A nucleus with mass number A and atomic number Z is represented as (A, Z).
Such isotopes are radioactive, and are referred to as “radioisotopes.” There are many types of emmitted particles and radiation that radioisotopes produce when they decay.
It was soon evident that Becquerel’s rays originated in the nuclei of the atoms.
The emission of these rays is called nuclear radioactivity, or simply radioactivity. A nucleus that spontaneously destroys part of its mass to emit radiation is said to decay.
The Curies’ radium salt glowed visibly from the radiation.
This radiation took its toll on them and other unaware researchers, though.
Thus, (A − 1, Z) means that the mass number is one less than before, but the atomic number didn’t change.