Thallium (Tl) - Atomic Structure & Chemical Properties

Atomic Model Details

Thallium (Tl) is a heavy metallic element located in Group 13 and Period 6 of the periodic table.

• Atomic Number (Z): 81
• Atomic Mass: 204.38 u (average atomic mass)
• Protons: 81
• Electrons: 81 (in a neutral atom)
• Neutrons: For its most abundant isotopes:
  • Thallium-205 (²⁰⁵Tl): 205 - 81 = 124 neutrons
  • Thallium-203 (²⁰³Tl): 203 - 81 = 122 neutrons
• Atomic Size:
  • Covalent Radius: Approximately 156 pm.
  • Metallic Radius: Approximately 170 pm.
  • Thallium’s atomic radius is larger than that of lighter Group 13 elements (Boron, Aluminium, Gallium, Indium). However, the increase in size from Indium to Thallium is less significant than expected due to the lanthanide contraction (poor shielding by 4f electrons) and relativistic effects of the inner core electrons, which lead to an increased effective nuclear charge experienced by the valence electrons.

Electronic Configuration & Shell Diagram Representation

• Electronic Configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p¹
  • This configuration indicates that Thallium has a completely filled 4f and 5d subshells, followed by 6s and 6p subshells.
  • The outermost shell contains 3 electrons (2 in 6s and 1 in 6p), making its valence shell configuration 6s² 6p¹.
• Shell Diagram Representation (Electron Distribution by Shell):
  • K-shell (n=1): 2 electrons
  • L-shell (n=2): 8 electrons
  • M-shell (n=3): 18 electrons
  • N-shell (n=4): 32 electrons (2s², 2p⁶, 3s², 3p⁶, 3d¹⁰, 4s², 4p⁶, 4d¹⁰, 4f¹⁴)
  • O-shell (n=5): 18 electrons (5s², 5p⁶, 5d¹⁰)
  • P-shell (n=6): 3 electrons (6s², 6p¹)

Periodic Trends (Group 13 Context)

• Ionization Enthalpy (IE):
  • First Ionization Enthalpy: Approximately 589.4 kJ/mol.
  • Trend: While ionization enthalpy generally decreases down a group due to increasing atomic size and shielding, Thallium exhibits a slightly higher first ionization enthalpy than Indium. This anomalous behavior is attributed to the inert pair effect and poor shielding by the intervening 4f and 5d electrons, leading to a stronger attraction of the 6s electrons by the nucleus.
• Electronegativity:
  • Pauling Scale: Approximately 1.62.
  • Trend: Electronegativity generally decreases down a group. However, due to the factors mentioned above (poor shielding, increased effective nuclear charge from lanthanide and relativistic effects), Thallium’s electronegativity is higher than that of Indium, making it less electropositive than expected.
• Electron Gain Enthalpy:
  • Value: Approximately -19 kJ/mol.
  • Trend: Electron gain enthalpy values generally become less negative (or more positive) down a group as the atomic size increases, and the added electron is further from the nucleus. Thallium has a relatively low (less negative) electron gain enthalpy, indicating a weak tendency to accept an extra electron.
• Atomic Radius:
  • Trend: As mentioned previously, atomic radii generally increase down a group. While Thallium is larger than Indium, the increase is less pronounced than expected due to lanthanide contraction and relativistic effects, which increase the effective nuclear charge on the valence electrons.

Key Physical Properties

• Density: 11.85 g/cm³ (at 20 °C). It is one of the densest common elements.
• State at Room Temperature: Solid.
• Color: Silvery-white, lustrous metal when freshly cut. It quickly tarnishes and turns bluish-grey upon exposure to air due to the formation of thallium oxide.
• Melting Point (MP): 304 °C. It has a relatively low melting point for a metal.
• Boiling Point (BP): 1473 °C.