Actinium is a very radioactive metal with the chemical symbol Ac and the atomic number of 89. It was first isolated in 1899 by the French chemist Andre Debierne from pitchblende residues left after the discovery of radium by Marie Curie. It was the first element in what is now known as the Actinide series, which includes lutetium and lawrencium. Its name is derived from the Greek word aktinos, meaning “ray.”

The atomic number of an atom gives a clue to its physical properties and to some of its chemical behavior. It also tells us how many protons are in its nucleus. The protons are the core particles of the atom, which are positively charged. There are also negatively charged electrons surrounding the protons in the nucleus. The total number of protons and neutrons in the nucleus of an atom is called its mass number. This value, which is given in units of kilograms per mole, is determined by multiplying the atomic number by the mass of an atom of carbon-12.

There are 36 isotopes of actinium, with most of them varying in their number of neutrons and electrons. The most stable isotope of actinium has 89 protons and 138 neutrons, giving it an atomic mass of 227. This isotope is the most common in natural and nuclear reactors, where it acts as a neutron source. It is also used as a therapeutic agent in targeted alpha therapy (TAT). Its radioactivity makes it an excellent source of beta and gamma radiation for treating cancerous cells.