3.1.1 Atomic Structure - Mass number and isotopes


Students should:
  • be able to recall the meaning of mass number (A) and atomic (proton) number (Z)
  • be able to explain the existence of isotopes
  • understand the principles of a simple mass spectrometer, limited to ionisation, acceleration, deflection and detection
  • know that the mass spectrometer gives accurate information about relative isotopic mass and also about the relative abundance of isotopes
  • be able to interpret simple mass spectra of elements and calculate relative atomic mass from isotopic abundance, limited to mononuclear ions
  • know that mass spectrometry can be used to identify elements (as used for example in planetary space probes)
  • know that mass spectrometry can be used to determine relative molecular mass

Isotope definition

Isotopes are atoms of a specific element that have a definite number of neutrons and consequently a different mass. In effect all atoms are isotopes of one element or another.

Most elements have several isotopes, some of which are stable, and others that spontaneously break apart releasing radioactivity.

Example: Describe the isotopes of hydrogen

The element hydrogen has three isotopes, 1H, 2H and 2H. 1H is the most common of the isotopes and makes up 99.99% of any sample of hydrogen. 2H is also called deuterium and comprises the other 0.01% of naturally occurring hydrogen. The third isotope is called tritium and is not very common.

Tritium is radioactive and breaks apart spontaneously releasing radioactivity, in this case, a fast moving electron.

Note that the product of this radioactive process is helium. Effectively, one of the neutrons in the tritium nucleus has emitted an electron (called a beta particle) and turned into a proton.

Summary of the hydrogen isotopes
Isotope name mass protons neutrons electrons
1H hydrogen 1 1 0 1
2H deuterium 2 1 1 1
3H tritium 3 1 2 1


Relative abundance of isotopes

The percentage of an isotope occurring in a natural sample of an element is called its natural abundance. The natural abundance of each isotope on the earth are usually very similar wherever the sample is obtained. However, there are circumstances where these values are variable, such as in meteorites or areas of unusual volcanic activity etc.

Chlorine gas is mainly composed of two isotopes, 35Cl and 37Cl

These have natural abundances of roughly 75% and 25% respectively.


Calculating relative mass from natural isotopic abundance

The 'relative abundance' of an isotope means the percentage of that particular isotope that occurs in nature. Most elements are made up of a mixture of isotopes. The sum of the percentages of the specific isotopes must add up to 100%.

The relative atomic mass is the weighted average of the isotopic masses.

Example: Chlorine has two isotopes 35Cl and 37Cl, with relative abundance of 75% and 25% respectively.

This means that in any naturally occurring sample of chlorine 75% of the atoms are Cl-35 atoms and 25% of the atoms are chlorine-37 atoms.

Calculating natural isotopic abundances from relative atomic mass. The relative abundances can be obtained using simultaneous equations (don't be afraid, it's fairly easy)


Mass spectrometer

The mass spectrometer is an instrument used for analysing samples of elements and compounds. It consists of six basic stages.

The sample is injected into the instrument. It may need vaporising by heating in which case the initial stage also has an oven. The sample in gaseous state then passes into an ionising beam of electrons which knock electrons off the sample creating positive ions.

These positive ions are then accelerated by some electrostatically charged plates into magnetic field, which then deflects the particles according to their mass/charge ratio.

The deflected ions then arrive at the detector.

The six stages are then:

The mass spectrometer

Isotopic abundance from the mass spectrometer

The mass spectrometer is a very sensitive instrument and can detect particles with very small differences in relative mass. Isotopes have mass numbers that differ by at least one atomic mass unit and so are easily differentiated. The intensity of the signal in a spectrum is directly proportional to the amount of that species in the sample.

Hence, the spectrum tells us both which isotopes are present and their relative proportions.

mass spectrum of neon

The molecular ion

In mass spectrometry of compounds the highest peak is called the molecular ion. It corresponding to the entire molecule with one electron dislodged bythe ionising beam. This allows us to find the relative mass of a compound very easily.

This has double use, because the individual masses of atoms are slightly different, so the relative mass of the molecular ion can also give information about the elemental constituents of the molecule.

The molecular ion


quick test