bei48482_FM

390 Chapter Eleven

Table 11.1Some Masses in Various Units

Particle Mass (kg) Mass (u) Mass (MeV/c^2 )

Proton 1.6726  10 ^27 1.007276 938.28

Neutron 1.6750  10 ^27 1.008665 939.57

Electron 9.1095  10 ^31 5.486  10 ^4 0.511

(^11) H atom 1.6736  10  (^27) 1.007825 938.79
mass units(u), which are so defined that the mass of a^126 C atom, the most abundant
isotope of carbon, is exactly 12 u. The value of a mass unit is
Atomic mass unit 1 u1.66054 10 ^27 kg
The energy equivalent of a mass unit is 931.49 MeV. Table 11.1 gives the masses of
the proton, neutron, electron, and^11 H atom in various units, including the MeV/c^2. The
advantage of using this unit is that the energy equivalent of a mass of, say, 10 MeV/c^2
is simply Emc^2 10 MeV.
Table 11.2 gives the compositions of the isotopes of hydrogen and chlorine. Chlo-
rine in nature consists of about three-quarters of the^35 Cl isotope and one-quarter of
the^37 Cl isotope, which yields the average atomic mass of 35.46 u that chemists use
(see Table 7.2). The chemical properties of an element are determined by the number
and arrangement of the electrons in its atoms. Since the isotopes of an element have
almost identical electron structures in their atoms, it is not surprising that the two iso-
topes of chlorine, for instance, have the same yellow color, the same suffocating odor,
the same efficiency as poisons and bleaching agents, and the same ability to combine
with metals. Because boiling and freezing points depend somewhat on atomic mass,
they are slightly different for the two isotopes, as are their densities. Other physical
Mass spectrometer being used to study the composition of semiconductor crystals.
bei48482_ch11.qxd 2/4/02 11:56 AM Page 390