Which of the following best describes the mass spectrum of fluorine, F₂? A) It consists of two peaks, appearing at 19 daltons and 38 daltons. B) It is similar to the spectra of bromine and chlorine. C) It consists of only one peak, since it is monoisotopic. D) It consists of two peaks, separated by two daltons. Or E) It consists of three peaks, appearing at 20, 21, and 22 daltons.
The mass spectrum of any material is the relative abundance of ions by the mass-to-charge ratio. Let’s look at this example peak on the mass charge axis. An ion 𝑥 with a mass of 10 daltons would appear here if it had a charge of plus. But a diatom of the same material with a charge of two plus would also appear here. However, in mass spectrometry, it’s much more likely, we’ll be analyzing singly positive ions. Generally speaking, it costs less energy to turn something into a positive ion than to turn a positive ion into a doubly positive ion. Now, let’s have a look at fluorine.
Since fluorine is already a gas, it will be passed directly into the ionization chamber. Once it’s in the ionization chamber, it will be ionized and in some cases it will fragment. These are the ions we are likely to get: F₂⁺ and F⁺. These ions will go into other stages and eventually be detected and registered on a computer. Fluorine, unlike bromine and chlorine, has only a single abundant isotope on Earth. That’s fluorine 19. We can use the mass number of this isotope, 19, to get a good estimate of the mass of a single atom, which is 19 daltons. You might also see this written with the units u as in unified atomic mass units or amu, atomic mass units.
Mass spectrometry involves electric fields, which help to move ions around. As such, mass spectrometry can only detect ions. So for a sample of fluorine gas, these are the ions we would detect. The F₂ parent ion and the F ion. Removing an electron from a fluorine atom isn’t going to change the mass very much. So we would expect this to have a mass of 19 daltons. This would give us a mass-to-charge ratio of 19. The parent iron F₂⁺ has a mass of two times 19 daltons, giving it a total mass of 38 daltons and a mass-to- charge ratio of 38.
Now, it’s not possible to predict the exact ratio of these ions. So the spectrum I’m going to draw is only a guess. So we have our two peaks at 19 and 38, one for F⁺ and one for F₂⁺. Now that we’ve had a good look at what the fluorine mass spectrum would look like, let’s look at the statements and see which one applies. Bear in mind that we’re looking for the best description of the fluorine spectrum, which means some of these statements may be true. But they may not be the best description. So they won’t be correct answers.
Statement A says that our mass spectrum of fluorine gas, F₂, will consist of two peaks. The spectrum we’ve predicted looks that way. So let’s continue. Statement A also says that the spectrum will show peaks at 19 daltons and 38 daltons, which indeed it does. So far, so good. Statement A is definitely true. But we’re gonna find out whether it’s the best description by looking at the other four first.
Statement B says that our spectrum for F₂ should be similar to the spectrums of Br₂ and Cl₂. In this respect, bromine isn’t very much like fluorine at all. It consists of two isotopes: bromine 79 and bromine 81, both with a natural abundance of about 50 percent. The presence of two isotopes, which differ in their masses by two daltons, would produce a spectrum like this, where we have a mixture of signals due to bromine 79 and bromine 81 and their various combinations. So the spectrum of fluorine wouldn’t be particularly similar to that of bromine.
So what about chlorine? Chlorine on Earth can mainly be found as two stable isotopes: chlorine 35 or chlorine 37. Chlorine 35 makes up about 75.8 percent of all chlorine on Earth, while chlorine 37 makes up 24.2 percent. This balance of isotopes will produce a mixture of signals due to chlorine 35 and chlorine 37 and their combinations. So the spectrum of fluorine isn’t that close to that of chlorine either. So B is definitely an incorrect answer. Before we move on to statement C, let’s just refresh our graph.
Statement C says that the spectrum for fluorine consists of only one peak. We can dismiss the statement straightaway because we know it has two. Spectrum C says that fluorine is monoisotopic. That’s true, but it’s not the point. We would only see one peak for any substance that was monoisotopic if it never formed molecules. So statement C is definitely not the correct answer.
Statement D says that the spectrum for fluorine has two peaks, which is true. However, it also says that those peaks are separated by only two daltons. But, in fact, they’re separated by 19. So we can move on to the last statement.
Statement E says that the mass spectrum of fluorine consists of three peaks at 20, 21, and 22 daltons. Neither of those parts are true because we have two peaks at 19 and 38 daltons.
So of the five statements given, the one that best describes the mass spectrum of fluorine, F₂, is that it consists of two peaks appearing at 19 daltons and 38 daltons.