Actinium (Ac)

Element 89 · actinide · group 3, period 7

• 89 protons
• 2/3 valence e−
• +3
• f-block

Properties

Electron configuration of actinium

Orbitals fill from the lowest energy upward. Writing that out for actinium gives 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 6d¹ 7s², which is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2 in plain text.

Everything up to Rn is the same as in that noble gas, so it is usually abbreviated to [Rn] 6d¹ 7s². The part outside the bracket is what does the chemistry.

Actinium ion charge

Actinium commonly forms ions with a charge of +3.

Common questions

How many valence electrons does actinium have?

There is no single agreed answer for actinium. The f-block elements draw on three subshells, so a single valence count is not meaningful for them. Check which convention your course uses before answering.

What is the electron configuration of actinium?

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2 6p6 6d1 7s2, or [Rn] 6d1 7s2 written from the previous noble gas. Orbitals fill lowest energy first.

What is the charge of actinium?

Actinium commonly forms ions with a charge of +3. Transition metals have several accessible oxidation states, which is why more than one charge is listed.

How many protons, electrons and neutrons does actinium have?

A neutral actinium atom has 89 protons and 89 electrons. Its standard atomic weight is 227, so the most common isotope has about 138 neutrons.

Nearby elements

• Lutetium (Lu)above it in group 3
• Thorium (Th)below it in group 3
• Radium (Ra)one place to its left
• Scandium (Sc)same group, same valence count
• Yttrium (Y)same group, same valence count
• Lanthanum (La)same group, same valence count

See the ionic charge of every element →