Fermium (Fm): Bohr Model, Protons, Neutrons & Electrons
Fermium is element number 100, an actinide in the f-block (the actinide row drawn below the table), period 7. Its atom has 100 protons, 157 neutrons and 100 electrons arranged in 7 shells: 2, 8, 18, 32, 30, 8, 2.
Protons, neutrons and electrons in fermium
The atomic number of fermium is 100, so every fermium atom has 100 protons. A neutral atom has the same number of electrons, 100. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For fermium-257, that is 257 − 100 = 157 neutrons. Every isotope of fermium is radioactive; 257 is the mass number of its most stable one.
How to draw the Bohr model of fermium
- Draw the nucleus and write 100 p⁺ and 157 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 32 electrons on it.
- Draw the fifth shell and put 30 electrons on it.
- Draw the sixth shell and put 8 electrons on it.
- Draw the seventh shell and put 2 electrons on it.
- Check: 2 + 8 + 18 + 32 + 30 + 8 + 2 = 100 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, 32, 30, 8, 2) is the one to draw.
Electron configuration
The electron configuration of fermium is [Rn] 7s² 5f¹². As an actinide it fills an inner f-subshell, and its common ion is variable.
Key facts about fermium
| Symbol | Fm |
|---|---|
| Atomic number | 100 |
| Relative atomic mass | [257] |
| Group · period · block | f-block · period 7 |
| Type | Actinide |
| State at 25 °C | unknown |
| Electron shells | 2, 8, 18, 32, 30, 8, 2 |
| Electron configuration | [Rn] 7s² 5f¹² |
| Valence electrons | varies (outer shell: 2) |
| Common ion | — |
| Electronegativity (Pauling) | 1.30 |
| First ionisation energy | 6.50 eV (627 kJ/mol) |
| Atomic radius (empirical) | — |
Where fermium sits on the periodic table
Frequently asked questions
How many protons, neutrons and electrons does fermium have?
Fermium has 100 protons and 100 electrons (atomic number 100). Its most common isotope, fermium-257, has 257 − 100 = 157 neutrons.
What is the electron configuration of fermium?
[Rn] 7s² 5f¹². In shells that is 2, 8, 18, 32, 30, 8, 2.
How many valence electrons does fermium have?
Fermium is an actinide, so its valence electrons are spread over two shells; its outermost shell holds 2.
Where is fermium on the periodic table?
In the f-block (the actinide row drawn below the table), period 7, in the f-block. It is an actinide.
