Isotope Pattern Practice
Chlorine and bromine sign their names in every mass spectrum: a second peak two mass units above the molecular ion, at a height set by their natural isotope ratios. Read the cluster and call the halogen content, or use the M+1 peak to count carbons. The compound behind each pattern appears after you answer.
Isotope patterns are simulated from natural isotopic abundances for a real published compound; intensities are calculated, not measured.
Common Questions
Why does chlorine give a 3:1 M/M+2 pattern and bromine a 1:1 pattern?
Natural chlorine is 76% ³⁵Cl and 24% ³⁷Cl (roughly 3:1), so one chlorine puts a partner peak two mass units above M at about a third of its height. Bromine's isotopes ⁷⁹Br and ⁸¹Br are nearly 50:50, so one bromine gives an almost equal doublet. Two of either element spread the pattern binomially: Cl₂ gives 9:6:1 across M/M+2/M+4, Br₂ gives about 1:2:1.
Why do fluorine and iodine show no isotope pattern?
¹⁹F and ¹²⁷I are each essentially 100% of their element's natural abundance — they are monoisotopic. A compound loaded with fluorine or iodine therefore shows a flat M+2, indistinguishable at a glance from a halogen-free compound. Only Cl and Br announce themselves in the cluster; the absence of an M+2 partner never rules F or I out.
How do I count carbons from the M+1 peak?
¹³C is 1.1% abundant, so each carbon contributes about 1.1% to M+1: divide the M+1 intensity (as % of M) by 1.1. A molecular ion with M+1 = 13% suggests about 12 carbons. Nitrogen adds a small extra (0.37% per N), and past about C15 the estimate loosens; treat it as ±1–2 carbons, which is exactly how this page grades it.