Fermium (Fm): electron configuration, atomic mass and facts
Fermium (Fm) is element 100 of the periodic table, an actinide in period 7 (f-block). Its electron configuration is [Rn] 5f12 7s2, with electrons arranged in shells 2, 8, 18, 32, 30, 8, 2, and its most stable isotope has mass number 257.
▶ Watch Fermium’s electrons orbit in the interactive periodic tableFermium at a glance
| Symbol | Fm |
|---|---|
| Atomic number | 100 |
| Atomic mass | [257] u (most stable isotope) |
| Category | Actinide |
| Group | f-block (no group number) |
| Period | 7 |
| Block | f-block |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f12 6s2 6p6 7s2 |
| Noble-gas shorthand | [Rn] 5f12 7s2 |
| Electrons per shell | 2, 8, 18, 32, 30, 8, 2 |
| Electronegativity (Pauling) | 1.3 |
| State at 20 °C | solid |
Where is Fermium on the periodic table?
Fermium is in period 7 of the f-block, the actinide row printed below the main table. It has 7 electron shells, and this row is where the 5f subshell fills.
How to work out the electron configuration of Fermium
Put 100 electrons into subshells in order of rising energy, filling each before starting the next: 1s² (2), 2s² (4), 2p⁶ (10), 3s² (12), 3p⁶ (18), 4s² (20), 3d¹⁰ (30), 4p⁶ (36), 5s² (38), 4d¹⁰ (48), 5p⁶ (54), 6s² (56), 4f¹⁴ (70), 5d¹⁰ (80), 6p⁶ (86), 7s² (88), 5f¹² (100). In shorthand, [Rn] stands for the 86 electrons of radon, giving [Rn] 5f¹² 7s².
How electronegative is Fermium?
Fermium has a Pauling electronegativity of 1.3, ranking 62 of the 100 elements with a known value (fluorine is highest at 3.98). That is low: it tends to give up electrons and form positive ions.
Frequently asked questions
What is the electron configuration of Fermium?
The electron configuration of Fermium (Fm) is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f12 6s2 6p6 7s2. In noble-gas shorthand it is written [Rn] 5f12 7s2.
How many electrons does Fermium have in each shell?
Fermium has 100 electrons arranged in shells as 2, 8, 18, 32, 30, 8, 2.
What is the atomic mass of Fermium?
Fermium has no stable isotopes. Its most stable isotope has a mass number of 257.
Is Fermium a metal?
Yes. Fermium is an actinide.
Explore nearby elements
More actinides: Berkelium, Californium, Einsteinium, Mendelevium, Nobelium, Lawrencium. See all elements on the periodic table.