Isotopes

It is interesting to note that atoms of a given atomic number can have different number of neutrons.

Some examples are listed below:

Hydrogen

Hydrogen atom (Z=1) has no neutrons.

Number of protons = 1

Number of electrons = 1

Number of neutrons = 0

It has been reported that the hydrogen element has atoms with mass number 2 and 3 also i.e.,

Atoms of elements having the same atomic number with different mass numbers are called isotopes.

Protium Deuterium Tritium

At.mass = 1 At.mass = 2 At.mass = 3

Nuclear composition of isotopes of chlorine:

Nuclear composition of isotopes of carbon:

Characteristics of isotopes

• All isotopes of an element have the same number of valence electrons thus have identical chemical properties.

• The physical properties of the isotopes are different due to the difference in the number of neutrons in their nuclei. The densities, melting points and boiling points etc., are slightly different.

Reason for fractional atomic masses of elements

Atomic masses of many elements are in fractions not in whole numbers.

Example

Cl - 35.5

Cu = 63.5

The fractional atomic masses of elements are due to the existence of isotopes having different masses.

Example

Natural chlorine consists of two isotopes:

Calculate the average atomic mass of chlorine.

Try it out!

• A naturally occurring sample of Lithium contains 7.42% of and 92.58% of . The relative mass of is 6.015 and that of is 7.016. Calculate the atomic mass of a naturally occurring sample of lithium.

Solution

• Which two of the following nuclei are isotopes of each other?

The two isotopes are: