Selenium (Se): Properties, Reactions, and Importance

Introduction

Selenium (Se), a p-block element in Group 16 (Chalcogens), exhibits fascinating properties that bridge non-metals and metalloids. Its unique photoconductive and semiconducting nature underpins its use in various technologies, while its biological role as an essential trace element is critical for human health. Understanding Selenium’s chemistry is vital for both theoretical examinations and appreciating its real-world impact.

CBSE/JEE Quick Revision Notes

Atomic Number (Z): 34
Atomic Mass (Ar): 78.96 u
Group: 16 (Chalcogens)
Period: 4
Block: p-block
Valency: Typically -2, +2, +4, +6
Common Oxidation States: -2, 0, +2, +4, +6
Nature: Non-metal, exhibiting allotropy (e.g., red amorphous, grey metallic, black vitreous forms). Grey ‘metallic’ selenium is the most stable and conductive form.
Electronegativity: 2.55 (Pauling scale)
First Ionization Enthalpy: 941 kJ/mol

Electron Configuration & Bonding Behavior

Electronic Configuration

The electronic configuration of Selenium is [Ar] 3d¹⁰ 4s² 4p⁴.

Valence Shell Configuration

The valence shell configuration is 4s² 4p⁴.

Bonding Behavior

Selenium primarily forms covalent compounds due to its relatively high ionization energy and electronegativity.

Oxidation State -2: Forms selenide ions (Se²⁻), though less stable than sulfide ions (S²⁻).
Oxidation State +2: Observed in compounds like SeCl₂.
Oxidation State +4: Most common positive oxidation state. Involves the excitation of one 4s electron and two 4p electrons, leading to sp³d hybridization (e.g., SeF₄).
Oxidation State +6: Achieved by promoting both 4s electrons and all three 4p electrons to empty 4d orbitals, resulting in sp³d² hybridization (e.g., SeF₆, H₂SeO₄).

Crucial Chemical Reactions

1. Reaction with Hydrogen

Selenium reacts with hydrogen to form hydrogen selenide, a highly toxic gas. H₂ (g) + Se (s) → H₂Se (g)

2. Reaction with Oxygen

Selenium burns in oxygen to form selenium dioxide. Se (s) + O₂ (g) → SeO₂ (s)

3. Reaction with Halogens

Selenium reacts directly with halogens to form halides.

With Fluorine: Se (s) + 2F₂ (g) → SeF₄ (l) (Selenium tetrafluoride) Se (s) + 3F₂ (g) → SeF₆ (g) (Selenium hexafluoride)
With Chlorine: Se (s) + 2Cl₂ (g) → SeCl₄ (s) (Selenium tetrachloride)

4. Reaction with Concentrated Nitric Acid (Oxidation)

Concentrated nitric acid oxidizes selenium to selenous acid. 3Se (s) + 4HNO₃ (conc.) → 3H₂SeO₃ (aq) + 4NO (g)

5. Reaction with Strong Bases (Disproportionation)

When heated with strong bases like NaOH, selenium undergoes disproportionation, forming selenide and selenite. 3Se (s) + 6NaOH (aq) → Na₂SeO₃ (aq) + 2Na₂Se (aq) + 3H₂O (l)

Industrial and Biological Importance

Industrial Importance

Photoconductivity: Grey amorphous selenium exhibits photoconductivity, meaning its electrical conductivity increases significantly upon exposure to light. This property is utilized in photocells, light meters, solar cells, and xerography (photocopying machines).
Semiconductors: Used in rectifiers to convert alternating current (AC) to direct current (DC).
Glass Industry: Selenium is used as a decolorizing agent in glass manufacturing, counteracting the green tint caused by iron impurities. It also imparts a red color to glass, enamels, and glazes.
Metallurgy: Added to stainless steel and copper alloys to improve their machinability.
Pigments: Cadmium sulfoselenide compounds produce vibrant red, orange, and maroon pigments.

Biological Importance

Essential Trace Element: Selenium is an essential micronutrient for humans and animals. It is a vital component of several enzymes known as selenoenzymes, most notably glutathione peroxidase (GPx), which protects cells from oxidative damage caused by free radicals.
Antioxidant Role: Through selenoenzymes, selenium plays a critical role in antioxidant defense mechanisms and immune function.
Thyroid Hormone Metabolism: Selenoenzymes are involved in the metabolism of thyroid hormones.
Deficiency: Selenium deficiency can lead to various health issues, including Keshan disease (a type of cardiomyopathy) and Kashin-Beck disease (an osteoarthropathy).
Toxicity: While essential, selenium is toxic in high doses. Excess intake can lead to selenosis, characterized by hair loss, nail brittleness, skin lesions, neurological damage, and gastrointestinal disturbances.