Tellurium (Te): Properties, Reactions, and Importance

Introduction

Tellurium (Te) is a brittle, silvery-white metalloid element found in Group 16 (Chalcogens) of the periodic table. While relatively rare, its unique semiconductor properties and ability to form a wide array of compounds make it crucial in various high-technology applications. Its chemical behavior shows characteristics intermediate between metals and nonmetals, aligning with its position as a metalloid.

CBSE/JEE Quick Revision Notes

• Atomic Number (Z): 52
• Atomic Mass (A): 127.60 u
• Symbol: Te
• Group: 16 (Chalcogens)
• Period: 5
• Block: p-block
• Classification: Metalloid
• Common Oxidation States: -2, +2, +4, +6
• Physical State at STP: Solid
• Nature: Brittle, lustrous, silvery-white. Exhibits polymorphism.
• Electronegativity (Pauling): 2.1
• Electronic Configuration: [Kr] 4d¹⁰ 5s² 5p⁴

Electron Configuration & Bonding Behavior

Tellurium has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁴ or in shorthand, [Kr] 4d¹⁰ 5s² 5p⁴.

• Valence Electrons: Six valence electrons (5s² 5p⁴).
• Bonding: Like other chalcogens, Tellurium primarily forms covalent bonds.
  • -2 Oxidation State: Achieved by gaining two electrons to complete its 5p subshell, typically seen in tellurides (e.g., H₂Te, Na₂Te).
  • +2 Oxidation State: Less common, but can occur, especially with halogens.
  • +4 Oxidation State: Most stable and common positive oxidation state, utilizing two 5s and two 5p electrons for bonding (e.g., TeO₂, TeCl₄).
  • +6 Oxidation State: Achieved by involving all six valence electrons, often seen with highly electronegative elements like oxygen or fluorine (e.g., H₂TeO₄, TeF₆).
• Metallic Character: Compared to oxygen, sulfur, and selenium, Tellurium exhibits more metallic character, reflected in its higher conductivity and amphoteric nature of its oxides.

Crucial Chemical Reactions

1. Reaction with Oxygen

Tellurium burns in air or oxygen to form tellurium dioxide. Te (s) + O₂ (g) → TeO₂ (s)

2. Reaction with Halogens

Tellurium reacts readily with halogens to form various halides, depending on the halogen and conditions. Higher oxidation states are favored with fluorine.

• With Fluorine (forming tellurium hexafluoride, Te(VI)): Te (s) + 3F₂ (g) → TeF₆ (g)
• With Chlorine (forming tellurium tetrachloride, Te(IV)): Te (s) + 2Cl₂ (g) → TeCl₄ (s)

3. Reaction with Acids

Tellurium is generally unreactive with non-oxidizing acids. It reacts with strong oxidizing acids.

• With Concentrated Nitric Acid: Te (s) + 4HNO₃ (conc) → H₂TeO₃ (aq) + 4NO₂ (g) + H₂O (l) (Tellurium is oxidized to tellurous acid, H₂TeO₃)

• With Hot Concentrated Sulfuric Acid: Te (s) + 2H₂SO₄ (conc, hot) → TeO₂ (s) + 2SO₂ (g) + 2H₂O (l) (In some cases, specific complex cations of Te like Te₄²⁺ can form, but TeO₂ is a common product).

4. Reaction with Alkalis

Tellurium undergoes disproportionation (simultaneous oxidation and reduction) in hot, concentrated alkali solutions.

3Te (s) + 6NaOH (aq) → Na₂TeO₃ (aq) + 2Na₂Te (aq) + 3H₂O (l) (Tellurium is oxidized to tellurite (TeO₃²⁻, Te(+4)) and reduced to telluride (Te²⁻, Te(-2))).

5. Formation of Tellurides

Tellurium reacts with active metals (e.g., alkali metals, alkaline earth metals) to form metal tellurides.

2Na (s) + Te (s) → Na₂Te (s)

Industrial and Biological Importance

Industrial Importance

• Alloys: Tellurium is added to steel, copper, and lead.
  • In steel, it improves machinability.
  • In copper, it enhances machinability without reducing electrical conductivity.
  • In lead, it increases strength and corrosion resistance, particularly for battery plates and cable sheathing.
• Semiconductors: Cadmium Telluride (CdTe) and Mercury Cadmium Telluride (HgCdTe) are vital semiconductors used in solar cells, infrared detectors, and thermoelectric devices.
• Thermoelectric Materials: Bismuth telluride (Bi₂Te₃) is a key component in thermoelectric generators and coolers (Peltier devices), converting heat directly into electricity or vice versa.
• Rubber Vulcanization: Tellurium compounds act as accelerators and antioxidants in the vulcanization of rubber.
• Optical Storage: Tellurium alloys are used in some phase-change optical data storage devices (e.g., rewritable CDs and DVDs).

Biological Importance

• Tellurium is not considered an essential element for humans or other higher organisms.
• In high concentrations, tellurium and its compounds are toxic. Exposure can cause a garlic-like odor on the breath (tellurium breath) due to the formation of volatile dimethyl telluride, (CH₃)₂Te, in the body.
• Some microorganisms possess the ability to metabolize tellurite (TeO₃²⁻) compounds, often reducing them to elemental tellurium, which can lead to the formation of black tellurium deposits.