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1.2Atomic Structure and Properties

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

5 min read 9 parts Lesson 8 of 16 1 quiz

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Lesson summary

1.2 Mass Spectra of Elements — Notes — AP CHEMISTRY notes: Explain the relationship between the mass spectrum of an element and the masses and abundances of its isotopes.A mass spectrum of a single element shows the isotopes of that element and their relative abundances in nature…

  • Fractional abundance
  • Mass spectrometer
  • Average atomic mass
  • Each peak corresponds to one isotope.The x-axis is mass (m/z) and the y-axis is relative abundance
  • For singly charged ions, m/z equals the mass in amu
  • The number of peaks equals the number of isotopes

Part 1 of 7What a mass spectrum shows

What a mass spectrum shows
A mass spectrum of a single element shows the isotopes of that element and their relative abundances in nature.
Each peak corresponds to one isotope.
  • The x-axis is mass (m/z) and the y-axis is relative abundance.
  • For singly charged ions, m/z equals the mass in amu.
The number of peaks equals the number of isotopes.

Part 2 of 7How the spectrometer works

1
The sample is vaporised and ionised.
2
The ions are accelerated and deflected according to their mass-to-charge ratio.
3
A detector records the relative abundance of each ion.
4
Mass spectra provide experimental evidence for the existence of isotopes.

Part 3 of 7Isotopes

Isotopes
Isotopes have the same number of protons but different numbers of neutrons, so different masses.
Isotopes of an element have identical chemical properties because they have the same electron configuration.

Part 4 of 7Average atomic mass

The average atomic mass on the periodic table is a weighted average: Σ(isotopic mass × fractional abundance).
Fractional abundance
Fractional abundance percentage ÷ 100, and all fractional abundances sum to 1.
If peak heights are given, divide each by the total height to obtain fractions.
The average is closest to the mass of the most abundant isotope.
It need not equal the mass of any real atom, for example Cl = 35.45 amu.

Part 5 of 7Calculating average atomic mass from a spectrum

1
Read the mass (m/z) of each peak from the x-axis.
2
Read each relative abundance from the y-axis.
3
Convert abundances to fractions.
4
Multiply each isotopic mass by its fractional abundance.
5
Add the products and check the result lies between the lightest and heaviest isotope masses.

Part 6 of 7Estimating abundances

  • With a known average mass and two isotopes, set x + y = 1 and solve the weighted-average equation.

Part 7 of 7Scope on the AP exam

  • Interpreting spectra of samples containing several elements, or peaks from species other than singly charged monatomic ions, is not assessed.

Key terms

TermMeaning
Mass spectrometerInstrument that separates ions by mass-to-charge ratio.
IsotopeAtom with the same Z but a different number of neutrons.
Mass spectrumPlot with a peak at each isotope mass and height showing abundance.
Average atomic massWeighted mean of isotope masses.
Relative abundancePercentage of each isotope.
m/zMass-to-charge ratio of an ion.

Avoid these mistakes

1
Using the simple mean of isotope masses instead of the weighted mean
2
Reading peak height as mass — height shows abundance
3
Forgetting to convert percentages to fractions
4
Expecting the average atomic mass to be a whole number

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