Francium

Francium is a highly radioactive alkali metal with atomic number 87. It occurs naturally only in trace amounts and is studied chiefly through atoms produced by radioactive decay.

Alkali metal: An element in Group 1 of the periodic table, characterized by a single outer electron and high chemical reactivity. Francium belongs to this group, whose shared chemistry helps predict its properties.
Actinium series: A radioactive decay chain that begins with uranium-235 and ends in stable lead-207. A branch of this chain produces naturally occurring francium-223.
Francium cooling and trapping: Experimental methods that use laser light and electromagnetic fields to slow and confine francium atoms. Trapping lets researchers study individual francium atoms before they decay.
Cesium: A soft, highly reactive alkali metal with atomic number 55 and one naturally occurring stable isotope. Cesium is a stable, experimentally accessible alkali-metal comparison for francium.
Atomic number: The number of protons in an atom’s nucleus, which determines the chemical element. Francium’s atomic number, 87, defines its identity.
Francium-223: The longest-lived francium isotope, with a half-life of about 22 minutes. It is the principal naturally occurring francium isotope and a common subject of study.
Atomic parity violation: A weak-interaction effect in atoms that makes mirror-reflected atomic transitions differ slightly. Francium’s large nucleus enhances effects sought in precision tests of the weak force.
Rubidium: An alkali metal with atomic number 37 that has both stable and radioactive isotopes. Rubidium shares francium’s group chemistry but is far less difficult to obtain.
Periodic table: A tabular arrangement of chemical elements ordered by atomic number and recurring properties. Francium occupies the final position in Group 1.
Alpha decay: Radioactive decay in which a nucleus emits a helium-4 nucleus, reducing its proton number by two. Francium-223 decays predominantly by alpha emission to astatine-219.

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