The Joy of Chemistry: The Amazing Science of Familiar Things

First Chapter

THE JOY OF CHEMISTRY

Prometheus Books

Chapter One

In the above epigraph, Emerson was not intimating that science is sexy; he meant that we base our scientific theory on the input of our senses: touch, taste, hearing, sight, and smell. But when it comes to the theory of atoms, our senses fail. We can't see an individual atom, we can't taste one, we can't feel one, we can't hear one, and we can't smell just one. If someone hits us on the head with a single atom of tungsten (fairly hefty as far as atoms are concerned), we remain blissfully unaware.

The reason we are so insensitive to single atoms is that a single atom is extremely small. Some ten million individual atoms would have to line up to span the length of a grain of rice. The parts that compose atoms are smaller yet. The nucleus, or center of an atom, is made up of protons and neutrons, and the radius of a proton is on the order of a femtometer, which is a quadrillionth of a meter, or a millionth of a billionth of a meter. Mighty small. The protons and neutrons have a mass of about a septillionth of a gram (a trillionth of a trillionth), which is diminutive in anybody's book. The electrons are about two thousand times less massive than the protons and neutrons. Electrons are to protons as fleas are to an elephant-the proton being the elephant. When we calculate the mass of an elephant, we don't add on the mass of the elephant's fleas, and when we calculate the mass of an atom, we don't add the mass of the electrons.

So the question naturally arises: If atomic particles are so small we can't use our senses to detect them, how do we know they are there? By inference. Humankind learned long ago that the input of the senses can be flawed. Optical, olfactory, and tactile illusions abound. So to discover the nature of those parts of the world that cannot be smelled, touched, and seen, people have learned to look at secondary effects and infer their causes. The concept was well captured by the venerated scientist Ernest Rutherford in the advice he gave to James Chadwick when Chadwick was looking for evidence for the neutron. Rutherford advised,

How could you find the Invisible Man in Piccadilly Circus? ... [B]y the reactions of those he pushed aside.

Similarly, J. J. Thomson, making inferences from his own work and the work of others, declared the existence of electrons in 1897. By consensus, electrons were assigned a negative charge. Thomson was able to determine the amount of charge on a given mass of electrons by bending a beam of electrons in a magnetic field.

This interaction of electrons with a magnetic field could cause a television picture to distort in the presence of a magnet. The electron beam in the cathode ray tube (CRT or television tube), which causes the phosphorescence on the screen, bends in a magnetic field. Of course, one should not bring a magnet up to a TV screen unless one has a dispensable TV because the interaction could do permanent damage to the electronics. But if one has a dispensable TV, it is an interesting effect to witness.

However, J. J. Thomson did not irrefutably establish the particulate nature of matter. It remained until 1909 for Jean Perrin to provide the definitive evidence for atoms, which he did by measuring the motion of microscopic pollen particles suspended in water. His detailed observations of this Brownian motion (named after the botanist Robert Brown) could be explained if it were assumed they were being buffeted about by moving atoms. His observations convinced the scientific community of the validity of the atomic model. Of course, they had been using the 46 THE JOY OF CHEMISTRY: The Amazing Science of Familiar Things model successfully before Perrin, but it was nice to have such elegant confirmation.

In 1910 Ernest Rutherford realized that atoms must be composed of a central, dense nucleus surrounded by a lot of empty space. He fired some atomic-sized particles at an ultrathin gold foil and found that most of the particles passed through the foil, but a few bounced back. Always able to turn an interesting phrase, Rutherford commented, "It was ... as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

Rutherford suggested that this nucleus at the center of the atom was composed of densely packed positively charged particles. Soon after, Henry Moseley, before his early death at Gallipoli in World War I, supplied experimental evidence for these particles, the protons. The other particles in the nucleus, the neutrons, proved a bit harder to pin down because they have no charge. But James Chadwick, taking Rutherford's advice, finally confirmed their existence in 1932. Chadwick measured the rebound of certain radiation from nitrogen and helium and found it corresponded to a neutral particle with about the same mass as a proton.

