Actinium,  ₈₉Ac

General properties
Pronunciation	/ækˈɪniəm/ ​(ak-TIN-ee-əm)
Appearance	silvery-white, glowing with an eerie blue light; sometimes with a golden cast
Mass number	227 (most stable isotope)
Actinium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson La ↑ Ac ↓ (Uqt) radium ← actinium → thorium
Atomic number (Z)	89
Group	group 3
Period	period 7
Block	f-block
Electron configuration	[Rn] 6d1 7s2
Electrons per shell	2, 8, 18, 32, 18, 9, 2
Physical properties
Phase at STP	Ac: Solid
Melting point	1500 K ​(1227 °C, ​2240 °F) (estimated)
Boiling point	3500±300 K ​(3200±300 °C, ​5800±500 °F) (extrapolated)
Density (near r.t.)	10 g/cm3
Heat of fusion	14 kJ/mol
Heat of vaporization	400 kJ/mol
Molar heat capacity	27.2 J/(mol·K)
Atomic properties
Oxidation states	+2, +3 (a strongly basic oxide)
Electronegativity	Pauling scale: 1.1
Ionization energies	1st: 499 kJ/mol 2nd: 1170 kJ/mol 3rd: 1900 kJ/mol (more)
Covalent radius	215 pm
Spectral lines of actinium
Other properties
Natural occurrence	Ac: From decay
Crystal structure	​face-centered cubic (fcc)
Thermal conductivity	12 W/(m·K)
CAS Number	7440-34-8
History
Discovery and first isolation	Friedrich Oskar Giesel (1902)
Named by	André-Louis Debierne (1899)
Main isotopes of actinium
Iso­tope Abun­dance Half-life (1/2) Decay mode Pro­duct 225Ac trace 10 d α 221Fr 226Ac syn 29.37 h β− 1.117 226Th ε 0.640 226Ra α 5.536 222Fr 227Ac trace 21.772 y β− 227Th α 223Fr
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Actinium is chemical element 89 on the periodic table. Its symbol is Ac. Actinium's mass is 227 g/mol.

Actinium is a silver radioactive, solid metal in actinide group. It is so radioactive that it glows in the dark. Even a small amount of actinium is dangerous to people.