Symbol

Ac

Atomic Number

89

Atomic Mass

227.0278

Family

Group 3 (IIIB) Transition metal

Pronunciation

ack-TIN-ee-um

Overview

Actinium is the third element in Row 7 of the periodic table, a chart that shows how the chemical elements are related to each other. Some chemists place it in Group 3 (IIIB), with scandium and yttrium. Other chemists call it the first member of the actinides. The actinides are the 14 elements that make up Row 7 of the periodic table. They have atomic numbers from 89 to 103 and are all radioactive. A radioactive atom is unstable and tends to throw off particles and emit energy in order to become stable. Either way of classifying actinium is acceptable to most chemists.

Actinium has chemical properties like those of lanthanum (number 57), the element just above it in the periodic table. Actinium is also similar to radium, the element just before it (number 88) in Row 7.

Naturally occurring actinium is very...