Americium at a glance
- Atomic number
- 95
- Symbol
- Am
- Atomic mass
- 243 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 7 7s 2
- State at 25 °C
- Solid
- Melting point
- 1176 °C
- Boiling point
- 2607 °C
- Density
- 12 g/cm³
- Electronegativity
- 1.3
- 1st ionization energy
- 578 kJ/mol
- Oxidation states
- +3
- Discovered
- 1944
Atomic structure
A neutral americium atom has 95 protons and 95 electrons, and its most common isotope carries about 148 neutrons. One atom weighs about 4.04 × 10⁻²⁵ kg, so a mole of americium (6.022 × 10²³ atoms) weighs 243 g. That nucleus holds 1.56 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 8, Q 2), which gives the configuration [Rn] 5f 7 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 americium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) americium is a solid. It melts at 1176 °C and boils at 2607 °C, so it stays liquid across a 1431-degree range. It stays solid in a candle flame (about 1000 °C) but would melt in molten iron (1538 °C). Its density is 12 g/cm³, 12 times as dense as water. Its atoms have a radius of about 175 pm (Plutonium next door is 175 pm). 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 578 kJ/mol. A bond between americium 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 Am³⁺. It shares much of its chemistry with the other actinides, especially its neighbours Neptunium, Plutonium, Curium and Berkelium. 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 the wartime Metallurgical Laboratory in Chicago by having plutonium capture neutrons in a reactor. Its longest-lived known isotope is Am-243, about 7,400 years. It was first identified in 1944, making it the 94th element to be discovered (counting the 11 known since antiquity together).