89 Ac

Actinium (Ac)

Element 89 · Actinide · Solid at 25 °C · Period 7 (f-block)

Actinium gives the actinide series its name and is so radioactive that it glows faintly blue in the dark.

On the periodic table, actinium sits in the f-block in period 7. It is one of 15 elements classified as an actinide, and at room temperature (25 °C) it is a solid.

The actinides fill the 5f orbitals in the bottom row of the table, and every one of them is radioactive. Only thorium and uranium occur on Earth in significant quantities; the rest are made in reactors or particle accelerators. Their nuclear properties — fission above all — shaped the twentieth century.

Properties of actinium

Properties of Actinium
Atomic number (Z)	89
Symbol	Ac
Atomic mass	(227) (mass number of the most stable isotope; this element has no standard atomic weight)
Category	Actinide
Group	— (f-block, associated with group 3)
Period	7
Block	f-block
Metal / nonmetal	Metal
State at 25 °C	Solid
Electron configuration	[Rn] 6d1 7s2
Electronegativity (Pauling)	1.1
Melting point	1,050 °C (1,323.15 K / 1,922 °F)
Boiling point	3,200 °C (3,473.15 K / 5,792 °F)
Density	10.07 g/cm³

Position in the periodic table

Actinium belongs to the actinides, the detached f-block row shown beneath the main table. These elements all fit between groups 2 and 4 of period 7, and are pulled out into their own row only to keep the table a manageable width.

Physical properties

Actinium melts at 1,050 °C (1,323.15 K / 1,922 °F) and boils at 3,200 °C (3,473.15 K / 5,792 °F), giving it a liquid range of about 2,150 °C at standard pressure.

Its density is 10.07 g/cm³.

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.