Americium

Made during World War II, first revealed on a children's radio quiz show, and now sitting in millions of smoke detectors.

At a glance

Symbol: Am
Atomic number: 95
Mass number of its longest-lived isotope: [243]
Group: none; it sits in the actinide row below the table
Period: 7
Family: actinide
Electron configuration: [Rn] 5f⁷ 7s²
Electronegativity: 1.30
Common ions: Am³⁺
Isotopes: none are stable; ²⁴³Am lasts longest, and ²⁴¹Am, with a half-life of 432 years, is the one in smoke detectors

On the table

Period 7, in the actinide row set below the main table. The other actinides are tinted, and every cell opens its own article.

Electrons and bonding

Radioactive, like every actinide. It gives up electrons to form positive ions.

Properties

State at 25 °C: solid
What it looks like: silvery, radioactive metal
Melting point: 1176 °C
Boiling point: 2011 °C
Density: 13.7 g/cm³

Where it is found

Not found in nature. It is made in nuclear reactors from plutonium.

Discovery and name

Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso made it in 1944 at the University of Chicago's wartime Metallurgical Laboratory. Seaborg first revealed it on the children's radio show Quiz Kids in November 1945, days before the official announcement. It was named after the Americas, just as europium above it had been named after Europe.

What it is used for

Ionization smoke detectors hold a fraction of a microgram of americium-241, whose alpha particles let a tiny current flow until smoke interrupts it. Americium-beryllium neutron sources measure moisture in soil and probe the rock around oil wells.

In compounds

AmO₂americium dioxide, in smoke detectorsAmCl₃americium chloride

Handle with care

Radioactive. The source in a smoke detector is safe while sealed, but it should never be taken out or broken open.