Among the following compounds, which will show $\mathrm{M^{+}}$ and $\mathrm{M+2}$ peaks in intensity ratio 1:1 in its EI mass spectrum?

Question and options

Among the following compounds, which will show

\mathrm{M^{+}}

and

\mathrm{M+2}

peaks in intensity ratio 1:1 in its EI mass spectrum?

Correct answer: B - $\mathrm{CH_3Br}$

Bromine has two isotopes,

\mathrm{^{79}Br}

and

\mathrm{^{81}Br}

, in roughly 1:1 abundance, so

\mathrm{CH_3Br}

shows M and

\mathrm{M+2}

peaks of nearly equal height. Cl gives 3:1, and F and I are monoisotopic.