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How do we know?

What evidence do scientists use to determine the existence of isotopes and the atomic weights of elements?

The existence of different isotopes of an element in a sample is primarily determined using a technique called mass spectrometry. Mass spectrometry is an important analytical tool in chemistry that indicates the mass of atoms or molecules in a sample.

    Schematic of mass spectrometer

  • Ionization: a vapourized sample is injected into the mass spectrometer (see the schematic to the right) and a stream of electrons bombards the particles in the sample. When this occurs, each particle ejects an electron and becomes a cation.
  • Acceleration: The cations are accelerated through a magnetic field.
  • Deflection: This magnetic field causes the cations to be deflected. As a result, the cations are differentiated based on their masses.
  • Detection: The mass spectrometer generates a spectrum displaying various peaks corresponding to the mass-to-charge (m/z) ratio of the particles that were present in the sample. Usually, if an atom is analyzed by mass spectrometry, the m/z ratio is equal to the mass number (A) of the atom. For example, hydrogen-1 would be found at an m/z ratio of 1.

Each of the peaks in a mass spectrum of a pure element corresponds to an isotope of the element. Therefore, the number of peaks indicates the number of isotopes and the m/z ratio on the X-axis of the mass spectrum represents the atomic mass of each of the isotopes of an element. Use the Mass Spectrometer Learning Tool to explore how many isotopes that three elements other than oxygen have.

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