ISOTOPES3.Atoms in a naturally occurring sample of the pure element (or of its com-
pounds) may consist of one or more isotopes. The abundance of an isotope is
defined as the percentage of atoms of that isotope in a sample of the element. The
abundance of isotopes in nature (their natural abundance) is fixed. For example,
about 75% of all chlorine atoms in nature are atoms of^3157 Cl (Table 3.2). This per-
centage does not change appreciably whether the chlorine occurs as chlorine gas, as
sodium chloride or as hydrochloric acid, or (in the case of chlorine compounds)
whether they were mined in Norway, Australia or Canada (see Fig. 3.3).
33Table 3.2Isotopic and atomic data for selected elements
Element Isotope Natural abundance/% Isotopic mass/u Atomic mass of element/u
Hydrogen^11 H 99.985 1.0078 1.008
(^21) H 0.01492 2.0140
(^31) H 0 3.01605
Chlorine^3517 Cl 75.770 34.9689 35.45
(^3717) Cl 24.229 36.9659
Uranium^23592 U 0.7205 235.0439 238.0
(^23892) U* 99.274 238.0508
Carbon^126 C 98.893 12.0000 12.01
(^136) C 1.107 13.0034
Oxygen^168 O 99.759 15.9949 16.00
(^178) O 0.0374 16.9991
(^188) O 0.2039 17.9992
Fluorine^199 F 100 18.9984 18.9984
With the exception of fluorine, the atomic masses of the elements are expressed to four significant
figures. Abundances and isotopic masses are taken from the American Handbook of Physics.
- shows that the isotope is radioactive.
Fig. 3.3A collection of hydrogen chloride molecules
(HCl) as found in nature. About 75% of the HCl
molecules contain the^35 Cl isotope.
Isotopes
(i)Use Table 3.2 to answer the following:
(a) Which isotope of uranium is the most abundant in nature?
(b) Which element exists only as one isotope in nature?
(c) Which isotope of hydrogen is radioactive?
(d) Which isotope of hydrogen does not occur in nature?
(ii)Bromine consists of two naturally occurring isotopes,^8315 Br and^7395 Br, and 50.69% of all bromine
atoms are of mass number 79. What percentage of bromine atoms has a mass number of 81?
Exercise 3D