Selenium: Element Overview

Basic Information

• Symbol: Se
• Atomic Number: 34
• Atomic Weight: 78.96 u
• Element Category: Non-metal (also considered a metalloid in some contexts)
• Group: 16 (Chalcogens)
• Period: 4

Physical Properties

• Phase at Room Temperature: Solid
• Melting Point: 221 °C (430 °F)
• Boiling Point: 685 °C (1265 °F)
• Density: 4.81 g/cm³ (gray selenium)
• Color: Varies with form: red, gray, and black are common allotropes.
• Crystal Structure: Hexagonal for gray selenium; amorphous for red and black forms.

Chemical Properties

• Electronegativity: 2.55 (Pauling scale)
• Ionization Energies:
  • First: 941 kJ/mol
  • Second: 2045 kJ/mol
  • Third: 2973.7 kJ/mol
• Oxidation States: -2, +4, +6 (most common are -2, +4, +6)
• Reactivity: Reacts with halogens and concentrated nitric acid but is generally resistant to attack by non-oxidizing acids.

Isotopes

• Stable Isotopes: Selenium has six stable isotopes: Se-74, Se-76, Se-77, Se-78, Se-80, and Se-82.
  • Se-74: 0.89%
  • Se-76: 9.37%
  • Se-77: 7.63%
  • Se-78: 23.77%
  • Se-80: 49.61%
  • Se-82: 8.73%
• Radioisotopes: Several radioisotopes, including Se-75, which is used in industrial radiography.

Occurrence and Production

• Natural Abundance: Selenium is the 67th most abundant element in the Earth’s crust.
• Sources: Found in minerals such as selenite (SeO₃²⁻), selenide, and selenate, often as a byproduct of copper refining.
• Production: Obtained as a byproduct of refining sulfide ores of copper, nickel, or lead.

Uses and Applications

• Glass Production: Used to decolorize glass and to make red-colored glasses and enamels.
• Electronics: Used in photocells, light meters, and solar cells due to its photoconductive properties.
• Alloys: Used to improve the machinability of stainless steel and copper alloys.
• Pigments: Used in pigments for ceramics, plastics, paints, and inks.
• Nutritional Supplements: An essential trace element in small amounts for animal and human nutrition.
• Medical: Used in antidandruff shampoos and in some medical treatments for skin diseases.

Safety and Precautions

• Reactivity: Selenium compounds, particularly hydrogen selenide (H₂Se), are highly toxic and pose significant health risks.
• Handling: Should be handled with care, using appropriate protective equipment to avoid inhalation and skin contact.
• Toxicity: Chronic exposure to selenium can lead to selenosis, characterized by gastrointestinal distress, hair loss, and neurological damage.

Biological Role

• Essential Element: Selenium is essential for human and animal health.
• Enzyme Function: Plays a crucial role in the functioning of several enzymes, including glutathione peroxidase, which protects cells from oxidative damage.
• Health: Required for proper thyroid function and immune system support.

Historical Background

• Discovery: Discovered by Jöns Jacob Berzelius in 1817.
• Naming: Named after the Greek word “selene,” meaning moon.

Interesting Facts

• Photoconductivity: Selenium’s electrical conductivity increases with exposure to light, making it useful in photocopying and light-sensing applications.
• Glass Industry: Selenium is used to remove the green tint caused by iron impurities in glass production.
• Allotropic Forms: Selenium exists in several allotropic forms, each with distinct properties and uses.

Summary

Selenium is a versatile non-metal with applications in electronics, glass production, and nutrition. It is essential for human health, playing a crucial role in various enzymatic processes. Discovered in the early 19th century, selenium is primarily obtained as a byproduct of copper refining. Its unique properties make it valuable in diverse industrial and technological applications.

