Uranium at a glance
- Atomic number
- 92
- Symbol
- U
- Atomic mass
- 238.03 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 3 6d 1 7s 2
- State at 25 °C
- Solid
- Melting point
- 1132 °C
- Boiling point
- 4131 °C
- Density
- 19.1 g/cm³
- Electronegativity
- 1.38
- 1st ionization energy
- 598 kJ/mol
- Oxidation states
- +6, +4
- Discovered
- 1789
Atomic structure
A neutral uranium atom has 92 protons and 92 electrons, and its most common isotope carries about 146 neutrons. One atom weighs about 3.95 × 10⁻²⁵ kg, so a mole of uranium (6.022 × 10²³ atoms) weighs 238.03 g. That nucleus holds 1.59 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 21, P 9, Q 2), which gives the configuration [Rn] 5f 3 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. None of its isotopes is truly stable, but U-235 and U-238 decays so slowly that it is still found in nature.
Physical properties
At room temperature (25 °C) uranium is a solid. It melts at 1132 °C and boils at 4131 °C, so it stays liquid across a 2999-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 19.1 g/cm³, 19.1 times as dense as water. Its atoms have a radius of about 175 pm (Protactinium next door is 180 pm). In the solid, its atoms pack into an orthorhombic lattice.
Chemical behaviour
Its electronegativity is 1.38 on the Pauling scale. Pulling off its outermost electron takes 598 kJ/mol. A bond between uranium and chlorine would be mostly ionic, with an electronegativity difference of 1.8. Its common oxidation states are +6, +4, so in compounds you will usually meet it as U⁶⁺ and U⁴⁺. It shares much of its chemistry with the other actinides, especially its neighbours Thorium, Protactinium, Neptunium and Plutonium. Handle with care: as a pure element it is radioactive.
Where it comes from
It makes up 2.7 mg/kg of Earth's crust, the 49th most abundant element there. It was first identified in 1789, making it the 28th element to be discovered (counting the 11 known since antiquity together).