OCR AS Chemistry F331: Isotopes

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  • Created on: 22-08-13 17:38
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Isotopes
Isotope: atom of the same element with different mass number
Most elements exist naturally as isotopes.
The relative atomic mass is an average of the masses of the isotopes taking into account
their abundances.
Isotopes of some elements are given below, together with their percentage abundance in a
naturally occurring sample.
Element Isotope Abundance
Chlorine 35Cl 75%
37Cl 25%
Iron 54Fe 5.8%
56Fe 91.7%
57Fe 2.2%
58Fe 0.3%
Bromine 79Br 50%
81Br 50%
Calcium 40Ca 96.9%
42Ca 0.7%
43Ca 0.1%
44Ca 2.1%
48Ca 0.2%
Spectrometry can be used to calculate the relative atomic mass of elements.
For example, the relative atomic mass of chlorine is:
(35.0 x 75) + (37.0 x 25) = 2625 + 925 = 3550 = 35.5
100 100 100
The relative atomic mass of chlorine = 35.5

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