Selenium (Se)

Atomic Structure

Selenium (Se) is a chemical element with atomic number 34, located in Group 16 (Chalcogens) and Period 4 of the periodic table.

Atomic Number (Z)

Z = 34: Indicates 34 protons in the nucleus of every Selenium atom.

Mass Number (A)

Average Atomic Mass: 78.96 u. This is the weighted average of the masses of its naturally occurring isotopes. The most abundant isotope is Selenium-80 (⁸⁰Se).

Subatomic Particles

Protons: 34 (Equal to the atomic number).
Electrons: 34 (In a neutral atom, equal to the number of protons).
Neutrons: Approximately 45 (Calculated as Average Atomic Mass - Atomic Number, i.e., 78.96 - 34 ≈ 45 neutrons for the average atom. For ⁸⁰Se, there are 80 - 34 = 46 neutrons).

Atomic Size

Covalent Radius: 115 pm.
Van der Waals Radius: 190 pm.

Electronic Configuration

The arrangement of electrons in the atomic orbitals of Selenium determines its chemical behavior.

Ground State Electronic Configuration

Full Configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁴
Condensed Configuration: [Ar] 3d¹⁰ 4s² 4p⁴
- The [Ar] represents the electronic configuration of Argon, the noble gas preceding Selenium.

Shell Diagram Representation

The electron distribution in different shells (K, L, M, N) for Selenium is:

K-shell (n=1): 2 electrons (1s²)
L-shell (n=2): 8 electrons (2s² 2p⁶)
M-shell (n=3): 18 electrons (3s² 3p⁶ 3d¹⁰)
N-shell (n=4): 6 electrons (4s² 4p⁴) - This is the valence shell, containing 6 valence electrons, which is characteristic of Group 16 elements.

Periodic Trends

Selenium’s position in the periodic table influences its properties, which follow distinct trends.

Ionization Enthalpy

First Ionization Enthalpy (IE₁): 941 kJ/mol. This is the energy required to remove the first electron from a gaseous Selenium atom.
Second Ionization Enthalpy (IE₂): 2045 kJ/mol. The energy required to remove the second electron.
- IE₁ for Se is lower than that of sulfur (999.6 kJ/mol) due to its larger atomic size and increased shielding effect.

Electronegativity

Pauling Scale: 2.55.
- Selenium is less electronegative than oxygen (3.44) and sulfur (2.58) but more electronegative than tellurium (2.1). This indicates its tendency to attract electrons in a chemical bond is moderate.

Electron Gain Enthalpy

Electron Gain Enthalpy (ΔegH): -195 kJ/mol.
- This negative value indicates that energy is released when a gaseous Selenium atom gains an electron to form a uninegative ion (Se⁻). Its value is less negative than oxygen (-141 kJ/mol) and sulfur (-200 kJ/mol), suggesting a slightly lower tendency to accept an electron compared to sulfur due to larger size and weaker nuclear attraction for an incoming electron, but still significant.

Atomic Radius

As mentioned above, Covalent Radius is 115 pm.
- Atomic radius generally increases down a group and decreases across a period. Selenium is larger than sulfur but smaller than tellurium in Group 16.

Key Physical Properties

Selenium exists in several allotropic forms, each with distinct physical properties. The most stable and common form is grey (metallic) selenium.

Density

Grey (metallic) Selenium: 4.81 g/cm³ at 25 °C.
Amorphous (red) Selenium: 4.28 g/cm³ at 25 °C.

State at Standard Temperature and Pressure (STP)

Solid. Selenium is a solid at 25 °C and 1 atm pressure.

Color

Grey (metallic) Selenium: Grey, lustrous solid. This form is a semiconductor.
Red Amorphous Selenium: Red powder.
Black Vitreous Selenium: Black, glass-like solid.

Melting Point

Grey (metallic) Selenium: 221 °C.
Amorphous Selenium: Variable, softens around 50-70 °C.

Boiling Point

Grey (metallic) Selenium: 685 °C.

Selenium (Se) - Atomic Structure and Chemical Properties