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Atomic Structure and Properties

Mass Spectra of Elements — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 16 Unit quiz

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Lesson summary

Mass Spectra of Elements — AP CHEMISTRY notes: Interpret the mass spectrum of an element and calculate average atomic mass from isotopic masses and abundances.A mass spectrometer measures the mass and relative abundance of each isotope in a sample.The result is displayed as a mas…

  • Substitute into average mass
  • Atomic mass unit
  • The result is displayed as a mass spectrum.Each peak is one isotope
  • Peak position gives the isotope mass and peak height gives the relative abundance
  • Chlorine shows peaks for chlorine-35 and chlorine-37, with chlorine-35 about three times as abundant (about 75% and 25%)
  • Isotopes have the same number of protons, a different number of neutrons and a different mass number

Part 1 of 6Mass spectrum of an element

Mass spectrum of an element
A mass spectrometer measures the mass and relative abundance of each isotope in a sample.
The result is displayed as a mass spectrum.
  • Each peak is one isotope.
  • Peak position gives the isotope mass and peak height gives the relative abundance.
Chlorine shows peaks for chlorine-35 and chlorine-37, with chlorine-35 about three times as abundant (about 75% and 25%).

Part 2 of 6Isotopes and relative abundance

Isotopes have the same number of protons, a different number of neutrons and a different mass number.
Most naturally occurring elements have more than one isotope.
Relative abundance
Relative abundance the percentage of each isotope in a sample.
Isotopes of an element have identical chemical behaviour because they have the same electron configuration.

Part 3 of 6Average atomic mass

Average atomic mass
Average atomic mass the weighted average of isotopic masses = Σ(isotope mass × relative abundance).
Abundances are converted from percentages to decimals by dividing by 100, so they sum to 1.
The result lies between the lightest and heaviest isotope masses and near the mass of the most abundant isotope.
The average need not equal the mass of any real atom, as chlorine's 35.45 amu shows.

Part 4 of 6Identifying an element from a spectrum

1
Estimate the weighted average from the peak masses and heights.
2
The average must fall between the lightest and heaviest isotopes.
3
It is close to the tallest peak but skewed toward the masses of other isotopes of significant abundance.
4
Compare the estimate with the atomic masses of the candidate elements.

Part 5 of 6Finding abundances from the average mass

1
For two isotopes let the fractions be x and y with x + y = 1.
2Substitute into average mass
(mass1)x + (mass2)y and solve.

Part 6 of 6Mass spectrometry in compounds

Mass spectrometry in compounds
When a compound such as AgF is analysed, subtract the known mass of the other element from each peak mass to obtain isotope masses of the element.
Only elements with a single isotope, such as fluorine, can be subtracted this way without extra peaks.

Key terms

TermMeaning
Mass spectrometerInstrument that measures isotope masses and abundances.
IsotopeAtom of the same element with a different number of neutrons.
Mass spectrumPlot of relative abundance against mass.
Relative abundancePercentage of each isotope in a sample.
Average atomic massWeighted average of isotope masses.
Atomic mass unitOne twelfth of the mass of a carbon-12 atom.

Avoid these mistakes

1
Using the simple mean of isotope masses — the mean must be weighted by abundance
2
Entering percentages as whole numbers without dividing by 100
3
Expecting the average mass to be a whole number
4
Thinking peak height shows mass — height shows relative abundance

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