What is the atomic number of Actinium?

Actinium has atomic number 89, which means a neutral atom has 89 protons and 89 electrons.

What is the chemical symbol for Actinium?

The chemical symbol for Actinium is Ac.

What category is Actinium in?

Actinium is classified as a actinide and sits in period 7 of the periodic table.

Element 89

Actinium (Ac)

Actinium is a chemical element with symbol Ac and atomic number 89. It is the first element of the actinide series. Actinium gave the name to the actinide series, a group of 15 similar elements between actinium and lawrencium in the periodic table.

Atomic Number

Atomic Mass

227 u

Period

Group

Actinide series

Phase

solid

Density

10.07 g/cm3

Electron Shells

2, 8, 18, 32, 18, 9, 2

Atomic Structure

Protons: 89. Electrons: 89. Neutrons: Varies by isotope.

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

Why Actinium Matters

Actinium helps connect atomic number 89, actinide behavior, period 7 trends, and electron shell structure. Its solid phase, atomic mass of 227 u, and electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 6d¹ make it useful for comparing bonding patterns across the periodic table.

Discovery and Occurrence

Actinium was discovered 1899 by André-Louis Debierne. Actinium is a chemical element with symbol Ac and atomic number 89. It is the first element of the actinide series. Actinium gave the name to the actinide series, a group of 15 similar elements between actinium and lawrencium in the periodic table.

Safety and Handling Context

Actinium has radioactive isotopes or is commonly discussed with radioactive materials. Use this page as an educational reference; real samples require qualified supervision and controlled handling.

Related Elements

Frequently Asked Questions

What is the atomic number of Actinium?: Actinium has atomic number 89, which means a neutral atom has 89 protons and 89 electrons.
What is the chemical symbol for Actinium?: The chemical symbol for Actinium is Ac.
What category is Actinium in?: Actinium is classified as a actinide and sits in period 7 of the periodic table.

Explore Actinium in 3D

Use the interactive table above to rotate atomic models, compare periodic trends, and move from Actinium to neighboring elements without losing your place.