Thulium (Tm)

Atomic Model Details

Thulium (Tm) is a chemical element with atomic number 69. It is a rare earth element and belongs to the lanthanide series.

Atomic Number (Z): 69
- Indicates 69 protons in the nucleus.
- Indicates 69 electrons in a neutral thulium atom.
Mass Number (A): Approximately 169 (specifically, the most stable isotope, Thulium-169, has an atomic mass of 168.93421 amu).
Number of Neutrons: For the stable isotope $^{169}\text{Tm}$, the number of neutrons is A - Z = 169 - 69 = 100.
Atomic Size (Metallic Radius): 172 pm (picometers). This value reflects the radius of the thulium atom in its metallic crystal structure.

The electronic configuration of Thulium provides insight into its chemical behavior.

Electronic Configuration: $[\text{Xe}] 4\text{f}^{13} 6\text{s}^2$
- The noble gas core $[\text{Xe}]$ represents the configuration of Xenon ($1\text{s}^2 2\text{s}^2 2\text{p}^6 3\text{s}^2 3\text{p}^6 3\text{d}^{10} 4\text{s}^2 4\text{p}^6 4\text{d}^{10} 5\text{s}^2 5\text{p}^6$).
- The outermost electrons are in the $4\text{f}$ and $6\text{s}$ subshells.
Shell Diagram Representation (Electrons per shell):
- K-shell (n=1): 2 electrons ($1\text{s}^2$)
- L-shell (n=2): 8 electrons ($2\text{s}^2 2\text{p}^6$)
- M-shell (n=3): 18 electrons ($3\text{s}^2 3\text{p}^6 3\text{d}^{10}$)
- N-shell (n=4): 31 electrons ($4\text{s}^2 4\text{p}^6 4\text{d}^{10} 4\text{f}^{13}$)
- O-shell (n=5): 8 electrons ($5\text{s}^2 5\text{p}^6$)
- P-shell (n=6): 2 electrons ($6\text{s}^2$)

Thulium exhibits trends characteristic of metals, particularly lanthanides.

Ionization Enthalpy: The energy required to remove an electron from a gaseous atom.
- First Ionization Enthalpy (IE$_1$): 596.7 kJ/mol
- Second Ionization Enthalpy (IE$_2$): 1163 kJ/mol
- Thulium readily loses its two $6\text{s}$ electrons and often one $4\text{f}$ electron to form a stable +3 oxidation state, typical for lanthanides.
Electronegativity (Pauling Scale): 1.25
- This low value indicates that Thulium has a low tendency to attract electrons and prefers to lose them, forming positive ions.
Electron Gain Enthalpy: Not well-defined for most metals and typically a positive value, meaning energy is required to add an electron. This reflects its metallic character and tendency to lose rather than gain electrons.
Atomic Radius: 172 pm
- As a lanthanide, its atomic radius is influenced by the lanthanide contraction, which results in a gradual decrease in atomic radii across the series.

Thulium is a soft, malleable, and ductile metal.