The 15 metallic elements with atomic numbers 89–103. Actinium (⁸⁹Ac) is the first and gives its name to the group. The others are: thorium (⁹⁰Th), proactinium (⁹¹Pa), uranium (⁹²U), neptunium (⁹³Np), plutonium (⁹⁴Pu), americium (⁹⁵Am), curium (⁹⁶Cm), berkelium (⁹⁷Bk), californium (⁹⁸Cf), einsteinium (⁹⁹Es), fermium (¹⁰⁰Fm), mendelevium (¹⁰¹Md), nobelium (¹⁰²No), and lawrencium (¹⁰³Lr). All the actinides are radioactive, highly electropositive, tarnish when exposed to air, and react with water, releasing hydrogen.