The Foundations of Chemistry

22
10 Ne. In Figure 5-9 we see that the isotope

20
10 Ne, mass 19.99244 amu, is the most abun-
dant isotope (has the tallest peak). It accounts for 90.48% of the atoms.^2210 Ne, mass
21.99138, accounts for 9.25%, and^2110 Ne, mass 20.99384, for only 0.27% of the atoms.
Figure 5-10 shows a modern mass spectrometer. In nature, some elements, such as
fluorine and phosphorus, exist in only one form, but most elements occur as isotopic
mixtures. Some examples of natural isotopic abundances are given in Table 5-3. The
percentages are based on the numbers of naturally occurring atoms of each isotope, not
on their masses.

Isotopes are two or more forms of

atoms of the same element with

different masses; the atoms contain the

same number of protons but different

numbers of neutrons.

5-8 Mass Spectrometry and Isotopic Abundance 187

Figure 5-8 The mass spectrometer. In the mass spectrometer, gas molecules at low
pressure are ionized and accelerated by an electric field. The ion beam is then passed
through a magnetic field. In that field the beam is resolved into components, each
containing particles of equal charge-to-mass ratio. Lighter particles are deflected more
strongly than heavy ones with the same charge. In a beam containing^1206 Cand^42 Heions,
the lighter^42 Heions would be deflected more than the heavier^1206 Cions. The
spectrometer shown is adjusted to detect the^1206 Cions. By changing the magnitude of the
magnetic or electric field, we can move the beam of^42 Heions striking the collector from B
to A, where it would be detected. The relative masses of the ions are calculated from the
changes required to refocus the beam.

Figure 5-9 Mass spectrum of neon (1ions only). Neon consists of three isotopes, of
which neon-20 is by far the most abundant (90.48%). The mass of that isotope, to five
decimal places, is 19.99244 amu on the carbon-12 scale. The number by each peak
corresponds to the fraction of all Neions represented by the isotope with that mass.

A

B

Slit

Collector

Detector

Electron Magnet

gun

Gas

inlet

Accelerating

plates

+

Beam of^126 C+ ions

Beam of^42 He+ ions

20

0.9048

0.0027 0.0925

21 22 23

Mass

Relative abundance