Isotopes are atoms with the same number of protons and different number of neutrons. Atomic number is the number of protons in an atom and atomic mass is the sum of protons and neutrons. Isotopes are considered as element with same atomic number but different mass number.

Atomic number is the number of protons in the nucleus and it is also the number of electrons in an unionised atom. For example the atomic number of carbon is 6 and it has three isotopes carbon-12, carbon-13 and carbon-14 with mass numbers 12, 13 and 14.


An isotope is specified by the name of a particular element followed by a hyphen and a mass number. For example ‘C’ for carbon and the mass number is indicated with a subscript on the upper left of the chemical symbol. The atomic number is indicated on the lower left. It is common to write only the mass number in the subscript and leave out the atomic number. For example 12C.

Types of isotopes

Some isotopes of an element are radioactive. They are also sometimes referred to as radioactive isotopes or radionuclides. Some have never been observed to decay radioactively and are therefore called stable isotopes or stable nuclides. For example 14C is radioactive form of carbon and 12C and 13C are stable isotopes.

The existence of isotopes was first suggested based on the studies of radioactive chains that refers to 40 different radioactive elements between uranium and lead. Though the periodic table shows only 11 elements between them.

Did you know: The concept of isotope was introduced to explain radioactivity by F. Soddy. J. J. Thomson discovered the first stable isotope of Neon.

Primordial nuclides exist since the solar system formation. There are 339 naturally occurring nuclides out of which 286 are primordial nuclides. Primordial nuclides include 32 nuclides with long half life and 254 are considered stable nuclides as they do not decay. If the elements have stable isotopes the isotopes predominate in the elemental abundance. Whereas in three elements tellurium, indium and rhenium the isotope found in nature is actually radioisotope of the element though these elements have one or two stable isotopes.

Did you know: The predicted half life of some of the nuclides exceeds the age of the universe.

There are about 27 known radionuclides with half-lives longer than that of the universe.  

The first evidence of multiple isotopes of a stable element was found during the explorations into the composition of canal rays. Many multiple isotopes of stable elements were discovered using mass spectrograph.

Whole number rule of isotopes masses

The whole number rule states that the masses of the isotopes are whole number multiples of the mass of the hydrogen atom. In other words, atomic weights are multiples of the weight of the hydrogen atom.

Properties of isotopes

Isotopes have similar electronic structure and nearly identical chemical behaviour. The largest number of stable isotopes observed for any element is ten. No element has nine stable isotopes.

Isotope examples

Carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively.
Majority of hydrogen atoms consist of one proton and one electron, but some also have one or two neutrons.

Chlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons


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