Isotopes

• Isotopes are atoms of the same element having the same atomic number but different mass numbers.
• Isotopes of an element have the same atomic number because they contain the same number of protons (and electrons) but different number of neutrons in their nuclei.

For example:

• All the chlorine atoms contain 17 protons, so the atomic number of all the chlorine is 17.
• Some chlorine atoms have 18 neutrons whereas other chlorine atoms contain 20 neutrons.
• Mass numbers of chlorine atoms 17 + 18 = 35 or 17 + 20 = 37
• Thus, chlorine has two isotopes of mass numbers 35 and 37 respectively.

The two isotopes of chlorine can be written as:

₁₇Cl³⁵      and     ₁₇Cl³⁷

• Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance. The chemical properties of isotopes are similar but their physical properties are different.
• The mass of an atom of any natural element is taken as the average mass of all the naturally occurring atoms of that element.
• If an element has no isotopes, then the mass of its atom would be same as the sum of protons and neutrons in it.
• But if an element occurs in isotopic forms, then the average mass is calculated by taking the percentage of each isotopic form.
• The average atomic mass of chlorine atom is:

₁₇Cl³⁵ percentage occurrence 75%

₁₇Cl³⁷ percentage occurrence 25%

• This does not mean that any one atom of chlorine has a fractional mass of 35.5 u. It means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass is 35.5 u.

1. Isotopes of Hydrogen

₁H¹   ₁H²   ₁H³

2. Isotopes of Carbon

₆C¹²  and  ₆C¹⁴

3. Isotopes of Oxygen

₈O¹⁶,  ₈O¹⁷ and ₈O¹⁸