Many heavy isotopes, most notably californium-252, also emit prompt neutrons among the products of a similar spontaneous radioactive decay process, spontaneous fission.
This period contains the actinides, which contains the heaviest naturally occurring element, californium; subsequent elements must be synthesized artificially.
Californium reacts when heated with hydrogen, nitrogen, or a chalcogen (oxygen family element); reactions with dry hydrogen and aqueous mineral acids are rapid.
Californium isotopes with mass numbers 249, 252, 253, and 254 have been observed in the radioactive dust collected from the air after a nuclear explosion.
All actinides are radioactive, paramagnetic, and, with the exception of actinium, have several crystalline phases: plutonium has seven, and uranium, neptunium and californium three.