Plutonium at a glance
- Atomic number
- 94
- Symbol
- Pu
- Atomic mass
- 244 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 6 7s 2
- State at 25 °C
- Solid
- Melting point
- 639.4 °C
- Boiling point
- 3228 °C
- Density
- 19.82 g/cm³
- Electronegativity
- 1.28
- 1st ionization energy
- 585 kJ/mol
- Oxidation states
- +4
- Discovered
- 1940
Atomic structure
A neutral plutonium atom has 94 protons and 94 electrons, and its most common isotope carries about 150 neutrons. One atom weighs about 4.05 × 10⁻²⁵ kg, so a mole of plutonium (6.022 × 10²³ atoms) weighs 244 g. That nucleus holds 1.6 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 24, P 8, Q 2), which gives the configuration [Rn] 5f 6 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 plutonium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) plutonium is a solid. It melts at 639.4 °C and boils at 3228 °C, so it stays liquid across a 2589-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 19.82 g/cm³, 19.8 times as dense as water. Its atoms have a radius of about 175 pm (Neptunium next door is 175 pm). In the solid, its atoms pack into a monoclinic lattice.
Chemical behaviour
Its electronegativity is 1.28 on the Pauling scale. Pulling off its outermost electron takes 585 kJ/mol. A bond between plutonium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. Its common oxidation states are +4, so in compounds you will usually meet it as Pu⁴⁺. It shares much of its chemistry with the other actinides, especially its neighbours Uranium, Neptunium, Americium and Curium. 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 bombarding uranium-238 with deuterons. Its longest-lived known isotope is Pu-244, about 80 million years. It was first identified in 1940, making it the 93rd element to be discovered (counting the 11 known since antiquity together).