Thulium
Thulium (Tm, atomic number 69) is a Period 6 lanthanide (f-block, 168.93 u). Second rarest naturally occurring lanthanide; ¹⁷⁰Tm serves as a compact portable X-ray source and Tm³⁺-doped lasers operate in an eye-safe infrared window cleanly absorbed b
Thulium is a chemical element on the periodic table with the atomic symbol Tm, atomic number 69, and standard atomic weight 168.93 u. Classified as a Lanthanide within Group 3 and Period 6 (f-block), thulium possesses a ground-state electron configuration of [Xe] 4f13 6s2 distributed across 6 concentric Bohr electron shells (2 · 8 · 18 · 31 · 8 · 2) with 2 outer-shell electrons. Second rarest naturally occurring lanthanide; ¹⁷⁰Tm serves as a compact portable X-ray source and Tm³⁺-doped lasers operate in an eye-safe infrared window cleanly absorbed by water.
⚛️ Interactive Periodic Table Lab: Explore Thulium (Tm, Z = 69) in the CRV Science Dynamic Periodic Table & Bohr Orbital Lab to visualize its animated 2 · 8 · 18 · 31 · 8 · 2 electron shells, simulate live phase transitions from 0 K to 6,000 K, and compare periodic heatmaps across all 118 IUPAC elements.
Atomic Structure and Periodic Trends: How Do Quantum Orbitals Define Thulium?
Electron Shell Architecture and Ground-State Configuration
A neutral atom of thulium contains 69 protons in its atomic nucleus balanced by 69 electrons occupying quantized atomic orbitals. In the Bohr-Sommerfeld and quantum-mechanical orbital models, these electrons populate 6 principal energy shells (K (n=1): 2e−, L (n=2): 8e−, M (n=3): 18e−, N (n=4): 31e−, O (n=5): 8e−, P (n=6): 2e−), terminating in the seven-orbital f-subshell manifold (azimuthal quantum number l = 3) with ground-state configuration [Xe] 4f13 6s2. Removing the most loosely bound valence electron from an isolated gaseous thulium atom requires a first ionization energy of 597 kJ/mol, while its tendency to attract shared bonding electrons is quantified by a Pauling electronegativity of 1.25.
Periodic Table Position and Neighboring Elements
Within the periodic table, Thulium occupies Period 6, Group 3 of the f-block (Lanthanides). Along Period 6, it is flanked by Erbium (Er, Z = 68) and Ytterbium (Yb, Z = 70). Vertically within its periodic family, Thulium shares valence orbital symmetry and homologous chemical trends with Lanthanum (La), Cerium (Ce), Praseodymium (Pr), Neodymium (Nd). Learners can interactively compare Thulium's orbital radii, ionization energy, and Pauling electronegativity against all neighboring elements in the Dynamic Periodic Table Lab.
Physical and Thermodynamic Properties: How Does Thulium Behave Across Temperature Regimes?
Standard Phase, Melting Point, and Boiling Point Dynamics
At standard ambient temperature and pressure (298.15 K / 25 °C, 1 atm), elemental thulium exists as a crystalline solid. As thermal energy increases, its crystal or molecular lattice undergoes solid-to-liquid fusion at a melting point of 1,818 K (+1544.85 °C / +2812.7 °F) and transitions to the vapor phase at a normal boiling point of 2,223 K (+1949.85 °C / +3541.7 °F), spanning a liquid thermodynamic range of 405 K. In the CRV Science Dynamic Periodic Table, dragging the live temperature slider between absolute zero (0 K) and solar-photosphere temperatures (6,000 K) dynamically tracks Thulium's state transitions in real time alongside enthalpy and entropy phase equilibria.