It took until the 1930s to discover all the pieces of the atom, which shows how difficult it was. Even so, laying out all the pieces still didn't solve the puzzle. There are other features of the atom-besides its size-that boggle the imagination. For one, there is the density of the nucleus. The density of a substance is the mass of a given volume. For instance, a bushel of feathers and a bushel of pebbles have the same volume but a definitely different mass. The bushel of pebbles is heavier; therefore, the bushel of pebbles is denser. We mentioned that Rutherford found that atoms have densely packed nuclei, but we didn't say how dense. It turns out that a nucleus, for all its diminutive proportions, packs some whopping million trillion grams per cubic centimeter. Compare that, say, to the density of lead, which is about eleven grams per cubic centimeter. The difference is that a nucleus consists of closely packed particles, but an atom is mostly empty space. A comparison can be made with Earth and the sun. If the nucleus of a lead atom were Earth, then the distance to the nearest electron would be about the distance to the sun. That is to say, most of the atom is just empty space. If you were to pack just nuclei into a container, you would be packing in solid marbles of matter. When you pack lead into a container, you are, in essence, packing in bubbles of empty space.

The question that may immediately spring to mind is this: If matter is composed of tiny bubbles, then why don't we collapse into puddles? An unsatisfying answer is that the electrons stay where they are because they are in orbit around the nucleus. But, one might object, if the electrons have a negative charge and the protons have a positive charge, shouldn't these opposite charges attract and shouldn't the electrons come crashing into the proton?

As it turns out, this is no trivial question.

In the early 1900s, the noted physicist Niels Bohr helped to answer this question by showing that an electron could remain at a distance from a proton as long as it kept moving. An analogy could again be made with Earth and the sun. Gravity does pull these two bodies together, but Earth, by moving in its orbit, can keep falling toward the sun but never fall into it. A similar picture can be painted for the electron. The electron can be thought of as circling the nucleus and thereby being pulled toward the nucleus, but not into it.

The reader has no doubt noticed that the above paragraphs contain quite a few qualifying phrases, such as "Niels Bohr ... helped to explain" instead of "Niels Bohr explained" and "an electron can be thought of" rather than "an electron is." This approach is necessary because the analogies are just that-analogies-and they work only at a rather crude level. The analogies fail, and fail quickly, when any degree of precision is required. We can have no exact analogy because the physics at the atomic level is quite different from the physics we experience in our relatively gargantuan, everyday, macroscopic world.

So it is with this disclaimer in mind that we present the structure of the atom as it is currently understood. We first note the main features that are of common knowledge. The nucleus, or center of the atom, is composed of positively charged particles called protons and uncharged, neutral particles called neutrons. Electrons, as is customarily acknowledged, are the negatively charged particles that reside in "orbits" around the nucleus.

Here the term orbit is being used metaphorically, not literally. Though the common picture is to show electrons orbiting, satellite-like, around the nucleus, the space occupied by the electrons cannot be clearly delineated. The best we can do is to describe a sort of fuzzy region of probability in which we believe the electron may be found. To emphasize this difference, we call the space occupied by electrons around a nucleus an orbital, not an orbit. These orbitals can be spherical, a sort of dumbbell shape, or intricate structures of loops, doughnuts, and lobes. Once again, however, there is nothing in common experience that is really quite like them, so the analogies are not perfect.

The problem only becomes worse when you include more than one electron in the discussion, which is true for every element beyond hydrogen. Electrons are charged particles, and charged particles tend to be attracted to each other if they are of opposite charge and repelled from one another if they are of the same charge. A fairly good model for this behavior is found in the behavior of magnets. Like poles of magnets repel, while opposite poles attract. In an atom, the situation is much murkier because there are not just magnetlike interactions of one electron to one electron, or one electron to one proton, but a sea of negative and positive charges that are all interacting. The Nobel physicist Enrico Fermi once likened the situation to boats bobbing in a harbor. We know intuitively that the motion of one boat will influence the motion of all the others, and vice versa, but in ways so intertwined and convoluted that the final motion of just one boat cannot be predicted at any one time.

