M + 2 Peak (CIE A Level Chemistry)

• The presence of bromine or chlorine atoms in a compound gives rise to a [M+2] and possibly [M+4] peak

Chlorine

• Chlorine exists as two isotopes, ³⁵Cl and ³⁷Cl
• A compound containing one chlorine atom will therefore have two molecular ion peaks due to the two different isotopes it can contain
  • ³⁵Cl = M⁺ peak
  • ³⁷Cl = [M+2] peak
  • The ratio of the peak heights is 3:1 (as the relative abundance of ³⁵Cl is 3x greater than that of ³⁷Cl)

• A compound containing two chlorine atoms will have three molecular ion peaks due to the different combinations of chlorine isotopes they can contain
  • ³⁵Cl + ³⁵Cl = M⁺ peak
  • ³⁵Cl + ³⁷Cl = [M+2] peak
  • ³⁷Cl + ³⁷Cl = [M+4] peak
  • The ratio of the peak heights is 9:6:1

Mass spectrum of compounds containing one chlorine atom (1) and two chlorine atoms (2)

Bromine

• Bromine too exists as two isotopes, ⁷⁹Br and ⁸¹Br
• A compound containing one bromine atom will have two molecular ion peaks
  • ⁷⁹Br = M⁺  peak
  • ⁸¹Br = [M+2] peak
  • The ratio of the peak heights is 1:1 (they are of similar heights as their relative abundance is the same!)

• A compound containing two bromine atoms will have three molecular ion peaks
  • ⁷⁹Br + ⁷⁹Br= M⁺ peak
  • ⁷⁹Br+ ⁸¹Br = [M+2] peak
  • ⁸¹Br + ⁸¹Br= [M+4] peak
  • The ratio of the peak heights is 1:2:1

Mass spectrum of compounds containing one bromine atom

Worked example: Analysing bromine spectra

Answer

The correct answer is D

Bromine is a diatomic molecule there will be 5 peaks on the mass spectrum of bromine

Bromine consists of molecules, not individual atoms

When bromine is passed through the mass spectrometer, an electron is given off to give the molecular ion, Br₂⁺

Some of these will fragment to make Br + Br⁺

• • Br₂⁺ → Br + Br⁺

The Br atom passes through the machine, and the Br⁺ ions will give lines at 79 and 81

There will also be a line for the unfragmented Br₂⁺ ion

This will give 3 molecular ion peaks

• • Br₂⁺ ion containing the isotopes 79 + 79 = 158
  • Br₂⁺ containing the isotopes 79 + 81 = 160
  • Br₂⁺ containing the isotopes 81 + 81 = 162