T HE F OCAL P OINT OF C HEMISTRY

Like physics, chemistry is concerned with basic, underlying processes that explain how the universe works. Indeed, these two sciences, along with astronomy and a few specialized fields, are the only ones that address phenomena both on the Earth and in the universe as a whole. By contrast, unless or until life on another planet is discovered, biology has little concern with existence beyond Earth's atmosphere, except inasmuch as the processes and properties in outer space affect astronauts.

While physics and chemistry address many of the same fundamental issues, they do so in very different ways. To make a gross generalization—subject to numerous exceptions, but nonetheless useful in clarifying the basic difference between the two sciences—physicists are concerned with external phenomena, and chemists with internal ones.

For instance, when physicists and chemists study the interactions between atoms, unless physicists focus on some specialized area of atomic research, they tend to treat all atoms as more or less the same. Chemists, on the other hand, can never treat atoms as though they are just undifferentiated particles colliding in space. The difference in structure between one kind of atom and another, in fact, is the starting-point of chemical study.

S TRUCTURE OF THE A TOM

An atom is the fundamental particle in a chemical element, or a substance that cannot be broken down into another substance by chemical means. Clustered at the center, or nucleus, of the atom are protons, which have a positive electric charge, and neutrons, which possess no charge.

Spinning around the nucleus are electrons, which are negatively charged. The vast majority of the atom's mass is made up by the protons and neutrons, which have approximately the same mass, whereas that of the electron is much smaller. The mass of an electron is about 1/1836 that of proton, and 1/1839 that of a neutron.

It should be noted that the nucleus, though it constitutes most of the atom's mass, is only a tiny portion of the atom's volume. If the nucleus were the size of a grape, in fact, the electrons would, on average, be located about a mile away.

T HE MOST COMMON AND / OR IMPORTANT CHEMICAL ELEMENTS .

I SOTOPES

Atoms of the same element always have the samenumber of protons, and since this figure isunique for a given element, each element is assigned an atomic number equal to the number of protons in its nucleus. Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons. Such atoms are called isotopes.

Isotopes are generally represented as follows: where S is the symbol of the element, a is the atomic number, and m is the mass number—the sum of protons and neutrons in the atom's nucleus. For the stable silver isotope designated as for instance, Ag is the element symbol (discussed below); 47 its atomic number; and 93 the mass number. From this, it is easy to discern that this particular stable isotope has 46 neutrons in its nucleus.

Because the atomic number of any element is established, sometimes isotopes are represented simply with the mass number thus: ⁹³ Ag. They may also be designated with a subscript notation indicating the number of neutrons, so that one can obtain this information at a glance without having to do the arithmetic. For the silver isotope shown here, this is written as Isotopes are sometimes indicated by simple nomenclature as well: for instance, carbon-12 or carbon-13.

I ONS

The number of electrons in an atom is usually the same as the number of protons, and thus most atoms have a neutral charge. In certain situations, however, the atom may lose or gain one or more electrons and acquire a net charge, becoming an ion. The electrons are not "lost" when an atom becomes an ion: they simply go elsewhere.

Aluminum (Al), for instance, has an atomic number of 13, which tells us that an aluminum atom will have 13 protons. Given the fact that every proton has a positive charge, and that most atoms tend to be neutral in charge, this means that there are usually 13 electrons, with a negative charge, present in an atom of aluminum. Aluminum may, however, form an ion by losing three electrons.

CATIONS.

After its three electrons have departed, the remaining aluminum ion has a net positive charge of 3, represented as +3. How do we know this? Initially the atom had a charge of +13 + (−13) = 0. With the exit of the 3 electrons, leaving behind only 10, the picture changes: now the charge is +13 + (−10) = +3.

When a neutral atom loses one or more electrons, the result is a positively charged ion, or cation (pronounced KAT-ie-un). Cations are represented by a superscript number and plus sign after the element symbol: Al ³⁺ , for instance, represents the aluminum cation described above. (Some chemists represent this with the plus sign before the number—for example, Al ⁺³ .) A cation is named after the element of which it is an ion: thus the ion we have described is called either the aluminum ion, or the aluminum cation.

ANIONS.

When a neutrally charged atom gains electrons, and as a result acquires a negative charge, this type of ion is known as an anion (AN-ie-un). Anions can be represented symbolically in much the same way of cations: Cl−, for instance, is an anion of chlorine that forms when it acquires an electron, thus assuming a net charge of −1. Note that the 1 is not represented in the superscript notation, much as people do not write 10 ¹ . In both cases, the 1 is assumed, whereas any number higher than 1 is shown.

The anion described here is never called a chlorine anion; rather, anions have a special nomenclature. If the anion represents, as is the case here, a single element, it is named by adding the suffix -ide to the name of the original element name: hence it would be called chloride. Other anions involve more than one element, and in these cases other rules apply for designating names. A few two-element anions use the-ide ending; such is the case, for instance, with a deadly mixture of carbon and nitrogen (CN ⁻ ), better known as cyanide.

For anions involving oxygen, there may be different prefixes and suffixes, depending on the relative number of oxygen atoms in the anion.