Chemistry

Structure of the Atom

Isotopes

In nature, most of the elements have a number of atoms which have the 'same atomic number' but 'different mass numbers'. Such atoms of an element are called 'isotopes'.Thus : 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).Isotopes of an element have different mass numbers because they contain different number of neutrons.It is clear that the isotopes of an element differ in the number of neutrons in their nuclei.Let us take an example to understand the meaning of isotopes more clearly.
All the chlorine atoms contain 17 protons, so the atomic number of all the chlorine atoms is 17. Now,some chlorine atoms have 18 neutrons whereas other chlorine atoms contain 20 neutrons. Chlorine atoms can, therefore, have mass numbers of 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 :

A = \begin{matrix} 35 \\ 17 \end{matrix}

Cl and

A = \begin{matrix} 37 \\ 17 \end{matrix}

Cl
Both of these chlorine isotopes have the same atomic number of 17 but different mass numbers of 35 and 37 respectively. The complete composition of the two isotopes of chlorine is given below :

Isotope	Protons	Neutrons	Electrons
$A = \begin{matrix} 35 \\ 17 \end{matrix}$ Cl $A = \begin{matrix} 37 \\ 17 \end{matrix}$ Cl	17 17	18 20	17 17

The diagrams of the two isotopes of chlorine are given below :
It should be noted that the mass number of an atom is equal to its atomic mass. So, we can also use the term "atomic mass" in place of "mass number" in the definition of isotopes and say that :isotopes are atoms of the same element having the same atomic number but different atomic masses.Please note that the difference in the masses of isotopes of an element is due to the different number of neutrons in their nuclei.For example, the two isotopes of chlorine contain different number of 18 and 20 neutrons,and hence they have different atomic masses of 35 u and 37 u respectively. It should be clear by now that whether we use the term "mass number" or "atomic mass" in the definition of isotopes, it means the same thing. Another point to be noted is that if they ask the nuclear composition of isotopes in the examination,then we should give the number of protons and neutrons present in the nucleus of each isotope. For example,
(i) Nuclear composition of

A = \begin{matrix} 35 \\ 17 \end{matrix}

Cl isotope = 17 protons + 18 neutrons
(ii) Nuclear composition of

A = \begin{matrix} 37 \\ 17 \end{matrix}

Cl isotope = 17 protons + 20 neutrons Please note that the isotopes of an element can also be represented by writing their mass numbers with the name of the element or symbol of the element. For example :
(i)The isotope of chlorine having mass number 35 can also be represented as chlorine-35 (or just Cl-35), and
(ii) The isotope of chlorine having mass number 37 can also be represented as chlorine-37 (or just Cl-37).
We will now give some more examples of isotopes of the various elements such as hydrogen, carbon,oxygen and neon.

Notes

Structure of the Atom - Notes

1. Drawback of Rutherford’s Model of the Atom