Question:What is meant by an atom being Radioactive?
If an atom is unstable it is said to be Radioactive.
Unstable atoms will spontaneously change into other types of atom.
The spontaneous change of a radioactive atom into a different kind of atom is called Radioactive Decay.
Radioactive decay is spontaneous and therefore random. It is not possible to predict when an individual atom will undergo radioactive decay, but if you have a large number of a particular type of radioactive atom then you can work out how many, on average, will have changed after a certain amount of time.
Although the individual atoms decay randomly, there is a certain probability of decay for each kind of atom.