85 At

Astatine (At)

Element 85 · Halogen · Solid at 25 °C · Group 17, Period 6

Astatine is the rarest naturally occurring element — probably less than a few grams exist in Earth's crust at any moment.

On the periodic table, astatine sits in group 17 — the halogens — in period 6. It is one of 6 elements classified as a halogen, and at room temperature (25 °C) it is a solid.

The halogens of group 17 are one electron short of a full outer shell, which makes them ferociously good at grabbing electrons from other elements. Their name means “salt-former”: they combine with metals to make salts such as sodium chloride. They exist as diatomic molecules, and the group is the only one to span gas, liquid, and solid at room temperature. Reactivity decreases down the group.

Properties of astatine

Properties of Astatine
Atomic number (Z)	85
Symbol	At
Atomic mass	(210) (mass number of the most stable isotope; this element has no standard atomic weight)
Category	Halogen
Group	17
Period	6
Block	p-block
Metal / nonmetal	Nonmetal
State at 25 °C	Solid
Electron configuration	[Xe] 4f14 5d10 6s2 6p5
Electronegativity (Pauling)	2.2
Melting point	302 °C (575.15 K / 575.6 °F)
Boiling point	337 °C (610.15 K / 638.6 °F)
Density	—

Position in the periodic table

Astatine shares group 17 — the halogens with Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Tennessine (Ts). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.

Physical properties

Astatine melts at 302 °C (575.15 K / 575.6 °F) and boils at 337 °C (610.15 K / 638.6 °F), giving it a liquid range of about 35 °C at standard pressure.

Data notes

The atomic mass is shown in parentheses because this element has no standard atomic weight; the number given is the mass number of its most stable known isotope.