We all know that atoms are made up of three subatomic particles called electrons, protons, and neutrons. Where the center of the atom called the nucleus is composed of protons and neutrons. If we talk about the charge of these particles, then the electron has negatively charged, proton has positively charged and neutron has no charge i.e neutral.
The ratio of the proton-to-electron is always one to one, so the atom as a whole has a neutral charge. For example, a carbon atom has 6 protons and 6 electrons. The properties of an atom can change considerably, depending on how many of each particle it has.
The changes in the number of neutrons in the nucleus of an atom produce different isotopes of that atom. For example - carbon has three isotopes:
1) carbon-12 (has six protons and six neutrons in its nucleus), It is a stable and commonly occurring form of the element.
2) carbon-13 (has six protons and seven neutrons in its nucleus), It is stable but rare.
3) carbon-14 (It has six protons and eight neutrons in its nucleus), It is unstable (i.e radioactive isotope) and rare.
The process of emitting these particles is known as radioactive decay. And scientists refer to the spontaneous emission of these particles as radiation.
Radioactive substances
The substances which decay (or disintegrate) by the spontaneous emission of radiations are called radioactive substances. Example - Some of the radioactive substances are Uranium (U), Radium (Ra), Thorium (Th), Polonium (Po), and Actinium (Ac).
Radioactive isotopes
The isotopes which decay (or disintegrate) by the spontaneous emission of radiations are called radioactive isotopes or radioisotopes. Example - Some of the radioactive isotopes are Uranium-238 (The best-known example of a naturally occurring radioisotope), Uranium-235, Molybdenum-99, Fluorine-18, etc.
Nature of radioactivity
Radioactivity is a nuclear phenomenon. Any physical changes (such as pressure and temperature) or any chemical changes (such as excessive heating, freezing, actions of strong electric and magnetic field, chemical treatment, oxidation, etc) can not change the rate of decay.
So it is clearly understandable that the radioactive phenomenon can not be due to the orbital electron which could easily be affected by such changes.