The Foundations of Chemistry

Solution

(a) For^3517 Cl: Atomic number17. There are therefore 17 protons per nucleus.

Mass number35. There are therefore 35 protons plus neutrons or, because

we know that there are 17 protons, there are 18 neutrons.

Because no charge is indicated, there must be equal numbers of protons and

electrons, or 17 electrons.

For^3717 Cl: There are 17 protons, 20 neutrons, and 17 electrons per atom.

These are isotopes of the same element. Both have 17 protons, but they differ in their numbers

of neutrons: one has 18 neutrons and the other has 20.

(b) For^6329 Cu: Atomic number29. There are 29 protons per nucleus.

Mass number63. There are 29 protons plus 34 neutrons.

Because no charge is indicated, there must be equal numbers of protons and

electrons, or 29 electrons.

For^6529 Cu: There are 29 protons, 36 neutrons, and 29 electrons per atom.

These are isotopes. Both have 29 protons, but they differ in their numbers of neutrons: one

isotope has 34 neutrons and the other has 36.

You should now work Exercises 16 and 18.

MASS SPECTROMETRY AND ISOTOPIC ABUNDANCE

Mass spectrometers are instruments that measure the charge-to-mass ratio of charged

particles (Figures 5-8). A gas sample at very low pressure is bombarded with high-energy

electrons. This causes electrons to be ejected from some of the gas molecules, creating

positive ions. The positive ions are then focused into a very narrow beam and accelerated

by an electric field toward a magnetic field. The magnetic field deflects the ions from

their straight-line path. The extent to which the beam of ions is deflected depends on

four factors:

1.Magnitude of the accelerating voltage (electric field strength).Higher voltages result in

beams of more rapidly moving particles that are deflected less than the beams of

the more slowly moving particles produced by lower voltages.

2.Magnetic field strength.Stronger fields deflect a given beam more than weaker fields.

3.Masses of the particles.Because of their inertia, heavier particles are deflected less

than lighter particles that carry the same charge.

4.Charges on the particles.Particles with higher charges interact more strongly with

magnetic fields and are thus deflected more than particles of equal mass with smaller

charges.

The mass spectrometer is used to measure masses of isotopes as well as isotopic abun-

dances, that is, the relative amounts of the isotopes. Helium occurs in nature almost

exclusively as^42 He. Its atomic mass can be determined in an experiment such as that illus-

trated in Figure 5-8.

A beam of Neions in the mass spectrometer is split into three segments. The mass

spectrum of these ions (a graph of the relative numbers of ions of each mass) is shown in

Figure 5-9. This indicates that neon occurs in nature as three isotopes:^2010 Ne,^2110 Ne, and

5-8

186 CHAPTER 5: The Structure of Atoms