Science Matters (17 page)

The question of why a particular material reacts with light in a particular way requires a complicated explanation, but it always depends on the way atoms and their electrons are linked. You can think of the process leading to color in this way: An incoming light-wave shakes an electron, accelerating it much as an ocean wave jostles a piece of driftwood. The accelerated electron in turn converts the energy it has received into another electromagnetic wave, which radiates away from the electron like radio signals from an antenna. The amount of original energy absorbed or radiated depends on the web of forces that holds the electron in place, and hence is extremely sensitive to small changes in the composition of the material.

It doesn’t take much to create vivid colors. Rubies, emeralds, and sapphires, the most prized of all colored gems, are all varieties of common colorless minerals. Adding a few atoms per thousand imparts a deep color (and high value) to otherwise bland rocks. A deep red ruby is nothing more than common aluminum oxide spiced with a trace of the element chromium (an element best known for its use in shiny chrome automobile bumpers). Chromium in a ruby soaks up a broad band of green energies; our eyes and brain interpret this lack of green as red. Most of the bright colors of objects you see around you arise from the same type of absorption. We “see” a color complementary to the one being absorbed.

In special instances, bright colors can arise from a different process. In so-called fluorescent materials, electrons absorb energetic radiation (commonly ultraviolet or “black light” radiation), raising an electron to a higher energy level. The energized electron soon drops down in a series of steps through lower
energy states, and in the process releases one or more photons at lower energy corresponding to visible light. These emitted photons give the material a vivid, bright color because they are concentrated in one very narrow range of wavelengths. When you look at a blue neon light, fluorescent paint, or Day-Glo colors, you are seeing such a narrow band of photons.

Melting and boiling aren’t the only kinds of phase transitions in nature. When a magnet forms in hot iron or a piece of cold metal becomes superconducting, more complex kinds of phase transitions occur. The study of these transitions occupies many theoretical physicists these days. They are finding that as different as these various phase transitions may appear to be, there are simple underlying regularities common to all of them. This suggests that when matter becomes more ordered (as in freezing) or more disordered (as in melting), it does so according to some as yet unknown general law. If we knew this law, we could understand exactly how it is that atoms establish themselves in a given order in materials.

Scientists look for patterns in nature—patterns to help us understand our universe and shape our environment. Countless thousands of materials have been synthesized and studied by materials researchers, who see again and again that atomic architecture dictates physical properties. If you want a strong fiber, use chains of carbon atoms. If you want a flexible electrical conductor, stick
with metals. If you want a tough electrical insulator, find a ceramic. We can use our understanding to design—atom by atom—new paints, new plastics, and new materials for golf clubs, window glass, and computer chips.

Look around you. Most of the objects you use in your home, at work, and school are the result of materials research. No one can predict the future, but you can be sure that in the coming months and years, scientists who engineer atoms will discover new materials that will change our world.

N
EXT TIME YOU FLIP
a light switch or turn on your TV, think for a moment about where the electricity you are using comes from. About 20 percent of the electricity generated in the United States comes from nuclear reactors, in whose core the incredible energies of the atom’s nucleus are harnessed for our needs.

In hospitals around the world those same energies are used to destroy tumors in patients who have contracted cancer, while in other parts of those same hospitals radioactive materials are injected into patients so that physicians can follow the movement of molecules inside the body and make diagnoses.

Meanwhile, in laboratories scientists measure the faint signals of decaying nuclei to estimate the dates at which the materials in which those nuclei appear were first made. The next time you see a mummy in a museum or hear someone talk about a dinosaur fossil, you can be sure that the scientists involved used the secrets of the atomic nucleus to obtain the knowledge they are passing on to you.

But whatever the use to which nuclear energy is put, it can always be understood in terms of a simple basic principle:

We now realize that mass is a very concentrated form of energy The nucleus of the atom, dense and heavy, accounts for most of the atom’s weight and almost none of its volume. The nucleus, which is responsible for nuclear energy and radioactivity, behaves independently of the atom’s far-flung electrons, which control chemical bonding. The nuclei of most familiar atoms are stable and do not change, but others disintegrate and send out energetic particles that we call radiation. In these reactions, some of the original mass converts to energy. Energy can also result from nuclear reactions like fission (the splitting of the nucleus) and fusion (the coming together of nuclei). In both cases, the energy obtained from a nucleus comes from the conversion of mass into energy.

The atom is almost all empty space. If the nucleus of a uranium atom were a bowling ball sitting in front of you right now, the electrons in orbit would be like 92 grains of sand scattered over an area equal to that of a good-sized city. Yet despite the small size of the nucleus, it ties up virtually all of the mass of an atom. In a rough way, you can say that the electrons determine the size of the atom, while the nucleus determines its weight. With so much mass crammed into such a tiny volume, the energy locked up in the nucleus is immense. This is why an atomic bomb, which works by rearranging things inside of nuclei, is so much more
destructive than ordinary chemical explosives, which work by rearranging electrons in their orbits.

The difference in size between the nucleus and the atom also helps explain an important characteristic of matter: what happens in the nucleus is largely independent of what happens to the electrons and vice versa. The electron in the suburbs doesn’t really care what the nucleus in the city is doing, and vice versa. Since chemistry involves the outer electrons, chemical reactions do not depend to any great extent on what happens in the nucleus. Similarly, the actions of the distant electrons do not affect what happens in the nucleus. These facts have extraordinary practical consequences.

