Americium (Am)

There is a synthetic element in most people's hallway: an ionisation smoke detector contains about 0.3 micrograms of americium-241, whose alpha particles ionise the air in a small chamber so that smoke entering it changes the current. Seaborg and his group announced americium and curium in November 1945 not in a journal but on a children's radio quiz show, five days before the formal presentation, after a young panellist asked whether any new elements had been discovered. Americium-241 is also the strongest common source of low-energy gamma rays used for calibration.

Properties

Atomic number

95

Symbol

Am

Mass number of most stable isotope

[243]

Category

Actinide

Period

7

Block

f-block

Electron configuration

[Rn] 7s² 5f⁷

Electrons per shell

2, 8, 18, 32, 25, 8, 2

Melting point

1,449 K — 1175.8 °C — 2148.5 °F

Boiling point

2,284 K — 2010.8 °C — 3651.5 °F

Density

13.69 g/cm³

Electronegativity

1.3 (Pauling)

Oxidation states

+3, +4, +5, +6

Discovered

1944 by Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁷

Noble gas shorthand

[Rn] 7s² 5f⁷

Shells

K: 2 · L: 8 · M: 18 · N: 32 · O: 25 · P: 8 · Q: 2

What it is used for

• Ionisation smoke detectors — the commonest consumer use of any synthetic element
• Thickness and density gauges in paper and steel mills
• Well-logging neutron sources in oil exploration
• Proposed long-life power source for European deep-space missions

Hazards

• Alpha emitter with a 432-year half-life; dangerous if the source is broken open and inhaled
• Intact smoke detectors are safe — the source is sealed in a metal foil

No stable isotope; the mass given is the mass number of americium-243, the longest-lived at 7,370 years. The smoke detector isotope is americium-241, half-life 432 years.

Neighbours in the table