Why does the relative atomic mass of chlotine is 35.5 ?
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What is chlotine?
Chlorine has isotopes with mass numbers ranging from 32 g mol−1 to 40 g mol−1. There are two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5.
Source:
http://en.wikipedia.org/wiki/Chlorine
The relative atomic mass of chlorine is 35.5 because it is a weighted average of the masses of its isotopes, 35Cl and 37Cl. Chlorine has two stable isotopes: 35Cl, which makes up about 75.77% of chlorine atoms, and 37Cl, which makes up about 24.23% of chlorine atoms.
To calculate the relative atomic mass of chlorine, you multiply the mass of each isotope by its relative abundance (expressed as a decimal), and then add them together.
For example:
(35 g/mol * 0.7577) + (37 g/mol * 0.2423) = 26.4575 + 8.9711 = 35.4286 g/mol
Rounding to the nearest tenth gives us 35.5 g/mol, which is the approximate relative atomic mass of chlorine.