Antimony (Sb): Properties, Reactions & Uses

Introduction

Antimony (Sb) is a metalloid element with significant applications across various industries. Its unique properties, combining characteristics of both metals and non-metals, make it indispensable in areas such as flame retardants, alloys, and semiconductor technology. Understanding Antimony’s chemical behavior and applications is crucial for competitive examinations.

CBSE/JEE Quick Revision Notes

• Symbol: Sb (from Latin stibium)
• Atomic Number: 51
• Atomic Mass: 121.76 u
• Group: 15 (Pnictogens)
• Period: 5
• Block: p-block
• Nature: Metalloid (exhibits both metallic and non-metallic properties)
• Common Oxidation States: +3, +5 (most stable), -3 (less common, usually in antimonides)
• Valency: 3, 5
• Physical State at STP: Solid
• Allotropes:
  • Metallic Antimony: The stable, most common form; silvery-white, brittle solid.
  • Yellow Antimony: Amorphous, unstable, formed by the reaction of stibine ($\text{SbH}_3$) with chlorine.
  • Explosive Antimony: Amorphous, unstable, formed by electrolysis of $\text{SbCl}_3$.

Electron Configuration & Bonding Behavior

Electron Configuration

• Full Configuration: $\text{1s}^2 \text{2s}^2 \text{2p}^6 \text{3s}^2 \text{3p}^6 \text{3d}^{10} \text{4s}^2 \text{4p}^6 \text{4d}^{10} \text{5s}^2 \text{5p}^3$
• Condensed Configuration: $\text{[Kr]} \text{4d}^{10} \text{5s}^2 \text{5p}^3$
• Valence Shell Configuration: $\text{5s}^2 \text{5p}^3$

Bonding Behavior

Antimony, being in Group 15, has 5 valence electrons.

• +3 Oxidation State: Formed by the loss or sharing of the three p-electrons, leaving the s-electron pair inert (inert pair effect). Examples: $\text{SbCl}_3$, $\text{Sb}_2\text{O}_3$.
• +5 Oxidation State: Formed by the loss or sharing of all five valence electrons ($\text{5s}^2 \text{5p}^3$). Examples: $\text{SbF}_5$, $\text{Sb}_2\text{O}_5$. The +5 oxidation state is less stable than +3 due to the inert pair effect, especially down the group.
• -3 Oxidation State: Exhibited in antimonides (e.g., $\text{Mg}_3\text{Sb}_2$) where Antimony accepts three electrons to achieve a stable octet, similar to non-metals.

Crucial Chemical Reactions

1. Reaction with Air/Oxygen

Antimony burns in air or oxygen to form antimony trioxide ($\text{Sb}_2\text{O}_3$). $4\text{Sb(s)} + 3\text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{Sb}_2\text{O}_3\text{(s)}$ Antimony trioxide is amphoteric, reacting with both acids and bases.

2. Reaction with Halogens

Antimony reacts vigorously with halogens to form trihalides or pentahalides.

• With Chlorine:
  • To form Antimony(III) chloride: $2\text{Sb(s)} + 3\text{Cl}_2\text{(g)} \rightarrow 2\text{SbCl}_3\text{(s)}$
  • With excess chlorine, to form Antimony(V) chloride: $2\text{Sb(s)} + 5\text{Cl}_2\text{(g)} \rightarrow 2\text{SbCl}_5\text{(l)}$

3. Reaction with Acids

Antimony does not react with non-oxidizing acids. It reacts with strong oxidizing acids.

• With Hot Concentrated Nitric Acid: Forms antimony pentoxide or meta-antimonious acid. $2\text{Sb(s)} + 10\text{HNO}_3\text{(conc.)} \rightarrow \text{Sb}_2\text{O}_5\text{(s)} + 10\text{NO}_2\text{(g)} + 5\text{H}_2\text{O(l)}$ Alternatively, $2\text{Sb(s)} + 5\text{HNO}_3\text{(conc.)} \rightarrow 2\text{HSbO}_3\text{(s)} + 5\text{NO}_2\text{(g)} + \text{H}_2\text{O(l)}$ (meta-antimonious acid)
• With Hot Concentrated Sulfuric Acid: Forms antimony(III) sulfate. $2\text{Sb(s)} + 6\text{H}_2\text{SO}_4\text{(conc.)} \rightarrow \text{Sb}_2(\text{SO}_4)_3\text{(aq)} + 3\text{SO}_2\text{(g)} + 6\text{H}_2\text{O(l)}$

4. Reaction with Metals

Antimony forms intermetallic compounds called antimonides when heated with active metals. $3\text{Mg(s)} + 2\text{Sb(s)} \xrightarrow{\text{Heat}} \text{Mg}_3\text{Sb}_2\text{(s)}$ (Magnesium antimonide)

Industrial and Biological Importance

Industrial Importance

• Alloys: Improves hardness and mechanical strength. Used in lead-acid batteries (increases durability), bullet manufacturing, plain bearings (Babbitt metal), and type metal.
• Flame Retardants: Antimony trioxide ($\text{Sb}_2\text{O}_3$) is a common synergistic agent in flame retardants for plastics, textiles, and rubber. It enhances the efficiency of halogenated flame retardants.
• Semiconductors: High-purity antimony is used in the manufacturing of diodes, infrared detectors, and Hall-effect devices. Indium antimonide ($\text{InSb}$) and Gallium antimonide ($\text{GaSb}$) are important semiconductor materials.
• Pigments and Ceramics: Antimony compounds are used as pigments in paints and glazes.
• Glass Clarifiers: Antimony oxide acts as a fining agent in glass manufacturing, removing gas bubbles.

Biological Importance

• Toxicity: Antimony and many of its compounds are toxic. Chronic exposure can lead to health issues.
• Medicinal Uses: Historically, antimony compounds (e.g., potassium antimony tartrate, “tartar emetic”) have been used as emetics and in the treatment of parasitic infections like leishmaniasis and schistosomiasis. However, their use is limited due to toxicity and the availability of less toxic alternatives.
• Environmental Impact: Antimony can be released into the environment through mining, smelting, and the degradation of products containing antimony, posing environmental concerns.