The highest mass peaks in the mass spectrum of Br2 occur at m / Z 158,160, and 162 . The ratio o

step 1 This question wants us to analyze the parent peaks for the mass spec data of bromine gas, given

The highest mass peaks in the mass spectrum of Br2 occur at...

The highest mass peaks in the mass spectrum of $\mathrm{Br}_{2}$ occur at $m / Z 158,160,$ and $162 .$ The ratio of intensities of these peaks is approximately $1: 2: 1 .$ Bromine has two stable isotopes, $^{79} \mathrm{Br}(50.7 \%$ abundance) and $^{81} \mathrm{Br}(49.3 \%$ abundance). (a) What molecular species gives rise to each of these peaks? (b) Explain the relative intensities of these peaks. (Hint: Consider the probabilities of each atom combination.

The highest mass peaks in the mass spectrum of $\mathrm{Br}_{2}$ occur at $m / Z 158,160,$ and $162 .$ The ratio of intensities of these peaks is approximately $1: 2: 1 .$ Bromine has two stable isotopes, $^{79} \mathrm{Br}(50.7 \%$ abundance) and $^{81} \mathrm{Br}(49.3 \%$ abundance).
(a) What molecular species gives rise to each of these peaks?
(b) Explain the relative intensities of these peaks. (Hint: Consider the probabilities of each atom combination.

Key Concepts

Molecular Ion Formation

The peaks at specific m/z values correspond to the formation of molecular ions with distinct isotopic compositions. For Br2, the ions are formed by the combination of the two bromine isotopes in all possible pairings, leading to different masses that correlate directly with the sum of the masses of the constituent isotopes.

Statistical Probability and Combinatorial Analysis

The relative intensities of peaks in a mass spectrum often reflect the statistical likelihood of forming each isotopic combination. When two atoms combine, the probability of each possible pairing depends on the isotopic abundances and the number of ways the isotopes can order themselves. For a diatomic molecule, one combination (different isotopes) can occur in two distinct ways, leading to approximately twice the intensity compared to the homonuclear combinations, which occur in only one way each.

Isotopic Composition

Elements that exist as multiple stable isotopes can lead to the formation of molecular ions with slightly different masses. In the context of molecular Br2, the presence of two stable isotopes of bromine results in different combinations that produce ions with distinct masses. This principle is central to interpreting the peaks observed in a mass spectrum.

Mass Spectrometry

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of ionized species, allowing the identification of molecules based on their mass. It provides a spectrum where each peak corresponds to a specific ionic fragment or molecular ion, and the position and intensity of the peaks carry information about the molecular composition.

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