Mercury (Hg) Study Guide: Properties, Reactions & Uses

Introduction: The Quicksilver Element

Mercury (Hg), traditionally known as “quicksilver,” is the only metallic element that is liquid at standard temperature and pressure. Its unique properties, including high density and excellent electrical conductivity, led to its widespread use in instruments and industrial processes. However, its significant toxicity has led to a global effort to reduce its use and mitigate environmental contamination. Understanding Mercury’s chemical behavior is crucial for addressing its environmental impact and for competitive examinations.

CBSE/JEE Quick Revision Notes

• Symbol: Hg (from Hydrargyrum, meaning “water-silver”)
• Atomic Number: 80
• Atomic Mass: 200.59 u
• Group: 12 (IIB)
• Period: 6
• Block: d-block (often considered a post-transition metal due to filled d-subshell)
• Nature at STP: Liquid metal
• Density: 13.534 g/cm³ at 25 °C (very high)
• Melting Point: -38.83 °C
• Boiling Point: 356.73 °C
• Common Oxidation States/Valencies: +1 (as dimeric Hg₂²⁺, mercurous) and +2 (as Hg²⁺, mercuric).
• Conductivity: Good electrical conductor, poor thermal conductor (compared to other metals).

Electron Configuration & Bonding Behavior

• Electron Configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s²
• Reason for Liquid State: The electron configuration of Mercury, with a completely filled 5d¹⁰ subshell and a filled 6s² subshell, results in weak metallic bonding. The d electrons are not easily promoted or involved in metallic bonding, leading to weak interatomic forces and thus a low melting point, making it liquid at room temperature.
• Oxidation States:
  • +2 Oxidation State (Mercuric): Formed by the loss of the two 6s electrons, leading to the stable Hg²⁺ ion. This is the most common oxidation state.
  • +1 Oxidation State (Mercurous): Exists primarily as a dimeric cation, Hg₂²⁺. In this ion, two Hg⁺ ions are covalently bonded together (⁺Hg—Hg⁺). This unusual dimeric form is due to the tendency of Hg⁺ to achieve a more stable electron configuration by pairing with another Hg⁺.
• Noble Character: Due to its high ionization energy and the stability of its filled electron shells, Mercury is relatively unreactive compared to other transition metals.

Crucial Chemical Reactions

1. Reaction with Oxygen/Air

• Mercury does not readily react with oxygen at room temperature.
• Upon heating, it reacts to form mercuric oxide. 2Hg(l) + O₂(g) → 2HgO(s) (at ~350 °C)
• Mercuric oxide decomposes back into mercury and oxygen at higher temperatures (~500 °C), a reaction historically used by Priestley to prepare oxygen. 2HgO(s) → 2Hg(l) + O₂(g) (at ~500 °C)

2. Reaction with Acids

• Non-oxidizing acids (e.g., HCl, dilute H₂SO₄): Mercury is unreactive with these acids.
• Oxidizing acids:
  • Dilute Nitric Acid: Forms mercurous nitrate. 6Hg(l) + 8HNO₃(dilute) → 3Hg₂(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)
  • Concentrated Nitric Acid: Forms mercuric nitrate. Hg(l) + 4HNO₃(conc) → Hg(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)
  • Hot Concentrated Sulfuric Acid: Forms mercuric sulfate. Hg(l) + 2H₂SO₄(conc, hot) → HgSO₄(s) + SO₂(g) + 2H₂O(l)

3. Reaction with Halogens

• Mercury reacts directly with halogens to form halides. The product (mercurous or mercuric) depends on the stoichiometry and conditions.
  • With excess mercury: Forms mercurous halides. 2Hg(l) + Cl₂(g) → Hg₂Cl₂(s) (Mercurous Chloride, Calomel)
  • With excess halogen: Forms mercuric halides. Hg(l) + Cl₂(g) → HgCl₂(s) (Mercuric Chloride, Corrosive Sublimate)

4. Reaction with Sulfur

• Mercury reacts with sulfur, even at room temperature, to form mercuric sulfide. This reaction is used to clean up mercury spills. Hg(l) + S(s) → HgS(s) (Mercuric Sulfide, Vermilion when red)

5. Amalgam Formation

• Mercury forms alloys called amalgams with most metals, including gold, silver, and sodium. Iron and platinum are notable exceptions. Hg(l) + Na(s) → Na-Hg (Sodium Amalgam)

Industrial and Biological Importance

Industrial Importance

• Historical Uses (decreasing due to toxicity):
  • Thermometers and Barometers: Due to its high coefficient of thermal expansion, high density, and non-wetting properties.
  • Dental Amalgams: Alloys of mercury with silver, tin, and copper used for dental fillings.
  • Electrical Switches and Relays: Its liquid state and conductivity make it useful.
  • Fluorescent and Mercury-Vapor Lamps: Generates UV light when ionized, which then excites phosphors.
• Chlor-Alkali Process (largely replaced): Historically, it was used as a flowing cathode to produce sodium hydroxide and chlorine from brine. This process, however, led to significant mercury pollution.
• Gold and Silver Extraction (highly restricted/obsolete): The amalgamation process used mercury to dissolve fine particles of gold and silver from their ores, forming an amalgam that could then be heated to recover the precious metals. This method caused widespread mercury contamination.
• Catalyst: Used in some specific chemical syntheses, such as the production of acetaldehyde from acetylene (though catalysts like palladium are now preferred).

Biological Importance

• Toxicity: Mercury and its compounds are highly toxic to living organisms. It has no known beneficial biological role in humans.
  • Elemental Mercury (Hg): Mercury vapor is readily absorbed through the lungs and can cross the blood-brain barrier, causing neurological damage. Liquid elemental mercury is poorly absorbed if ingested.
  • Inorganic Mercury Compounds (e.g., HgCl₂, HgS): Primarily affect the kidneys and gastrointestinal tract.
  • Organic Mercury Compounds (e.g., Methylmercury, CH₃Hg⁺): The most dangerous form. It is highly soluble in lipids, accumulates in the brain, and bioaccumulates in food chains (e.g., fish). It causes severe neurological damage, developmental abnormalities, and Minamata disease.
• Environmental Concern: Mercury released into the environment (from industrial emissions, coal combustion, mining) can be converted by microorganisms into methylmercury, which then enters the aquatic food chain and biomagnifies, posing a serious threat to wildlife and human health.