This problem is called the three-body problem by the people who study such things (the theoreticians of quantum mechanics). When you have two particles in motion that attract each other, you can describe the situation with an equation. But when you have three particles, and there are attractions and repulsions, and all these particles are in motion, there are too many things going on for one neat equation. The problem is one of clouds: a cloud exists and we can point to it and measure it, but to predict in advance just where it will be and what form it will take is not possible. There are too many factors, too many variables, many of which are unknown or unknowable. This problem is at the heart of the probability approach to atomic structure.

But luckily you don't have to know the position of every cloud to predict the weather. Based on the probability approach, the theory of quantum mechanics is able to accurately interpret and predict many of the properties of atoms and molecules and how they interact. Scientists have also been able to understand and work with another interesting beastie: the ion. An ion is an atom or molecule that has lost some electrons or gained some extra electrons, as our plastic spoon gained some electrons in the "Water Witch" demonstration. The fact that there are too few or too many electrons means that the positive charge from the protons is unbalanced and the ion has a net positive or negative charge. In the case of the plastic spoon, electrons were moved by friction, and the spoon acquired a net negative charge. Other charged species are able to perform other wonderful tricks. For instance, the miracle of photocopy machines.

For Example: Protons and Photocopiers

Okay, some people might consider it an exaggeration to call a photocopy machine a miracle, but for anyone who has dealt with mimeograph machines, miracle is almost too weak. Photocopy machines are nearly as integral to the information age as computers and satellite cell phones. For all the photocopiers' modern-age convenience and programming-as well as features that allow one to copy, collate, stack, staple, and punch holes-the technology underlying them is actually fairly straightforward. The basic principle behind the operation of photocopiers is that static charges attract.

The parts of a photocopier are familiar to everyone who has ever used one to any extent because anyone who has ever used one to any extent has had a paper jam and had to open the copier to remove the jammed paper. If you have somehow avoided this experience, then simply go to a machine and open it to familiarize yourself with the interior workings. Integral to the photocopier is a rotating drum, a movable light source, a black powdery mess called toner, a heat source, and an elaborate system of rollers.

The first step in the photocopy process is for the drum to acquire a uniform static electric charge. The method used is slightly fancier than the method by which the spoon acquired its static charge in the "Water Witch" demonstration, but just slightly. With this static charge, the drum can now attract toner, just as the spoon in the "Water Witch" attracted water. If this were the last step, however, then the paper produced would be a uniform black color, which is not the promised copy. To accomplish the copy, a very bright light passes under the paper to be copied.

Light is used because, as we will see, light and matter routinely interact: camera film reacts with light to form an image; digital sensors record light levels in a digital camera; photosensors detect the presence of a person in a light beam and trigger a door to open. When the light strikes a dark part of the paper, it is absorbed, but when the light strikes a white part of the paper, the light is reflected onto the drum. The interior material of the drum is photoconductive, which means light will cause the interior to eject an electron. Wherever an electron is ejected from the interior of the drum, it neutralizes the static charge at the surface of the drum in just that one spot. The rotation of the drum is synchronized with the movement of the light under the paper so the flat image can be transferred to the curved surface of the drum.

The light-exposed drum then turns past the toner, and toner is attracted to the still-charged portions of the drum. A piece of paper with a static charge is now passed over the surface of the drum and attracts the toner away from the drum. The paper is heated to fix the toner onto the paper, and the new copy is presented.

If the principle behind the operation of photocopiers-the attraction of materials to a static charge-is so basic and well understood, why did it take so long to produce a practical photocopier? The answer, as with many such innovations, is that the fundamental concept existed long before the materials necessary to implement the idea. Materials science is a discipline all to its own because of the virtually infinite variety of properties that the elements, as well as the substances derived from the elements, can display. Consider, for instance, the variation in the behavior of aluminum and copper (both of which are used in electrical wiring) and steel nails and galvanized nails (both of which are hit on the head). In the next chapter, we lay the differences on the table-the periodic table, that is.

(Continues...)

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Excerpted from THE JOY OF CHEMISTRY by CATHY COBB MONTY L. FETTEROLF Copyright © 2005 by Cathy Cobb and Monty L. Fetterolf. Excerpted by permission.
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