Neutrons and protons constitute the mass number of an atom. It was once believed that all the atoms of a given element have the same mass number, that is, the same number of protons and neutrons. This is not true for all elements.
For example, it was found that there are some atoms of chlorine with mass number of 35 while others have a mass number of 37.
Atoms of the same element having different mass numbers are called isotopes. Some elements that have isotopes are given in Table 1.2.
Did you know?
Tin (Sn) has ten isotopes of mass numbers 112, 114, 115, 116, 117, 118, 119, 120, 122 and 124.
What are the characteristics of isotopes?
Although the isotopes of an element have slightly different masses, chemically they all behave in almost the same way. Chlorine’s isotope of mass number 35 does not behave differently from its isotope of mass number 37. But the properties which depend on mass number are different. Physical properties such as melting points, boiling points and densities of isotopes may vary.
For many years, efforts were made to separate the isotopes of various elements. Substantial success has been achieved and pure separated isotopes of hydrogen and many other elements are now available on a commercial basis. But, these are still difficult to obtain and expensive.