Quantitative Atomic and Thermodynamic Parameter Matrix
| Parameter | Quantitative Value / Formula | Physical / Quantum Basis | Comparative Context in Group 3 / Period 6 |
|---|---|---|---|
| Atomic Number & Symbol | Z = 69 (Tm) | 69 nuclear protons; nuclear charge +69e | Located in Period 6, Group 3 (f-block) |
| Standard Atomic Weight | 168.93 u (g/mol) | Abundance-weighted isotopic nucleon mass | ~100 neutrons in dominant/reference mass number |
| Electron Configuration | [Xe] 4f13 6s2 | Shells: 2 · 8 · 18 · 31 · 8 · 2 (2 outer-shell e−) | Lanthanides valence orbital architecture |
| Electronegativity & Ionization | χ = 1.25; IE1 = 597 kJ/mol | Pauling bond polarity scale & first valence ionization enthalpy | Governs covalent vs. ionic bond character |
| Melting & Boiling Points | MP: 1818 K; BP: 2223 K | Cohesive lattice energy and intermolecular forces | STP (298 K) phase: SOLID (Liquid range: 405 K) |
| Principal Oxidation States | +3, +2 | Valence electron donation, sharing, or acceptance | Discovered / First Isolated: 1879 |
Chemical Reactivity, Bonding, and Oxidation States: How Does Thulium Form Compounds?
Characteristic Oxidation States and Coordination Chemistry
As a 4f-block inner transition metal (rare-earth lanthanide), progressive filling of radially contracted inner 4f orbitals—shielded by outer 5s² 5p⁶ shells—produces the trivalent (+3) lanthanide contraction and sharp f–f optical and magnetic transitions. In chemical compounds, thulium exhibits characteristic oxidation states of +3, +2. Depending on the electronegativity difference (Δχ) of its bonding partners—such as Oxygen (χ = 3.44), Fluorine (χ = 3.98), Chlorine (χ = 3.16), Nitrogen (χ = 3.04), Carbon (χ = 2.55), or Hydrogen (χ = 2.20)—thulium forms redox-specific oxides, halides, hydrides, or coordination complexes governed by Gibbs free energy (ΔG = ΔH − TΔS).
Nucleosynthesis, Discovery History, and Elemental Origins
First identified or isolated in 1879, Thulium carries a distinct cosmological and geochemical signature. With atomic number Z = 69 beyond the iron-56 binding-energy peak, thulium is synthesized astrophysically via neutron-capture nucleosynthesis—combining slow neutron capture (the s-process in asymptotic giant branch stars) and rapid neutron capture (the r-process in neutron-star mergers and core-collapse supernovae).
Biological Role and Scientific Applications: Why Is Thulium Important in Nature and Technology?
Biochemical Classification: Abiotic / Technological / Synthetic
In comparative biochemistry and systems physiology, Thulium is classified as Abiotic / Technological / Synthetic. No known essential biological role; important in planetary geochemistry, astrophysics, materials science, or nuclear physics. Across terrestrial and marine ecosystems, its chemical speciation interacts with thermodynamics, enthalpy, entropy, chemistry.
Key Mechanistic Functions and Technological Uses
Second rarest naturally occurring lanthanide; ¹⁷⁰Tm serves as a compact portable X-ray source and Tm³⁺-doped lasers operate in an eye-safe infrared window cleanly absorbed by water. Researchers and students can further investigate Thulium's functional role alongside all 118 elements by launching the Interactive Periodic Table & Element Detective Lab on CRV Science Learn.
Frequently Asked Scientific Questions
What is the electron configuration and valence shell structure of Thulium (Tm)?
Thulium (atomic number Z = 69) has the ground-state electron configuration [Xe] 4f13 6s2, distributing its 69 electrons across 6 shells as 2 · 8 · 18 · 31 · 8 · 2. Its outermost shell contains 2 electrons, placing Thulium in Group 3, Period 6 (f-block) as a member of the Lanthanides. You can watch these 69 electrons orbit the nucleus in real time in the Dynamic Periodic Table Bohr Lab.
What phase of matter is Thulium at room temperature, and when does it melt or boil?
At standard room temperature (298 K / 25 °C) and 1 atm pressure, Thulium is a crystalline solid. It melts at 1,818 K (+1544.85 °C / +2812.7 °F) and boils at 2,223 K (+1949.85 °C / +3541.7 °F). Using the 0–6,000 K temperature slider in the Interactive Periodic Table, you can observe Thulium's exact solid, liquid, and gas transitions relative to all other elements.
How does Thulium function in biochemistry, materials science, and technology?
Second rarest naturally occurring lanthanide; ¹⁷⁰Tm serves as a compact portable X-ray source and Tm³⁺-doped lasers operate in an eye-safe infrared window cleanly absorbed by water. In biological systems, it is categorized under Abiotic / Technological / Synthetic, with principal oxidation states of +3, +2 and a first ionization energy of 597 kJ/mol.