The primary constituents of the nucleus are the proton (with a positive charge) and the neutron (which is electrically neutral). The number of protons in the nucleus determines the number of electrons in orbit, and hence the chemical identity of the atom. A carbon atom, for example, will always have a nucleus with six protons. But the number of neutrons in the nucleus can vary without changing the chemical identity of the atom. A nucleus with 6 protons and 6 neutrons, for example, is called carbon-12, while a nucleus with 6 protons and 8 neutrons is called carbon-14, but both are atoms of carbon. Nuclei with the same number of protons and different numbers of neutrons are said to be isotopes of each other. All known chemical elements have many isotopes.

The two principal kinds of nuclear particles, protons and neutrons, are locked tightly into the structure of the nucleus. It takes tremendous amounts of energy to change this nuclear structure. In the outer regions of an atom, electrons emit visible light when
they move from one orbit to another. Inside the nucleus, a proton or neutron making a similar change emits a gamma ray with a million times as much energy as that contained in visible light. The energy available in the nucleus is much higher than that available in the rest of the atom.

Almost all nuclear energy comes from conversion of mass. The mass of a nucleus is typically somewhat less than the sum of the masses of the protons and neutrons that would be assembled to create it. The nucleus of carbon-12, for example, has a mass about 1 percent less than the mass of six protons and six neutrons. When a carbon-12 nucleus forms, the excess mass is converted into energy via the formula
E = mc
²
, and this energy holds the nucleus together.

There are two ways to tap the energy of the nucleus: fission and fusion. In both cases, the energy we get comes from the conversion of mass into energy, and in both cases the energy is available because the mass of the final state of the nuclear system is less than that of the original.

A nucleus undergoes fission when it splits into two or more fragments. Usually the combined masses of the fragments is greater than the mass of the original nucleus. You normally have to put energy into splitting the nucleus, just as a logger puts energy into swinging an ax to split a log. Occasionally, however, the sum of the masses of the fragments is less than the mass of the original nucleus. In this case, the fission releases energy that is usually called “nuclear energy.”

The most familiar nucleus that yields energy when it splits is an isotope of uranium called uranium-235 (92 protons, 143 neutrons). This isotope makes up less than 1 percent of naturally
occurring uranium (the most common form is uranium-238). If a slow-moving neutron collides with uranium-235, the nucleus splits into two roughly equal fragments and two or three more neutrons. The sum of these masses is less than that of the original nucleus, and the difference in mass is converted into the energy of motion of the fragments. This energy, eventually released as heat, is used to run commercial nuclear reactors and generate electricity—perhaps even the electricity that provides the light you’re using to read this book. As you read, atoms of uranium are dying to provide your light.

The heart of a nuclear reactor is the core, a large stainless steel vessel that holds several hundred fuel rods. These pencil-thin shafts of uranium, rich in the isotope uranium-235, are separated by a fluid (usually water) whose atoms collide with neutrons and slow them down. When a fission occurs in one fuel rod, the fast neutrons leave the rod, are slowed down in the water, and then enter another rod and cause more fissions. These fissions each produce two or three neutrons of their own, each of which can go on to cause still more fissions in other rods. Scientists call this proliferation of collisions a chain reaction, the rate of which is regulated by lowering control rods of neutron-absorbing materials down between the fuel rods.

Nuclear reactors contain a core with radioactive nuclear fuel rods. Nuclear chain reactions in this fuel heat a surrounding jacket of water that converts heat into steam. The steam in turn drives an electric generator
.

The energy of the fission fragments heats the water, which is pumped out of the core. In another part of the generating plant this heat produces steam, which runs a conventional generator and produces electricity. Thus, a nuclear reactor differs from a coal-or oil-powered plant only in the way it produces heat. Once the steam is made, everything else is the same.

The aspect of nuclear reactor operation that most often occupies public attention is the possibility of accidents. The names Three Mile Island and Chernobyl conjure up visions of radioactive nightmares. The most serious reactor accidents (which are also the most unlikely) involve loss of the fluid that separates fuel rods. (At Three Mile Island, a faulty pump caused a partial loss.) A reactor
can’t
explode like a bomb, because with the moderating fluid gone, neutrons are no longer slowed down and the chain reactions stop. The core of the reactor is still hot, in both the thermal and nuclear sense, however, and the heat can start to melt the metal in the core. The China Syndrome—molten nuclear fuel so hot it melts through the Earth to China—is an exaggerated reference to such melting. In reality, nuclear fuel never gets hot enough to melt very far through the Earth. At Three Mile Island, which like all American reactors is housed in a reinforced concrete containment building, the partial meltdown led to no radioactivity outside of the reactor building itself. There
were no measurable public health problems. At Chernobyl, where the reactor was separated from the environment by glass windows, the consequences were much more serious.

The question that faces us all is whether we as a society are prepared to accept the (admittedly small) risks associated with nuclear power to gain the advantages of electricity generated by reactors. This isn’t a scientific question, but one of values—of weighing costs versus benefits. But in order to make a decision, every citizen should know some basic facts about reactors and radioactivity. In the 1970s, the collective reaction of American citizens was that the risks of building reactors outweigh the benefits, and no reactors were built for the remainder of the twentieth century. Today, with increasing concern about global warming caused by the burning of fossil fuels like coal, this decision is being rethought, and several companies have started the initial licensing procedure to build another generation of reactors.