 |
| 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.
|
|
|
|
|
|
|
| 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:1
|
|
| Natural chlorine consists of two isotopes:
|
|
| Calculate the average atomic mass of chlorine.
|
|
|
|
|
|
| Example: 2
|
|
| A naturally occurring sample of Lithium contains 7.42% of 6Li and 92.58% of 7Li. The relative mass of 6Li is 6.015 and that of 7Li is 7.016. Calculate the atomic mass of a naturally occurring sample of lithium.
|
|
|
| Solution:
|
|
|
|
| Example: 3
|
|
| Which of the following two nuclei are isotopes of each other?
|
|
|
| Solution:
|
|
| The two isotopes are:
|
|
|
|
|
|
| Isobars
|
|
| The atoms of different elements, which have the same mass number but different atomic numbers are called isobars. These have different number of protons but equal sum of number of protons and neutrons.
|
|
|
| Some Typical Isobars
|
|
|
|
| Isotones
|
|
| The atoms of different elements, which have the same number of neutrons but different atomic numbers, are called isotones.
|
|
|
| Some Typical Isotones
|
|
|
|
|
|
| Before we end let us appreciate how far the theory of the atoms and their structure has progressed since Dalton made first proclamations on the existence of the atom. Here is a comparison of Dalton's atomic theory with the Modern theory of atoms.
|
|
|
| Comparison of Dalton's Atomic Theory and Modern Atomic Theory
|
|
|
|
|
|
|
|
|
|
| |
