1.2B Isotopes

What are Isotopes?

Isotopes are atoms of an element that have the same number of protons but a different number of neutrons.

Isotopes have very similar chemical properties because the number of protons and electrons are the same. Their atomic mass will differ because they have different numbers of neutrons. This difference in mass can lead to different physical properties (e.g. boiling points). Heavier isotopes move more slowly at a given temperature and these differences can be used to separate isotopes.

Relative Average Mass

Looking at the periodic table, we can see that some elements, like Chlorine, have decimal numbers in their atomic masses (35.45). But can an atom have 0.45 neutrons?

Determining Relative Atomic Mass

The relative average masses of elements are calculated using the relative abundance of isotopes. For chlorine, chlorine-35 has a relative abundance of 77.5% while chlorine-37 has a relative abundance of 22.5%.

Relative Average Mass = (% abundance of isotope 1)(mass of isotope1) + (% isotope2)(mass isotope2) + ...

R.A .M = (0.775)(35) + (0.225)(37)

R.A.M = 27.125 + 8.323

R.A.M = 35.45

Mass Spectra

An instrument known as a mass spectrometer can be used to measure the mass of individual atoms. The results are presented as a mass spectrum, where the percent abundance is plotted against the mass/charge ratio of different ions. The relative average mass can be calculated from this data.

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