In Dalton’s Atomic Theory, all the atoms of an element were considered alike in all properties including their masses. Later on, it was discovered that atoms of the same element can possess different masses but same atomic numbers. Such atoms of an element are called isotopes.

So what really are isotopes? Isotopes can be defined as, “Different kind of atoms of the same element having a same atomic number, but different atomic masses”. Having same atomic number, the isotopes of an element possess same chemical properties and the same position in the periodic table. However, the physical properties of isotopes vary since these properties often depend upon mass. Isotopes have the same number of protons and electrons but they differ in the number of neutrons present in their nuclei.

Isotope Word Origin and History

The word ‘isotope’ was first introduced by a renowned British chemist Frederick Soddy in the year 1913, as recommended by Margaret Todd. The term isotope means ‘having the same place’, which was isolated from the Greek words ‘isos’ which means ‘equal’ and ‘topos’ which means ‘place’.

Isotopes Examples

Carbon has three isotopes written as [latex]_{ 6 }{ C }^{ 12 }[/latex], [latex]_{ 6 }{ C }^{ 13 }[/latex], and [latex]_{ 6 }{ C }^{ 14 }[/latex]. Each of these has 6 protons and 6 electrons. However, these isotopes have 6, 7 and 8 neutrons respectively. Similarly, hydrogen has three isotopes [latex]_{ 1 }{ H }^{ 1 }[/latex], [latex]_{ 1 }{ H }^{ 2 }[/latex], and [latex]_{ 1 }{ H }^{ 3 }[/latex] called protium, deuterium and tritium. Moreover, uranium has two, oxygen has three, nickel has five, calcium has six, palladium has six, cadmium has nine and tin has eleven isotopes.

Relative Abundance of Isotopes

The isotopes of all the elements have their own natural abundance. The properties of a particular element, which are mentioned in the literature, mostly correspond to the most abundant isotope of that element. The relative abundance of the isotopes of elements can be determined by many ways but mass spectrometry is the most commonly used technique. The following table shows the natural abundance of some common isotopes:

Researchers have concluded that above 280 different isotopes occur in nature, which include 40 radioactive isotopes as well. Besides these, about 300 unstable radioactive isotopes have been produced through artificial disintegration. These unstable isotopes have been known to be beneficial for various medicinal and scientific purposes. The distribution of isotopes among the elements is varied and complex.

The elements like arsenic, fluorine, iodine and gold, etc. have only a single isotope. They are called mono-isotopic elements. In general, the elements of odd atomic number almost never possess more than two stable isotopes. The elements of even atomic number usually have a larger number of isotopes, and isotopes whose mass numbers are multiples of four are particularly much more abundant. For example, [latex]{ 16 }_{ O }[/latex], [latex]{ 24 }_{ Mg }[/latex], [latex]{ 28 }_{ Si }[/latex], [latex]{ 40 }_{ Ca}[/latex] and [latex]{ 56 }_{ Fe }[/latex] form nearly 50% of the earth’s crust. Out of these 280 isotopes that occur in nature 154 have an even mass number and even atomic number.

In a nutshell, isotopes exist because atoms are constantly changing blocks of matter.  The nuclei of these atoms are constantly switching from one state of energy to another, either absorbing the energy or sometimes emitting it. Isotopes are abundant in nature but the majority of them are not stable. These unstable isotopes are mostly radioactive and can be used for various medical and scientific purposes.