Questions and Answers about Selenium:

1. Q: What is the chemical symbol for Selenium?

   • A: Se

2. Q: What is the atomic number of Selenium?

   • A: 34

3. Q: What type of element is Selenium?

   • A: Non-metal (also considered a metalloid in some contexts)

4. Q: What is the atomic weight of Selenium?

   • A: 78.96 u

5. Q: What group is Selenium in?

   • A: Group 16 (Chalcogens)

6. Q: What period is Selenium in?

   • A: Period 4

7. Q: What is the phase of Selenium at room temperature?

   • A: Solid

8. Q: What is the melting point of Selenium?

   • A: 221 °C (430 °F)

9. Q: What is the boiling point of Selenium?

   • A: 685 °C (1265 °F)

10. Q: What is the density of Selenium at 20 °C?

    • A: 4.81 g/cm³

11. Q: What is the color of Selenium in its most stable form?

    • A: Gray, metallic luster

12. Q: What is the crystal structure of Selenium?

    • A: Hexagonal for gray selenium; amorphous for red and black forms

13. Q: What is the electronegativity of Selenium?

    • A: 2.55 (Pauling scale)

14. Q: What is the first ionization energy of Selenium?

    • A: 941 kJ/mol

15. Q: How many stable isotopes does Selenium have?

    • A: Six (Se-74, Se-76, Se-77, Se-78, Se-80, Se-82)

16. Q: What is the most abundant isotope of Selenium?

    • A: Se-80

17. Q: How is Selenium commercially produced?

    • A: As a byproduct of refining sulfide ores of copper, nickel, or lead

18. Q: What is the natural abundance of Selenium in the Earth’s crust?

    • A: 67th most abundant element

19. Q: What common minerals contain Selenium?

    • A: Selenite (SeO₃²⁻), selenide, selenate

20. Q: What industry uses Selenium extensively for glass production?

    • A: Glass industry

21. Q: What is one use of Selenium in electronics?

    • A: Used in photocells and solar cells

22. Q: What role does Selenium play in the production of pigments?

    • A: Used in pigments for ceramics, plastics, paints, and inks

23. Q: Why is Selenium used in nutritional supplements?

    • A: An essential trace element in small amounts for human and animal nutrition

24. Q: What is the biological role of Selenium in the body?

    • A: Plays a crucial role in the functioning of several enzymes, including glutathione peroxidase

25. Q: Who discovered Selenium?

    • A: Jöns Jacob Berzelius

26. Q: In what year was Selenium discovered?

    • A: 1817

27. Q: What is the origin of the name “Selenium”?

    • A: Named after the Greek word “selene,” meaning moon

28. Q: What property makes Selenium valuable in photocopying and light-sensing applications?

    • A: Its photoconductivity

29. Q: What health hazard is associated with Selenium compounds?

    • A: Some selenium compounds are highly toxic and pose significant health risks

30. Q: How should Selenium and its compounds be handled?

    • A: With care, using appropriate protective equipment

31. Q: What is the second ionization energy of Selenium?

    • A: 2045 kJ/mol

32. Q: What is the third ionization energy of Selenium?

    • A: 2973.7 kJ/mol

33. Q: Is Selenium essential for plant health?

    • A: Selenium is not essential for all plants but is beneficial in trace amounts for some

34. Q: What is the significance of Selenium in thyroid function?

    • A: Selenium is required for proper thyroid function

35. Q: What is the role of Selenium in the production of special glass types?

    • A: Used to decolorize glass and make red-colored glasses and enamels

36. Q: How does Selenium benefit the electronics industry?

    • A: Used in semiconductors and various electronic components

37. Q: What is Selenium’s role in medical treatments?

    • A: Used in antidandruff shampoos and some medical treatments for skin diseases

38. Q: What is the environmental impact of Selenium mining?

    • A: Generally low, as it is produced as a byproduct of other metal refining processes

39. Q: What color are Selenium compounds typically?

    • A: They can exhibit various colors, with red and black being common for different allotropes

40. Q: What is Selenium’s role in the immune system?

    • A: Essential for the proper functioning of the immune system