Revision: Substances in Common Use Science SSC (English Medium) 9th Standard Maharashtra State Board

Electrons in outer orbits are weakly bound with the nucleus. In solids these weakly bound electrons leave their individual atom and become a part of it. These electrons are known as free electrons.

As nucleus is positively charged it strongly attracts the negative charged electrons. The electron orbit close to the nucleus are tightly bound by strong attractive force of nucleus. These electrons are known as bound electrons.

The phenomenon of spontaneous disintegration of an unstable nucleus of a naturally occurring isotope accompanied by emission of active radiations, α particles, β particles and γ radiations is called radioactivity.

Radioactivity is the spontaneous emission of high-energy radiation from the nucleus of an unstable atom.

A radioactive substance is a substance whose nuclei are unstable and therefore emit radiation on their own.