Explain why the relative atomic mass of chlorine is not a whole number?

1 Answer
Rick Sanchez · 1s2s2p
Apr 28, 2018

The answer to why chlorine has an atomic weight of 35.5 not 35 is because of something called isotopes.

Explanation:

Chlorine-35 has 17 protons and 18 neutrons and occurs in nature about 75% of the time. Chlorine-37 has 17 protons and 20 neutrons and occurs in nature about 25% of the time. So, when the mass of chlorine is measured using an instrument called a mass spectrometer the result is a mass of 35.5. The atomic weight, or as it is properly called, the RELATIVE atomic mass, is a weighted average of the masses of all the isotopes of chlorine. #"Relative Atomic Mass" = (75/100 xx 35) + (25/100 xx 37) = 35.5#

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