A Nuclear Science Primer

The unique identity of every chemical element is established by the number of positively charged protons in the nucleus—not the number of its negatively charged orbiting electrons, which can vary. An "ordinary" neutral hydrogen atom, atomic number one, is orbited by a single electron; a ordinary uranium atom, atomic number 92, is orbited by 92 electrons. But while nature is always natural, it's only ordinary part of the time. Atoms frequently lose electrons or acquire extra ones, thus becoming positively or negatively charged. Atoms with net electric charge are called ions.

While a nucleus can't lose or acquire protons without changing into a different element altogether, the number of its neutrons can vary—resulting in changes of mass and other interesting effects. Nuclei of the same element with the same atomic number (charge) but different masses are called isotopes of that element.

Up to atomic number 20, the common nuclei typically have the same number of neutrons as protons—one exception being hydrogen, about 99.99 percent of which has no neutron at all. The carbon isotope 12C, for example, with six protons and six neutrons, makes up all but one percent of natural carbon. Thereafter, as atomic number increases, the number of neutrons in the nucleus increases faster yet. Silver, a middleweight element with 47 protons in its nucleus, has two almost equally common isotopes, 107Ag with 60 neutrons and 109Ag with 62. In the heaviest elements, the ratio of neutrons to protons is skewed: the most common isotope of uranium, 238U, has 92 protons and a whopping 146 neutrons.

And That's Just the Beginning...
How and why nuclei fall apart is just one of many fascinating threads of explanation followed in the nuclear science wall chart in separate areas of concentration. Other sections of the chart are devoted to the expansion of the universe after the Big Bang; phases of nuclear matter such as the nucleon gas and the quark-gluon plasma; pathways to controlled nuclear energy; and applications of nuclear science, from medicine to the humble smoke detector. The chart of the nuclides shows atomic and neutron numbers for all known nuclei and indicates their predominant decay modes and regions of greater nuclear binding energy ("magic numbers"). Brief discussions of unstable nuclei and the newly discovered element 112 are included as well.

For too long the phrase "nuclear science" has conjured, in the popular imagination, little more than fears of mushroom clouds and reactors run amok. By improving the teaching of high school physics, the new Contemporary Physics Education Project wall chart, inspired by physicists at Berkeley Lab, shows one of the best ways for people engaged in the scientific enterprise to combat a rising tide of pseudoscience and prejudice.

What Holds It All Together
The inequity of neutron-to-proton ratio in massive nuclei has to do with the force that holds the nucleus together. Because like electrical charges repel each other, scientists realized early on that packed protons—those little nuggets of positive charge—should cause any nucleus with two or more protons to blow itself apart. A quite different force was required, which researchers understandably named the "strong nuclear force."

It turns out that both protons and neutrons consist of smaller entities called quarks. The strong force is what holds quarks together; it is the leftover, "residual" strong force that binds the nuclei. Neutrons and protons are both affected by the strong force, so adding more neutrons effectively increases the mutual attraction within a given nucleus and at the same time spaces the charged protons farther apart.

At around the atomic mass of uranium, however, the collection of hundreds of protons and neutrons in the nucleus gets too unwieldy for the residual strong force to keep things together very long. Therein lies another tale, that of the different modes of radioactive decay. . . .

Next: The Nucleus

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Charting Nuclear Science | A Nuclear Science Primer | The Nucleus

Research Review Fall '98 Index | Berkeley Lab