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# Can someone explain the mass spectra of Cl2 and Br2 ? watch

1. Thanks!
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2. (Original post by Mina_)
Thanks!
The link tells you why they have the spectra that they do!

Read what it says about bromine and apply it to the chlorine MS
3. (Original post by charco)
The link tells you why they have the spectra that they do!

Read what it says about bromine and apply it to the chlorine MS
I understand it for Br2 as the relative abundance for both isotopes is approx. 50% but I dont really understand why the Cl2 isotope relative abundance height is in a 91 ratio
4. (Original post by Mina_)
I understand it for Br2 as the relative abundance for both isotopes is approx. 50% but I dont really understand why the Cl2 isotope relative abundance height is in a 91 ratio
It's the statistical probability of each possible molecule occuring.

If you choose two chlorine atoms at random there is a 3/4 probability of it being a 35Cl atom and a 1/4 probability of it being a 37Cl atom

So if you consider the four possible combinations:

35Cl35Cl probability 3/4 x 3/4 = 9/16
35Cl37Cl probability 3/4 x 1/4 = 3/16
37Cl35Cl probability 1/4 x 3/4 = 3/16
37Cl37Cl probability 1/4 x 1/4 = 1/16

The middle two molecules are the same mass hence there is a 6/16 probability of mass 72

All of the probabilities add up to 1 (as they should) so you hae a ratio:

mass 70 = 9
mass 72 = 6
mass 74 = 1

And this is the intensity of the signals observed
5. (Original post by charco)
It's the statistical probability of each possible molecule occuring.

If you choose two chlorine atoms at random there is a 3/4 probability of it being a 35Cl atom and a 1/4 probability of it being a 37Cl atom

So if you consider the four possible combinations:

35Cl35Cl probability 3/4 x 3/4 = 9/16
35Cl37Cl probability 3/4 x 1/4 = 3/16
37Cl35Cl probability 1/4 x 3/4 = 3/16
37Cl37Cl probability 1/4 x 1/4 = 1/16

The middle two molecules are the same mass hence there is a 6/16 probability of mass 72

All of the probabilities add up to 1 (as they should) so you hae a ratio:

mass 70 = 9
mass 72 = 6
mass 74 = 1

And this is the intensity of the signals observed
This was extremely helpful! Thank you

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