Most of the elements present on this planet have few or no naturally occurring unstable isotopes, except for the very long-lived ones like uranium and thorium isotopes and an interesting pair of isotopes which illustrate how elements may be transformed into other elements by "building up" rather than decay processes.

Carbon-14 and Hydrogen-3 (tritium) are both formed in the upper atmosphere as a result of collisions with neutrons from solar radiation which reaches the earth: