Californium at a glance
- Atomic number
- 98
- Symbol
- Cf
- Atomic mass
- 251 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 10 7s 2
- State at 25 °C
- Solid
- Melting point
- 899.9 °C
- Boiling point
- 1470 °C
- Density
- 15.1 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 608 kJ/mol
- Oxidation states
- +3
- Discovered
- 1950
Atomic structure
A neutral californium atom has 98 protons and 98 electrons, and its most common isotope carries about 153 neutrons. One atom weighs about 4.17 × 10⁻²⁵ kg, so a mole of californium (6.022 × 10²³ atoms) weighs 251 g. That nucleus holds 1.56 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 28, P 8, Q 2), which gives the configuration [Rn] 5f 10 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 californium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) californium is a solid. It melts at 899.9 °C and boils at 1470 °C, so it stays liquid across a 570-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 15.1 g/cm³, 15.1 times as dense as water. In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 1.3 on the Pauling scale. Pulling off its outermost electron takes 608 kJ/mol. A bond between californium 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 Cf³⁺. It shares much of its chemistry with the other actinides, especially its neighbours Curium, Berkelium, Einsteinium and Fermium. 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 at Berkeley by striking curium-242 with alpha particles. Its longest-lived known isotope is Cf-251, about 900 years. It was first identified in 1950, making it the 98th element to be discovered (counting the 11 known since antiquity together).