Curium at a glance
- Atomic number
- 96
- Symbol
- Cm
- Atomic mass
- 247 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 7 6d 1 7s 2
- State at 25 °C
- Solid
- Melting point
- 1340 °C
- Boiling point
- 3110 °C
- Density
- 13.51 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 581 kJ/mol
- Oxidation states
- +3
- Discovered
- 1944
Atomic structure
A neutral curium atom has 96 protons and 96 electrons, and its most common isotope carries about 151 neutrons. One atom weighs about 4.1 × 10⁻²⁵ kg, so a mole of curium (6.022 × 10²³ atoms) weighs 247 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 25, P 9, Q 2), which gives the configuration [Rn] 5f 7 6d 1 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. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. All of its isotopes are radioactive, so any curium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) curium is a solid. It melts at 1340 °C and boils at 3110 °C, so it stays liquid across a 1770-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 13.51 g/cm³, 13.5 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 581 kJ/mol. A bond between curium 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 Cm³⁺. It shares much of its chemistry with the other actinides, especially its neighbours Plutonium, Americium, Berkelium and Californium. 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 plutonium-239 with alpha particles. Its longest-lived known isotope is Cm-247, about 15.6 million years. It was first identified in 1944, making it the 95th element to be discovered (counting the 11 known since antiquity together).