Thulium (Tm): Bohr Model, Protons, Neutrons & Electrons
Thulium is element number 69, a lanthanide in the f-block (the lanthanide row drawn below the table), period 6. Its atom has 69 protons, 100 neutrons and 69 electrons arranged in 6 shells: 2, 8, 18, 31, 8, 2.
Protons, neutrons and electrons in thulium
The atomic number of thulium is 69, so every thulium atom has 69 protons. A neutral atom has the same number of electrons, 69. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For thulium-169, that is 169 − 69 = 100 neutrons.
How to draw the Bohr model of thulium
- Draw the nucleus and write 69 p⁺ and 100 n⁰ inside it.
- Draw the first shell and put 2 electrons on it (the first shell holds at most 2).
- Draw the second shell and put 8 electrons on it.
- Draw the third shell and put 18 electrons on it.
- Draw the fourth shell and put 31 electrons on it.
- Draw the fifth shell and put 8 electrons on it.
- Draw the sixth shell and put 2 electrons on it.
- Check: 2 + 8 + 18 + 31 + 8 + 2 = 69 electrons, the same as the number of protons.
From potassium onwards the shells don’t simply fill as 2, 8, 8: the fourth shell starts before the third is full, so the real arrangement above (2, 8, 18, 31, 8, 2) is the one to draw.
Electron configuration
The electron configuration of thulium is [Xe] 6s² 4f¹³. As a lanthanide it fills an inner f-subshell, and its common ion is Tm³⁺.
Key facts about thulium
| Symbol | Tm |
|---|---|
| Atomic number | 69 |
| Relative atomic mass | 168.93 |
| Group · period · block | f-block · period 6 |
| Type | Lanthanide |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 31, 8, 2 |
| Electron configuration | [Xe] 6s² 4f¹³ |
| Valence electrons | varies (outer shell: 2) |
| Common ion | Tm³⁺ |
| Electronegativity (Pauling) | 1.25 |
| First ionisation energy | 6.18 eV (596 kJ/mol) |
| Atomic radius (empirical) | 175 pm |
Where thulium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does thulium have?
Thulium has 69 protons and 69 electrons (atomic number 69). Its most common isotope, thulium-169, has 169 − 69 = 100 neutrons.
What is the electron configuration of thulium?
[Xe] 6s² 4f¹³. In shells that is 2, 8, 18, 31, 8, 2.
How many valence electrons does thulium have?
Thulium is a lanthanide, so its valence electrons are spread over two shells; its outermost shell holds 2.
Where is thulium on the periodic table?
In the f-block (the lanthanide row drawn below the table), period 6, in the f-block. It is a lanthanide.
