Actinium

Element 89 on the periodic table, symbol Ac, group 3, period 7.

Actinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.

Properties

Atomic Number: 89
Atomic Mass: 227.0000 u
Group: 3
Period: 7
Block: f-block
Category: Actinide
State at 25 °C: Solid
Melting Point: 1,500 K · 1,226.9 °C
Boiling Point: 3,500 K · 3,226.9 °C
Density: 10 g/cm³
Atomic Radius: 195 pm
Electronegativity: 1.1
Oxidation States: +3
Abundance in Crust: 5.50e-10 mg/kg
Discovered: 1899
Discovered By: Friedrich Oskar Giesel

Electron Configuration

[Rn] 6d1 7s2

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 6d1

Ionisation Energies (kJ/mol)

1. 499
2. 1,170

Elements near Actinium

Elements in the same group share the number of outer electrons and behave in similar ways. Elements in the same period share a shell, so they change steadily from left to right.