All elements are made up of tiny invisible particles called atoms. According to the scientists, at the centre of an atom there is a core called the nucleus around which the electrons revolve in different orbits. The nucleus is made up of two types of particles called protons and neutrons. All atoms of a particular element have the same number of protons in the nucleus. The number of protons in an atom, is called its atomic number while the total number of protons and neutrons is called its mass number or atomic weight. Chemical properties of a substance depend upon the electrons outside the nucleus while the physical properties on its mass number. Atoms of the same element having the same atomic number but different mass numbers are called isotopes. For example, hydrogen has three isotopes – protium, deuterium and tritium. Protium has only one proton, deuterium or heavy hydrogen has one proton and one neutron while tritium has one proton and two neutrons. Tritium does not exist naturally. It is produced artificially.
All the isotopes of same element have the same chemical properties but they differ in physical properties. Almost all elements are mixtures of isotopes. Oxygen and chlorine have two isotopes each. Two of the isotopes of Uranium are U235 and U238. Isotopes of some elements are radioactive in nature, i.e. they emit invisible radiations and decay into isotopes of other elements. Isotopes of an element are separated by an instrument called mass spectrograph. Particle accelerators are used to prepare ‘radio isotopes’. Manufacturers of precision instruments use some isotopes to detect tiny flaws in metal castings and to make extremely fine measurements.
