Fermium at a glance
- Atomic number
- 100
- Symbol
- Fm
- Atomic mass
- 257 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 12 7s 2
- State at 25 °C
- Solid
- Melting point
- 1527 °C
- Boiling point
- n/a
- Density
- 9.7 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 627 kJ/mol
- Oxidation states
- +3
- Discovered
- 1952
Atomic structure
A neutral fermium atom has 100 protons and 100 electrons, and its most common isotope carries about 157 neutrons. One atom weighs about 4.27 × 10⁻²⁵ kg, so a mole of fermium (6.022 × 10²³ atoms) weighs 257 g. That nucleus holds 1.57 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 7 shells (K 2, L 8, M 18, N 32, O 30, P 8, Q 2), which gives the configuration [Rn] 5f 12 7s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. All of its isotopes are radioactive, so any fermium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) fermium is a solid. It melts at about 1527 °C; its boiling point has not been measured. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 9.7 g/cm³ (predicted), 9.7 times as dense as water.
Chemical behaviour
Its electronegativity is 1.3 on the Pauling scale. Pulling off its outermost electron takes 627 kJ/mol. A bond between fermium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. Its common oxidation states are +3, so in compounds you will usually meet it as Fm³⁺. It shares much of its chemistry with the other actinides, especially its neighbours Californium, Einsteinium, Mendelevium and Nobelium. Handle with care: as a pure element it is radioactive.
Where it comes from
It does not occur in nature in useful amounts; it is made in nuclear reactors or particle accelerators. It was first made in the same 1952 test debris as einsteinium by uranium soaking up many neutrons in a split second. Its longest-lived known isotope is Fm-257, about 100 days. It was first identified in 1952, making it the 100th element to be discovered (counting the 11 known since antiquity together).