100Fm[257]
Fermium (Fm)
Element 100 · Actinide
Fermium (Fm) is a chemical element with atomic number 100. It is classified as an actinide in period 7 of the periodic table (the f-block). Its most stable known isotope has mass number 257 and its ground-state electron configuration is [Rn] 5f¹² 7s².
Fermium facts
| Atomic number (Z) | 100 |
|---|---|
| Symbol | Fm |
| Mass number of most stable isotope | [257] |
| Category | Actinide |
| Group | — (f-block) |
| Period | 7 |
| Block | f-block |
| Electron configuration | [Rn] 5f¹² 7s² |
| Electronegativity (Pauling) | 1.3 |
| State at 20 °C | Unknown (synthetic) |
| Protons / electrons | 100 / 100 |
| Neutrons (most common isotope, approx.) | 157 |
| Molar mass | 257 g/mol |