Science Matters (16 page)

All of these adjectives—flexible, strong, light, colorful—refer
to physical properties that you can measure. Scientists organize the dozens of different physical characteristics into a few basic categories, including mechanical, magnetic, and optical properties. All of these properties are determined by the atoms and how they fit together.

Mechanical properties include a host of characteristics that de scribe how a material withstands stress and strain. Does it scratch easily? Does it break when you squeeze, twist, or stretch it? Mechanical properties are critical to thousands of day-to-day applications. You want your floor to be strong, your bed soft, and your toothbrush bristles flexible. Thousands of materials scientists spend their professional lives looking for “new and improved” products with useful mechanical properties.

Some materials are strong and others weak; some boughs bend while others break. These profound differences in material properties reflect equally profound differences in atomic structures. When two objects, like your feet and the floor, come into contact, two forces are involved. The force of gravity pushes you down, but an equal and opposite electrostatic force acts as electrons in feet and floor try to occupy the same space. Any time two chunks of matter are forced together, both objects must experience some deformation, even if only a subtle one. (Your feet become a little flatter, for example, and the floor bends just a bit.) Under mild stress most solids like feet and floors deform elastically; when stress is released they spring right back to their
original shape. If too much force is applied, however, materials reach their elastic limit and a permanent deformation occurs: metals bend, paper rips, glass shatters, fruit squishes.

Elasticity and strength have their origins in atomic structure. We already have part of the story in the relative strength of different chemical bonds. Clearly a material can’t be strong if all its bonds are weak. But even compounds with the strongest bonds can be weak if the atoms aren’t properly arranged. The way bonds are assembled into a three-dimensional structure makes all the difference. To grasp this point, consider again the two forms of pure carbon, diamond and graphite. Diamond is the hardest, strongest material known. In a diamond crystal, strong covalent bonds link each carbon atom to four neighbors. The entire carbon array forms a rigid three-dimensional framework.

Diamond and graphite are both crystals formed entirely from carbon, but their atomic structures are strikingly different. In diamond, each carbon atom is surrounded by four neighbors in a rigid three-dimensional framework. Graphite, on the other hand, is a layer structure with each carbon atom near only three others
.

Carbon-carbon bonds also dominate the graphite structure, but each atom is tightly bound to only three neighbors in layers of atoms. Graphite layers are held together by van der Waals forces so weak that graphite is one of the softest known minerals. Every time you use a “lead” pencil, black layers of graphite shear off the end and transfer to paper because the small force you exert on the pencil is enough to break the van der Waals force. Graphite illustrates the common adage that a chain is only as strong as its weakest link. The distribution of strong and weak bonds, rather than the type of atom, makes the critical difference in mechanical properties.

Carbon-carbon bonds hold together the strongest known fibers. Long chains of carbon atoms are essential structural elements of spiderwebs, nylon fibers, wood, and a host of plastics. These chain molecules, or polymers, can also make the best elastic bands when carbon-carbon chains adopt a folded or zigzag form, straightening only under tension.

Grab a hammer and strike a piece of fine china with all your might. The ceramic shatters into a thousand pieces, each retaining the shape of a fragment of the original dish. Do the same thing to a lump of lead. The lead deforms, absorbing the blow by changing shape. These differences reflect the diversity of atomic bonding.

China has ionic bonds, with alternating plus and minus ions that cause inflexible links in fixed directions. Attractive forces line up exactly along atomic pairs. Ionic crystals are like models
constructed of rigid balls and sticks. Once a few sticks come loose, the results are often catastrophic. If bent too far, the sticks break and are not easily re-formed.

Bonds in metals, with their freely roving electrons, are much less directional. Metal atoms are something like marbles in a bowl of molasses. Each round marble fits neatly into the array, surrounded by its neighbors. Molasses, like the metal’s electron sea, keeps everything stuck together. Tilt the bowl, however, and the marbles slowly slide past one another, adopting a new arrangement. The efficient packing of marbles is retained, but individual marbles shift. The same sort of thing happens when a metal is hammered or bent. Layers of atoms slide against each other, yielding a new shape to the metal object, but the bonding is preserved.

Rubber balls adopt a different strategy in response to a blow. Individual covalent carbon-carbon bonds in rubber are strong, but they have a great deal of directional leeway. The stress of a bat or foot deforms the ball, forcing the atoms closer together momentarily. This process parallels what happens when you squeeze a spring: you do work, adding energy to the system, which is stored as elastic potential energy in the spring. But though the bonds may bend, they do not break. The stored energy reconverts to kinetic energy as the bonds straighten out and the ball snaps back into shape and zooms off. Most sports would be a lot less interesting if we had only metal or ceramic balls!

If you’ve performed any sort of do-it-yourself work around your home you’ve probably run into composite materials, one of the hallmarks of modern materials science. Composites overcome the shortcomings of one substance by combining it with others.
Shatterproof car windshields, integrated circuits, and steel-reinforced concrete are all examples of the new technology.

Plywood is the classic composite material. It consists of thin layers of wood glued and pressed together, with the grain direction alternating layer by layer. The resulting lumber has no weak direction due to the grain, so plywood is stronger than traditional boarding of the same thickness. Furthermore, it can be fabricated in large sheets from relatively small trees, because the wood veneers can be sliced off a rotating tree like paper towels off a roll.

