The atomic mass of fluorine is 18.998 amu, and its mass spectrum
...

Question

The atomic mass of fluorine is 18.998 amu, and its mass spectrum
shows a large peak at this mass. The atomic mass of chlorine
is 35.45 amu, yet the mass spectrum of chlorine does not
show a peak at this mass. Explain the difference.

Accepted Answer

Welcome back everyone in this example, we're told that when a sample of iodine is analyzed using mass spectrometry, the mass spectrum shows a large peak at 1 26.904. However, the mass spectrum of bromine does not show a peak at 79.904 which is the atomic mass of bromine. And we need to explain why this is so so what we're going to do is sketch out a mass spectrum here and sorry about that. So we have our Y axis and our X axis here and on our Y axis, we should recall that we have our relative abundance of our isotopes of our atom and then on our X axis we have their their atomic masses which we should recall is represented by m divided by Z. So first considering our I dine here, we'll use the color black for iodine. So according to the prompt we have a large peak at the value 1 26.904. So we'll draw that in here. So it's a large peak here. And actually let's make it scale to our graph. So large peak with a value of 1 26.904. And we should recognize that that would be the atomic mass of iodine. Then according to the prompt, we have our bro mean which does not show a peak at 79.904. But when we recall about the isotopes of bromine, We should recall that we have bro mean 79 and Bro mean 81. So these are going to be the isotopes of grooming. And we would recall that expressing these or the abundances of these isotopes on our mass spectrum, we would produce two smaller peaks where we have a peek at a value of around 78 .918 For our roaming 79. And then we would have a peak showing up at a value of around 80.916 for our bro mean 81. And so as you can see, we have these two smaller peaks produced. However, according to the prompt at the value 79.904, we don't have a peak produced for bromine. And so we want to recognize that for our iodine, we only have this one large peak. And versus our bro mean, we would recall that our iodine is going to be something known as mono isotopic. And this means that it only can form one naturally occurring stable isotope And in this case for iodine, that would be iodine 127. Now we should recall that mono isotopic elements can also have synthetic isotopes. And so with regard to our iodine, we would also recall that we have iodine 1 29 which is a radioactive isotope of iodine that's only present in trace amounts. And so that's why according to our mass spectrum, we only have this one large peak at 1 26.904 because we just have this one naturally occurring, stable isotope of iodine, which makes iodine mono ice a topic. And so to explain our answer, we would say that therefore bro mean is not a mono ice a topic element because as we explained above, it has two isotopes, which is why it produces two peaks on our spectrum and does not produce a peak at this value given in the prompt. And so this will actually complete our answer here. So because bromine is a or not a mono ice, a topic element would explain why our mass spectrum appears the way it does. So this would correspond to choice be on our multiple choice. So I hope that everything that I explained was clear. If you have any questions, please leave them down below and I will see everyone in the next practice video.

The atomic mass of fluorine is 18.998 amu, and its mass spectrum shows a large peak at this mass. The atomic mass of chlorine is 35.45 amu, yet the mass spectrum of chlorine does not show a peak at this mass. Explain the difference.

Watch next