# What do we learn from an isotope peak (or M+2 peak) in a mass spectrum?

Mass spectrum of a molecule showed molecular ion peak and its isotopic peak in 1:1 ratio. What is the possibility of the atom present in the molecule?

Since it is mentioned that it has a molecular ion peak it gives the fact that it must be aromatics, olefins and not alcohols or branched hydrocarbons since the latter have less probability to show molecular ion peaks . Since they contain isotopic peaks it must mean they have chloro or bromo compounds. But I do not understand how to use ratios to identify the atom present in the molecule

• The abundance of the isotopes of Chlorine 35:37 is approx 3:1 and is characteristic of chlorinated compounds. – Waylander Apr 30 '19 at 7:17

Each element has different natural abundances for its isotopes. Two peaks with approximately 1:1 ratio indicates the presence of bromine. $$\ce{^{79}Br}$$ is the most common isotope, corresponding to $$0.5069$$ of naturally occurring bromine. The isotopic composition for $$\ce{^{81}Br}$$ is $$0.4931$$.
What this means is that, for a molecule contaning a single bromine atom, approximately, half of the molecular ions will have $$\ce{^{79}Br}$$ and the other half will have $$\ce{^{81}Br}$$, resulting in two peaks with roughly the same intensity with $$m/Z$$ differing by two units.