Fiber composites, in which strong, flexible strands are embedded in epoxy resins, provide lightweight materials of unusual strength. Fiberglass, made of hairlike glass threads woven, molded, and impregnated with a resin cement, is still the most widespread of these materials. Engineers increasingly use carbon fiber composites in aircraft design, and they provide an extra punch to tennis rackets and golf clubs.

All magnetic fields are created by moving electric charge, but when you look at a refrigerator magnet it’s not obvious that anything is moving. The key to the magnetic properties of refrigerator decorations, like all other material properties, lies at the atomic scale.

Every electron revolves around the atomic nucleus. If you could stand at one point along the orbit and count, you would see an electron go by every so often. The single moving electron, then, is a tiny electric current. These orbiting electrons, like other electric currents, create magnetic fields, and their net effect is to provide the atom with a magnetic field as well. This means
that you can imagine replacing each atom in a material by a tiny bar magnet with its own north and south pole. In the vast majority of materials these atomic magnets point in random directions, and the magnetic fields associated with them cancel each other out, so they can’t exert a magnetic force.

In a few materials, notably some compounds of iron, nickel, and cobalt, the orientations of atoms and their orbiting electrons are not entirely random. In these materials the atomic magnets line up with each other. This alignment takes place in blocks several thousand atoms on a side, called domains. Within each domain, each tiny atomic magnetic field reinforces the whole. In a permanent magnet neighboring domains tend to line up and reinforce each other as well, and the material as a whole is capable of exerting a large magnetic field. If the material is heated, however, the alignment of the domains can be scrambled and the magnet reverts to being an ordinary piece of iron, even though a uniform alignment is retained in each domain.

The atomic-scale origin of iron’s magnetism explains why every permanent magnet has a north pole and a south pole and why no isolated magnetic poles exist. If you break apart a magnet you always get small pieces with north and south poles. We can now see that this result follows from the fact that an electron in orbit is like a loop of electrical current, and that such loops always produce dipole fields. This statement is valid down to the basic unit of magnetism, the atom. But take an atom apart and the magnet disappears.

We humans have a special interest in the way matter behaves when it meets the very narrow range of electromagnetic radiation
called visible light. There is nothing intrinsically different about visible light compared to any other part of the spectrum; we could just as easily talk about the interaction of matter with radio or gamma rays. But visible light is special to us because we have a pair of sophisticated light detectors—our eyes—that allow us to determine luster, transparency, color, and a variety of other optical properties.

The sensitivity of our eyes to red, yellow, green, and blue is not pure chance. The sun’s energy output peaks around visible light, and Earth’s atmosphere is transparent to those wavelengths. Coincidentally the energy of light is similar to that of many kinds of bonding electrons. When you observe colors and lusters, you are in effect probing the world of atomic bonding.

Light interacts with matter in several ways. Some light waves pass right through a crystal without any effect more noticeable than tides have on a ship at sea. Other waves do interact with atoms, but are absorbed and reemitted unchanged. The net result of these two processes is that light energy can travel through matter, a process we call transmission. Window glass, water, and the atmosphere all transmit light.

Light moves a little more slowly in matter (with atoms) than in a vacuum (with nothing), so transmitted light waves undergo a slight change of direction at the surface, a phenomenon called refraction. Familiar effects like the shimmering of air above a heated highway, the apparent distortion of someone standing in a swimming pool, and the concentration of light by a lens result from refraction. Dozens of everyday optical devices, from microscopes and telescopes to eyeglasses and cameras, rely on lenses that bend light rays to achieve their aims.

Some light waves bounce off solids like ripples off the side of a boat, a phenomenon called scattering or reflection. The angle at which light strikes a flat surface exactly matches the reflected
angle. Most surfaces are rough, however, so that each light wave scatters through a slightly different angle and no image is formed. On very smooth surfaces, such as polished metals or glass in a mirror, the scattering is more uniform.

Other light waves add energy to the crystal and disappear—the process of absorption. Ordinary sunlight (or white light) is a mixture of light at all wavelengths—or all colors. A material we perceive to be colored absorbs some visible wavelengths more than others. A leaf that absorbs red light looks green to us, for example, and a stained-glass window that absorbs blue looks orange. If an object absorbs all the wavelengths, we call it black.

Each tiny packet of light—each photon—has a choice. It can be transmitted, reflected, or absorbed. Each of these events is a different kind of interaction between light and atoms, and not all photons have to do the same thing. When you stand in front of a shop window you see your ghostly reflection, proving that some light is being reflected. But at the same time you can view the goods on display as some light passes through the glass and back out again. Water transmits most light, but dive below about 100 feet and things get pretty dark because each foot of water absorbs a small fraction of the light that comes to it.

Opaque colored materials, like dyed clothing, poster paper, or flowers, scatter the light they don’t absorb. Transparent colored materials, including stained glass, gemstones, and mixed drinks, transmit the unabsorbed light rays. Your body absorbs some light, but reflects some light both from the surface and from a fraction of an inch below the surface, thus giving healthy skin a kind of “glow.” Combinations of transmission, absorption, and scattering result in many distinctive surface lusters: greasy, metallic, waxy, or dull. For each visible wavelength, any fraction of photons may be transmitted, absorbed, or scattered, leading to an infinite variety of colors and lusters.