Page 8 - 11-chem-1

1.BASIC CONCEPTS eLearn.Punjab

Table (1.2) shows the natural abundance of some common isotopes.

Table (1.2) Natural abundance of some common isotopes.

Element Isotope Abundance (%) Mass (amu)

Hydrogen 1 H, 2H 99.985, 0.015 1.007825, 2.01410
Carbon 12 C, 13C 98.893, 1.107 12.0000, 13.00335
Nitrogen 99.634, 0.366 14.00307 15.00011
Oxygen 14 N, 15N 15.99491, 16.99914, 17.9916
Sulphur 16 O, 17O, 18O 99.759, 0.037, 0.204
Chlorine 32 S, 33S, 34S, 36S 31.97207, 32.97146, 33.96786, 35.96709
Bromine 36 Cl, 37Cl 95.0, 0.76, 4.22, 0.014 34.96885, 36.96590
75.53, 24.47 78.918, 80.916
79 Br, 81Br 50.54, 49.49

We know at present above 280 different isotopes occur in nature. They include 40 radioactive
isotopes as well. Besides these about 300 unstable radioactive isotopes have been produced
through artificial disintegration. The distribution of isotopes among the elements is varied and
complex as it is evident from the Table (1.2). The elements like arsenic, fluorine, iodine and gold,
etc have only a single isotope. They are called mono-isotopic elements.
In general, the elements of odd atomic number almost never possess more than two stable isotopes.
The elements of even atomic number usually have larger number of isotopes and isotopes whose
mass numbers are multiples of four are particularly abundant. For example, l6O, 24Mg, 28Si, 40Ca and
56Fe form nearly 50% of the earth’s crust. Out of 280 isotopes that occur in nature, 154 have even
mass number and even atomic number.

1.3.2 Determination of Relative Atomic Masses of Isotopes by Mass Spectrometry

Mass spectrometer is an instrument which is used to measure the exact masses of different
isotopes of an element. In this technique, a substance is first volatilized and then ionized with the
help of high energy beam of electrons. The gaseous positive ions, thus formed, are separated on
the basis of their mass to charge ratio (m/e) and then recorded in the form of peaks. Actually mass
spectrum is the plot of data in such a way that (m/e) is plotted as abscissa (x-axis) and the relative
number of ions as ordinate (y-axis).

8