Promethium (Pm): Bohr Model, Protons, Neutrons & Electrons
Promethium is element number 61, a lanthanide in the f-block (the lanthanide row drawn below the table), period 6. Its atom has 61 protons, 84 neutrons and 61 electrons arranged in 6 shells: 2, 8, 18, 23, 8, 2.
Protons, neutrons and electrons in promethium
The atomic number of promethium is 61, so every promethium atom has 61 protons. A neutral atom has the same number of electrons, 61. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For promethium-145, that is 145 − 61 = 84 neutrons. Every isotope of promethium is radioactive; 145 is the mass number of its most stable one.
How to draw the Bohr model of promethium
- Draw the nucleus and write 61 p⁺ and 84 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 23 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 + 23 + 8 + 2 = 61 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, 23, 8, 2) is the one to draw.
Electron configuration
The electron configuration of promethium is [Xe] 6s² 4f⁵. As a lanthanide it fills an inner f-subshell, and its common ion is Pm³⁺.
Key facts about promethium
| Symbol | Pm |
|---|---|
| Atomic number | 61 |
| Relative atomic mass | [145] |
| Group · period · block | f-block · period 6 |
| Type | Lanthanide |
| State at 25 °C | solid |
| Electron shells | 2, 8, 18, 23, 8, 2 |
| Electron configuration | [Xe] 6s² 4f⁵ |
| Valence electrons | varies (outer shell: 2) |
| Common ion | Pm³⁺ |
| Electronegativity (Pauling) | 1.13 |
| First ionisation energy | 5.58 eV (538 kJ/mol) |
| Atomic radius (empirical) | 185 pm |
Where promethium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does promethium have?
Promethium has 61 protons and 61 electrons (atomic number 61). Its most common isotope, promethium-145, has 145 − 61 = 84 neutrons.
What is the electron configuration of promethium?
[Xe] 6s² 4f⁵. In shells that is 2, 8, 18, 23, 8, 2.
How many valence electrons does promethium have?
Promethium is a lanthanide, so its valence electrons are spread over two shells; its outermost shell holds 2.
Where is promethium 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.
