Arsenic (As): Properties, Reactions, and Importance

Introduction: Arsenic’s Relevance

Arsenic (As) is a chemical element classified as a metalloid. Its significance stems from its unique properties, leading to applications in semiconductor technology, alloys, and historical use in pesticides. Crucially, its pronounced toxicity and carcinogenic nature make it a significant environmental and health concern. Understanding arsenic’s chemistry is vital for comprehending its roles in both beneficial technologies and detrimental environmental contexts.

CBSE/JEE Quick Revision Notes

• Symbol: As
• Atomic Number: 33
• Atomic Mass: 74.92 g/mol
• Group: 15 (Pnictogens)
• Period: 4
• Block: p-block
• Nature: Metalloid
• Common Oxidation States: -3, +3, +5
• Valency: 3, 5
• Electronegativity (Pauling): 2.18
• Allotropes: Grey (metallic), Yellow (molecular As₄), Black (amorphous)
• Density (Grey As): 5.72 g/cm³
• Melting Point (Grey As): 817 °C (at 28 atm)
• Sublimation Point: 615 °C (at 1 atm)

Electron Configuration & Bonding Behavior

Electronic Configuration

The ground state electronic configuration of Arsenic is: [Ar] 3d¹⁰ 4s² 4p³

Bonding Characteristics

• Valence Shell Configuration: 4s² 4p³
• Oxidation States:
  • +3 Oxidation State: Achieved by utilizing the three 4p electrons. This state is common and stable due to the “inert pair effect” where the 4s electrons are less available for bonding. Compounds like AsCl₃ and As₂O₃ exhibit this state. The geometry is typically trigonal pyramidal due to the presence of a lone pair on As.
  • +5 Oxidation State: Achieved by utilizing all five valence electrons (4s² 4p³). This requires promotion of one 4s electron to an empty 4d orbital, or involvement of all five valence electrons in bonding. Compounds like AsF₅ and As₂O₅ show this state. The geometry is often trigonal bipyramidal.
  • -3 Oxidation State: Observed in arsenides with highly electropositive metals (e.g., Na₃As), where arsenic gains three electrons to achieve a stable octet.
• Covalent Bonding: Arsenic primarily forms covalent compounds. It forms strong covalent bonds with less electronegative elements.
• Coordination Number: Can vary, commonly 3 or 5.

Crucial Chemical Reactions

1. Reaction with Oxygen (Air)

Arsenic readily combines with oxygen upon heating.

• Formation of Arsenic(III) oxide (Arsenic trioxide): 4As (s) + 3O₂ (g) → 2As₂O₃ (s)
• Formation of Arsenic(V) oxide (Arsenic pentoxide) at higher temperatures/oxygen pressures: 4As (s) + 5O₂ (g) → 2As₂O₅ (s)

2. Reaction with Halogens

Arsenic reacts vigorously with halogens, typically forming trihalides and pentahalides.

• With Chlorine (Trihalide): 2As (s) + 3Cl₂ (g) → 2AsCl₃ (l)
• With Fluorine (Pentahalide, due to high electronegativity of F): 2As (s) + 5F₂ (g) → 2AsF₅ (g)

3. Reaction with Metals

Arsenic forms arsenides with electropositive metals.

• With Sodium: 2As (s) + 6Na (s) → 2Na₃As (s)

4. Reaction with Acids

Arsenic is generally unreactive with non-oxidizing acids. It reacts with oxidizing acids.

• With Concentrated Nitric Acid: 2As (s) + 6HNO₃ (conc) → 2H₃AsO₃ (aq) + 6NO₂ (g) + 2H₂O (l) (Forms Arsenic(III) acid, then slowly oxidizes to Arsenic(V) acid)
• With Dilute Nitric Acid: 2As (s) + 10HNO₃ (dil) → 2H₃AsO₄ (aq) + 10NO (g) + 4H₂O (l) (Forms Arsenic(V) acid)

5. Marsh’s Test (Detection of Arsenic)

This is a highly sensitive method for detecting arsenic.

• Arsenic compounds are reduced by nascent hydrogen (generated by Zn + HCl) to arsine gas (AsH₃). As₂O₃ (s) + 6Zn (s) + 12HCl (aq) → 2AsH₃ (g) + 6ZnCl₂ (aq) + 3H₂O (l)
• Arsine gas is then passed through a heated tube, where it decomposes to elemental arsenic, forming a characteristic shiny black mirror. 2AsH₃ (g) --(heat)--> 2As (s, mirror) + 3H₂ (g)

Industrial and Biological Importance

Industrial Importance

• Semiconductor Industry: Gallium arsenide (GaAs) is a crucial semiconductor material used in high-speed integrated circuits, LEDs, laser diodes, and photovoltaic devices due to its superior electron mobility compared to silicon.
• Alloys: Arsenic is used to harden lead alloys for applications such as lead shot, battery grids, and cable sheathing.
• Wood Preservatives (Historical/Restricted): Chromated copper arsenate (CCA) was widely used to treat wood against decay and insects. Its use is now heavily restricted or banned in many regions due to environmental and health concerns.
• Pesticides and Herbicides (Historical/Restricted): Arsenical compounds (e.g., lead arsenate, sodium arsenite) were historically used as insecticides and herbicides. Most agricultural uses have been phased out due to toxicity.
• Glass Manufacturing: Arsenic compounds act as a refining agent and decolourizer in glass production, removing bubbles and neutralizing unwanted tints.

Biological Importance

• Toxicity: Arsenic compounds are notoriously toxic to most living organisms.
  • Mechanism: Inorganic arsenic interferes with cellular respiration, enzyme activity (especially those containing sulfhydryl groups), and oxidative phosphorylation, leading to multi-organ damage.
  • Carcinogenicity: Arsenic is classified as a Group 1 human carcinogen by the International Agency for Research on Cancer (IARC), associated with various cancers (skin, lung, bladder, liver, kidney).
• Medicinal Use: Despite its toxicity, arsenic trioxide (As₂O₃) has found a specific niche in medicine. It is approved for the treatment of acute promyelocytic leukemia (APL), a type of blood cancer, demonstrating its potential as a targeted therapeutic agent.
• Environmental Contamination: Natural geological sources can lead to high levels of arsenic in groundwater, posing a severe public health crisis in many parts of the world, particularly in South and Southeast Asia (e.g., Bangladesh, West Bengal in India). Chronic exposure through contaminated drinking water leads to arsenicosis, characterized by skin lesions, neurological effects, and increased cancer risk.