Recognised as Chemical element
Actinium
- Ac
- Z = 89
- actinide
- No stable isotope
Actinium (Ac) is element 89, an actinide in period 7. Its most stable isotope has mass number 227. Weights follow IUPAC standard atomic weights, 2021 revision.
Identity
SymbolAc
Mass number of the most stable isotope227this element has no stable isotope
Mass in kilograms3.7694237 × 10^-25
Categoryactinide
GroupLanthanide/actinide series, with no group number
Period7
Blockf-block
Electrons in a neutral atom89
Neutrons in the most common isotope138≈
Electron configuration
Configuration1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s²ground state, subshells written in order of n
Noble gas shorthand[Rn] 6d¹ 7s²
Electrons per shell2, 8, 18, 32, 18, 9, 2K, L, M and outward
Electrons in the outer shell2
Unpaired electrons1Hund's rule; unpaired electrons make the free atom paramagnetic
Madelung ruleBrokenthe measured ground state differs from the filling-order prediction, 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f¹ 6s² 6p⁶ 7s²
Quantities
Molar mass227 g/mol
Mass of one mole227 grams
Atoms in one gram2.6529254 × 10^21
Mass of one atom3.7694237 × 10^-22 g
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