Isotopes
Atoms of a given element which have same atomic number (nuclear charge) but different mass number are called isotopes. In orther words, isotopes are the atoms of the same element differing in mass number. Thus isotopes have same number of protons and electrons but different number of neutrons. They have same position in the periodic table, same chemical properties and same atomic charge. The term was first coined by soddy. However, isotopes were first separated by Aston using mass spectrometer (Ne20) and Nc22).
Examples:
(i) 1H1, 1H2, 1H3
(ii) 6C12, 6C13, 6C14
(iii) 8O16, 8O17, 8O18
(iv) 17Cl35 and 17Cl37
Of all the elements, tin has maximum number of stable isotopes (ten).
The fractional atomic weight (35.5) of chlorine is due to the fact that in the ordinary chlorine atom, Cl35 and Cl37 are present in the ratio of 3:1.
\(\therefore\) Average atomic weight of Cl = \(
\frac{3 \times 35 + 1\times 37}{4} = 35.5 \) amu
The percentage of a given isotope in the naturally occurring sample of an element is called its isotopic abundance. As the isotopic abundance of an element is constant irrespective of its source, atomic weight of an element is constant.
