in solution

Hg⁴⁺

[Xe] 4f¹⁴ 5d⁸

IE₄ extremely high — only in HgF₄ at cryogenic T

The Hg₂²⁺ dimer vs. a simple Hg⁺ monomer: Isolated Hg⁺ would have one unpaired 6s electron. Two Hg⁺ atoms pair their 6s¹ electrons into a σ bond, releasing ~230 kJ/mol of bond energy. Relativistic contraction of the 6s orbital makes this bond unusually strong for a metal–metal bond, stabilizing the dimer over the monomer. The resulting [Hg–Hg]²⁺ is diamagnetic (no unpaired electrons) — confirmed by its observed diamagnetic behavior.

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Summary

Mercury (Hg, atomic number 80) is a Group 12 d-block metal in Period 6 with the ground-state electron configuration [Xe] 4f¹⁴ 5d¹⁰ 6s². It is the only metallic element that is liquid at room temperature. Mercury displays three oxidation states: 0 (elemental), +1 (the mercurous ion, which is uniquely a Hg–Hg bonded dimer Hg₂²⁺), and +2 (mercuric, the most versatile state). Unlike its Group 12 congeners zinc and cadmium — which also strongly favor +2 — mercury's larger, more polarizable 5d orbitals and relativistic effects stabilize the Hg–Hg bond, making the +1 dimer a common and well-characterized species (e.g., Hg₂Cl₂, calomel). All mercury species are toxic, with methylmercury (from Hg²⁺ methylation) being the most bioaccumulative environmental form. This reference covers all oxidation states with electron configurations, common compounds, stability context, and toxicity notes.

How it works

Click a tab — Oxidation States, Compounds, Electron Config, or Physical Props — to navigate sections.
The Oxidation States panel shows all known states with stability badges, electron configs, and notes.
The Compounds panel lists real mercury compounds grouped by oxidation state with formulas and uses.
The Electron Config panel shows orbital filling for Hg(0), Hg⁺ (in Hg₂²⁺), and Hg²⁺ with ionization steps.
The Physical Props panel lists atomic and material data for quick reference.
Click any monospace table cell to copy its content to the clipboard.

Use cases

Students studying Group 12 chemistry and why mercury forms a stable +1 dimer.
Chemistry teachers explaining the Hg₂²⁺ ion and how it differs from a simple Hg⁺ monomer.
Toxicologists and environmental chemists tracking mercury oxidation states in contaminated sites.
Researchers working with organomercury compounds or mercury catalysis.
Anyone preparing for exams covering Period 6 transition metals and relativistic effects.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu