Actinium at a glance
- Atomic number
- 89
- Symbol
- Ac
- Atomic mass
- 227 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 6d 1 7s 2
- State at 25 °C
- Solid
- Melting point
- 1050 °C
- Boiling point
- 3198 °C
- Density
- 10.07 g/cm³
- Electronegativity
- 1.1
- 1st ionization energy
- 499 kJ/mol
- Oxidation states
- +3
- Discovered
- 1899
Atomic structure
A neutral actinium atom has 89 protons and 89 electrons, and its most common isotope carries about 138 neutrons. One atom weighs about 3.77 × 10⁻²⁵ kg, so a mole of actinium (6.022 × 10²³ atoms) weighs 227 g. That nucleus holds 1.55 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 9, Q 2), which gives the configuration [Rn] 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 actinium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) actinium is a solid. It melts at 1050 °C and boils at 3198 °C, so it stays liquid across a 2148-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 10.07 g/cm³, 10.1 times as dense as water. Its atoms have a radius of about 195 pm (Radium next door is 215 pm). In the solid, its atoms pack into a face-centred cubic lattice.
Chemical behaviour
Its electronegativity is 1.1 on the Pauling scale. Pulling off its outermost electron takes 499 kJ/mol. A bond between actinium and chlorine would be mostly ionic, with an electronegativity difference of 2.1. It releases 34 kJ/mol when it gains an extra electron. Its common oxidation states are +3, so in compounds you will usually meet it as Ac³⁺. It shares much of its chemistry with the other actinides, especially its neighbours Thorium and Protactinium. Handle with care: as a pure element it is radioactive.
Where it comes from
It makes up 5.5e-10 mg/kg of Earth's crust, the 86th most abundant element there. It was first identified in 1899, making it the 82nd element to be discovered (counting the 11 known since antiquity together).