Actinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.
Three reasons Actinium stands out
- 1
Radioactive, once used as neutron source and radiation therapy.
- 2
It glows with a pale blue light in the dark due to its radiation ionising the surrounding air.
- 3
Its radioactive isotope is used in a cutting-edge cancer therapy that delivers alpha radiation directly to tumour cells.
Blue light from invisible decay
Actinium is so radioactive that a sample can ionize surrounding air and appear to glow blue. It begins the actinide series and occurs only in traces in uranium ores. Actinium-225 is now prized for targeted therapies that deliver powerful alpha radiation over cellular distances.