Strong force does not differentiate whether the particle is a proton or a neutron, hence these particles together are referred to as nucleons. It is seen that in nature, for low mass atoms, N and Z are nearly equal. But as Z increases, N becomes more than Z, so as to counter balance the proton-proton Coulomb repulsion. But as mass of atoms becomes very large, they tend to be unstable and break up. The break up results in emission of energetic particles.
Unstable nucleus is called a radioactive nucleus.
The instability in the nucleus can be spontaneous or can be induced by a nuclear reaction. The energetic radiation given off by a radioactive nucleus is called radioactivity. Very rarely a radioactive atom may also emit neutrons. Radioactivity was discovered by accident by Henry Bacquerel in 1896. He had left a salt of uranium in one drawer of his desk and a photographic plate in another drawer.