[Alec]

Hello every one

I've been thinking about an exercise (brief answer is in the back of my book (so far so good)) to do with the natural occurring isotopes of bromium.

This was the instruction:

The bromine molecule, Br₂, is known to contain two atoms of bromine. A mass spectrum of the molecular ions formed showed three Br₂⁺ peaks, with mass numbers 158, 160, and 162. Use this information to determine which isotopes of bromine occur in nature.
(Taken from: Chemical Principles)

If no actual mass spectrum is included so that we can visually observe the occurrence, how can we determine solely on the basis of this textual instruction that it's 79 and 81?

I was thinking along the lines of the need for a covalent bond and this not being possible with an even atom number, I don't know.

Also have the feeling the + in "Br₂⁺ peaks" has part of the answer...

What do you think?

[Borek]

Technically you have to find M₁ and M₂ that satisfy a system of a linear Diophantine equations:

n₁M₁ + m₁M₂ = 158

n₂M₁ + m₂M₂ = 160

n₃M₁ + m₃M₂ = 162

where n_i+m_i=2.

This is actually a rather trivial exercise in logic, no chemistry required at all.

[Alec]

Thanks Borek for a quick response.

The difference between the three peaks I too observed to be two, but not in what manner it singles out Br-80...

[Borek]

Assuming Br-80 exists, what would be source of 158 peak? 162 peak?

[DrCMS]

If
A+A = 158
A+B = 160
B+B = 162

what are A and B?

[Alec]

If
A+A = 158
A+B = 160
B+B = 162

what are A and B?

Yes, that's very clear,

 

many thanks!