Terbium (Tb): Atomic Structure and Chemical Properties

Atomic Model Details

Terbium (Tb) is a chemical element with atomic number 65. It is a member of the lanthanide series, an f-block element.

• Atomic Number (Z): 65
• Atomic Mass (A): 158.925 u (unified atomic mass units)
• Protons: 65
• Electrons: 65 (in a neutral atom)
• Neutrons: 94 (for its most stable and naturally occurring isotope, Terbium-159)
• Atomic Size:
  • Metallic Radius: 178.2 pm
  • Ionic Radius (for Tb³⁺): 104 pm (for coordination number 6)

Electronic Configuration & Shell Diagram Representation

Terbium exhibits a characteristic electronic configuration reflecting its position in the f-block.

• Ground State Electronic Configuration: [Xe] 4f⁹ 6s²
  • This means it has the electron configuration of Xenon (Xe) followed by nine electrons in the 4f subshell and two electrons in the 6s subshell.
• Shell Representation (Electrons per shell):
  • K-shell (n=1): 2 electrons
  • L-shell (n=2): 8 electrons
  • M-shell (n=3): 18 electrons
  • N-shell (n=4): 27 electrons (2 in 4s, 6 in 4p, 10 in 4d, 9 in 4f)
  • O-shell (n=5): 8 electrons (2 in 5s, 6 in 5p)
  • P-shell (n=6): 2 electrons (2 in 6s)
• Common Oxidation States: The most common oxidation state for Terbium is +3, arising from the loss of the two 6s electrons and one 4f electron to achieve a relatively stable, though not half-filled or fully-filled, f-subshell configuration. A +4 oxidation state is less common but observed, for instance, in TbF₄.

Periodic Trends

Terbium’s position in the lanthanide series dictates its periodic trends.

• Ionization Enthalpy:
  • First Ionization Enthalpy (IE₁): 565.8 kJ/mol (Energy required to remove the first electron)
  • Second Ionization Enthalpy (IE₂): 1115 kJ/mol (Energy required to remove the second electron)
  • Third Ionization Enthalpy (IE₃): 2110 kJ/mol (Energy required to remove the third electron)
  • Trend: As a metal, Terbium has relatively low ionization enthalpies, allowing it to readily lose electrons and form positive ions. The successive ionization energies increase, as expected, due to increased effective nuclear charge on the remaining electrons.
• Electronegativity (Pauling Scale): 1.10
  • Trend: Terbium is a highly electropositive element, typical for f-block metals. Its low electronegativity indicates a strong tendency to lose electrons rather than gain them, forming ionic bonds.
• Electron Gain Enthalpy: Not typically reported or well-defined for highly electropositive metals like Terbium. Metals generally do not readily accept electrons, and thus, their electron gain enthalpies are usually positive or negligible, indicating energy absorption rather than release upon electron gain.
• Atomic Radius: 178.2 pm (metallic radius)
  • Trend: Within the lanthanide series, the atomic and ionic radii generally decrease gradually with increasing atomic number. This phenomenon is known as the lanthanoid contraction, caused by the poor shielding effect of the 4f electrons, leading to an increased effective nuclear charge and a contraction of the electron shells.

Key Physical Properties

Terbium is a silvery-white, malleable, and ductile metal.

• Density: 8.23 g/cm³ (at 25 °C)
• State at Room Temperature (25 °C, 1 atm): Solid
• Color: Silvery-white (lustrous, but tarnishes slowly in air)
• Melting Point: 1356 °C
• Boiling Point: 3230 °C