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3.2 Isotopes
Different atoms of the same element possessing different atomic masses but having same atomic number are known as Isotopes. Since the isotopic atoms have the same atomic number, they must contain an equal number of protons. As their atomic masses are different, there must be a difference in the number of neutrons they possess. They will show similar chemical properties but their physical properties will be different due to different masses. Also since all isotopes of the same element have the same atomic number they occupy the same place in the periodic table.
Fractional atomic masses can be explained from the study of isotopes.
Chlorine has two major isotopes having atomic masses 35
and 37 and present in the ratio 3 : 1. Hence atomic weight of chlorine
is neither 35 or 37 but the average of the two.
i.e. atomic weight of chlorine = 
= 35.5
Examples of isotopes of some common elements are illustrated in following table.
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Table 3 Isotopic Elements |
|
Element |
Atomic No. |
Mass No. |
Symbol |
No. of Protons |
No. of Neutrons |
Abudance |
|
Hydrogen
Deuterium
Tritium |
1
1
1 |
1.0078
2.0141
3.0161 |
1H1
1D2
1T3 |
1
1
1 |
0
1
2 |
99.99%
0.02%
rare |
|
Oxygen |
8
8
8 |
16
17
18 |
8O16
8O17
8O18 |
8
8
8 |
8
9
10 |
99.76%
0.04%
0.20% |
|
Chlorine |
17
17 |
34.97
36.97 |
17C135
17C137 |
17
17 |
18
20 |
76%
26% |
|
Neon |
16
16 |
20
22 |
16Ne20
16Ne22 |
16
16 |
4
6 |
90%
10% |
The above table can be used in the following way to find out the atomic weight of naturally occurring elements.
Example :
Here tritium is not considered as its abundance is very less.
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