Actinium is element number 89, with the symbol Ac. Its name comes from the Greek word for “ray.” It is the first of the actinoids.
Atomic number 89(227)
AcActiniumアクチニウム (akuchiniumu)
Symbol
Ac
Atomic weight
(227)
Place in the table
Period 7 · Actinoids
The number in parentheses is not an atomic weight. It is the mass number of one radioactive isotope of the element.
Atomic weight from the Chemical Society of Japan, “Table of Standard Atomic Weights with Four Significant Figures (2026)”.
The element of rays
Actinium was discovered in 1899, the year after the Curies found radium. Radium (Latin) and actinium (Greek) are both named for “ray.”
Where it sits on the table
The fifteen elements from actinium to lawrencium are called the actinoids. On the Room of the Elements table they are in the lower of the two rows set apart at the bottom.
The actinoids take their name from this first element, actinium.
Think about it
Q1Isn’t it interesting that the same meaning, “ray,” became two names, one Latin and one Greek?
Q2Count the fifteen actinoids on the Room of the Elements table.
Q3Why do you think elements linked to radioactivity were found one after another around 1898–1900?
There is no single right answer. Talking it over with someone at home is a good way to learn.
Did you know? — Elements with names linked to radioactivity: radium (1898), actinium (1899), radon (1900) and protactinium (1918).