bei48482_FM

388 Chapter Eleven

T

hus far we have been able to regard the nucleus of an atom merely as a tiny,

positively charged object whose only roles are to provide the atom with most

of its mass and to hold its electrons in thrall. The chief properties (except mass)

of atoms, molecules, solids, and liquids can all be traced to the behavior of atomic

electrons, not to the behavior of nuclei. Nevertheless, the nucleus turns out to be of

paramount importance in the grand scheme of things. To begin with, the very exis-

tence of the various elements is due to the ability of nuclei to possess multiple electric

charges. Furthermore, the energy involved in almost all natural processes can be traced

to nuclear reactions and transformations. And the liberation of nuclear energy in re-

actors and weapons has affected all our lives in one way or another.

11.1 NUCLEAR COMPOSITION

Atomic nuclei of the same element have the same numbers of protons

but can have different numbers of neutrons

The electron structure of the atom was understood before even the composition of its

nucleus was known. The reason is that the forces that hold the nucleus together are

vastly stronger than the electric forces that hold the electrons to the nucleus, and it is

correspondingly harder to break apart a nucleus to find out what is inside. Changes

in the electron structure of an atom, such as those that occur when a photon is emit-

ted or absorbed or when a chemical bond is formed or broken, involve energies of

only a few electronvolts. Changes in nuclear structure, on the other hand, involve

energies in the MeV range, a million times greater.

An ordinary hydrogen atom has as its nucleus a single proton, whose charge is e

and whose mass is 1836 times that of the electron. All other elements have nuclei that

contain neutrons as well as protons. As its name suggests, the neutron is uncharged;

its mass is slightly greater than that of the proton. Neutrons and protons are jointly

called nucleons.

The atomic numberof an element is the number of protons in each of its atomic

nuclei, which is the same as the number of electrons in a neutral atom of the element.

Thus the atomic number of hydrogen is 1, of helium 2, of lithium 3, and of uranium

92. All nuclei of a given element do not necessarily have equal numbers of neutrons.
    For instance, although over 99.9 percent of hydrogen nuclei are just single protons, a
    few also contain a neutron, and a very few two neutrons, along with the proton
    (Fig. 11.1). The varieties of an element that differ in the numbers of neutrons their
    nuclei contain are called its isotopes.
    The hydrogen isotope deuteriumis stable, but tritiumis radioactive and eventu-
    ally changes into an isotope of helium. The flux of cosmic rays from space continually
    replenishes the earth’s tritium by nuclear reactions in the atmosphere. Only about 2 kg
    of tritium of natural origin is present at any time on the earth, nearly all of it in the
    oceans. Heavy wateris water in which deuterium atoms instead of ordinary hydrogen
    atoms are combined with oxygen atoms.
    The conventional symbols for nuclear species, or nuclides,follow the pattern AZX,

where Xchemical symbol of the element

Zatomic number of the element

number of protons in the nucleus

Amass number of the nuclide

number of nucleons in the nucleus

Proton

Neutron

Electron

Ordinary

hydrogen Deuterium Tritium

Figure 11.1The isotopes of

hydrogen.

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