Astatine (At) is a radioactive chemical element atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. There are only 25 grams of Astatine in the Earth's crust at any given time. Astatine is an extremely radioactive element; all its isotopes have short half-lives of 8.1 hours or less. The isotope Astatine 213 has a half-life of only 125 nanoseconds.

The bulk properties of astatine are not known with any certainty. Research is limited by its short half-life, which prevents the creation of weighable quantities. A visible piece of astatine would immediately vaporize itself because of the heat generated by its intense radioactivity. It is a member of the halogens (the group of elements including fluorine, chlorine, bromine, and iodine).

Promethium (Pm) is a chemical element with an atomic number 61. All of its isotopes are radioactive; it too is extremely rare, with only about 500-600 grams naturally occurring in Earth's crust at any given time.

Technetium (Tc) is a chemical element with an atomic number 43. It is the lightest element whose isotopes are all radioactive; none are stable, only about 18,000 tons can be found at any given time in the Earth's crust.

Antimony (Sb). The abundance of antimony in the Earth's crust is estimated to be 0.2 to 0.5 parts per million. Even though this element is not abundant, it is found in more than 100 mineral species.

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