Thorium at a glance
- Atomic number
- 90
- Symbol
- Th
- Atomic mass
- 232.04 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 6d 2 7s 2
- State at 25 °C
- Solid
- Melting point
- 1842 °C
- Boiling point
- 4788 °C
- Density
- 11.72 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 587 kJ/mol
- Oxidation states
- +4
- Discovered
- 1829
Atomic structure
A neutral thorium atom has 90 protons and 90 electrons, and its most common isotope carries about 142 neutrons. One atom weighs about 3.85 × 10⁻²⁵ kg, so a mole of thorium (6.022 × 10²³ atoms) weighs 232.04 g. That nucleus holds 1.58 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 18, P 10, Q 2), which gives the configuration [Rn] 6d 2 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 Th-232 decays so slowly that it is still found in nature.
Physical properties
At room temperature (25 °C) thorium is a solid. It melts at 1842 °C and boils at 4788 °C, so it stays liquid across a 2946-degree range. It stays solid in molten iron (1538 °C) but would melt in a welding arc. Its density is 11.72 g/cm³, 11.7 times as dense as water. Its atoms have a radius of about 180 pm (Actinium next door is 195 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.3 on the Pauling scale. Pulling off its outermost electron takes 587 kJ/mol. A bond between thorium and chlorine would be mostly ionic, with an electronegativity difference of 1.9. It releases 113 kJ/mol when it gains an extra electron. Its common oxidation states are +4, so in compounds you will usually meet it as Th⁴⁺. It shares much of its chemistry with the other actinides, especially its neighbours Actinium, Protactinium and Uranium. Handle with care: as a pure element it is radioactive.
Where it comes from
It makes up 9.6 mg/kg of Earth's crust, the 38th most abundant element there. It was first identified in 1829, making it the 54th element to be discovered (counting the 11 known since antiquity together).