All matter, living or otherwise, is composed of elements, substances that cannot be broken down into simpler forms by chemical means. But of the more than 100 existing elements, only about 25 percent are common in living matter. Even more striking, just six elements-hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur-account for more than 90 percent of the weight of most organi.sms.
Atomic Structure and the States of Matter
The smallest amount of an element that can exist is an atom. Atoms are so small that it is hard to imagine the number of them in even the smallest bit of matter. To illustrate, the period at the end of this sentence probably contains as many as three or four billion atoms.
In a solid, atoms are stacked together in a very orderly way. Although atoms are in constant motion except at extremely low temperatures, such movements are restricted and hard to detect in a solid. But when energy in the form of heat is added, the energized atoms begin bouncing about rather more freely, until the formerly ordered arrangement of the solid is slowly destroyed. We call this process melting, and the result is the formation of a liquid.
As more heat is applied to a liquid, its atoms become increasingly turbulent and eventually reach a state so energetic that some of them pop free from the surface of the fluid. This boiling turns the liquid into a gas.
Under proper conditions, any substance can be converted from one physical state to another. Some substances require extreme, unearthly conditions if they are to undergo change, but water is one example of a common substance that exists in all three states (ice, liquid water, and water vapor) in our "ordinary" environment.
Substances that consist of a single element are unusual. The rarity of gold (gold atoms) and diamonds (carbon atoms) serves to illustrate the point quite well. Most substances are compounds, formed by chemical reactions between different elements. When substances, either elements or compounds, do not chemically react but merely physically intermingle, mixtures are formed. A rock run through by a vein of gold is a heterogeneous mixture. However, seawater is a solution-a homogeneous mixture of compounds, including water, salt, and many more.
Compounds, as we have said, are the result of chemical change. Changes of this type are brought about by chemical reactions whereby the atoms in substances (the reactants) undergo rearrangement to form entirely new substances (products). Since such chemical change underlies all biological processes, our immediate goal is to understand its basis. We will find that chemical reactivity depends on atomic structure, specifically on the number and arrangement of subatomic particles within an atom.
Of the many subatomic particles that scientists have postulated or identified, only three concern us here. Electrons are particles bearing a negative charge; protons possess a positive charge; and neutrons have no charge at all. Many properties of atoms are a reflection of the electrical properties of their subatomic particles, since particles with similar electrical charges repel one another and those with opposite electrical charges attract one another.
Protons and neutrons are always found in the central portion, or nucleus, of an atom, while the electrons spin around the nucleus within defined orbital shells. In an electrically neutral atom, the number of electrons must be equal to the number of protons. The electrons are maintained in orbit because the positively charged nucleus attracts the negatively charged electrons.
The ordinary hydrogen atom, with one proton and one electron, is the smallest and simplest atom.
All other atoms contain more protons and electrons plus some neutrons. In the naturally occurring elements, the number of electrons per atom ranges from one in hydrogen to 92 in uranium.
The Atomic Nucleus. Protons and neutrons are the largest and heaviest of the subatomic particles. Just how the uncharged neutrons and the positively charged protons are organized within the nucleus is a matter of conjecture. What is certain is that a powerfully attractive strong force must operate between the protons and neutrons of the nucleus. Otherwise the nucleus would blow apart because the similarly charged protons should push away from one another. The true nature of this strong force is still somewhat mysterious; we do know that it must be tightly internalized, since it does not affect the attraction of protons for electrons. The strength of the strong nuclear force is seen when a small portion is tapped in the release of atomic energy.
The number of protons in an atom of a single element is a constant characteristic and is given as the atomic number. The atomic weight is essentially equal to the number of protons plus the number of neutrons in the atomic nucleus. It is not the real weight of an atom, but its relative weight as compared with other atoms.
Although the number of protons in an atom of a single element is constant, the number of neutrons may vary slightly. Atoms with the same number of protons but different numbers of neutrons are isotopes of the same element. In nature, one isotopic form of an element usually predominates, and the others are found only in minute amounts. Not all elements have isotopic forms, yet some have many.
The presence or absence of a few neutrons does not affect the nature of an atom, so isotopes of an element have the same chemical properties. When it is useful, we can distinguish among isotopes by noting the variation in their atomic weights, as when we refer to carbon-12 and carbon-14.
You are probably aware that some isotopes are radioactive. The reason radioactive isotopes are radioactive is that they are atoms with too few or too many neutrons. As might be expected, this irregular condition in an atom sometimes causes the nucleus to be unstable. When the nucleus is unstable, it eventually disintegrates and releases radiation in the form of particles or energy or both. Perhaps the most widely recognized radioactive isotope is the unstable atom carbon-14. Not all isotopes are radioactive, however. For example, neither of the two heavy isotopes of oxygen is unstable.
The Arrangement of Electrons in an Atom. Even if we had a microscope powerful enough to enable us to see an atom, it is unlikely that we would actually see an individual negatively charged particle. Electrons are clouds of matter that ripple around the nucleus in their orbital shells. Pinpointing an electron at any given moment would be like trying to follow a lone spoke of a spinning bicycle wheel. What we might see instead is a blur, or what physicists call an electron cloud. But regardless of this fact, it is often convenient to illustrate electrons as discrete particles; we do so in this book.
For reasons of simplicity we are also going to ignore the fact that not all electron shells are completely circular. Some of them are elongated into an ellipse or swing back and forth in the shape of a figure 8. If you are worried that electrons may smash into one another as they scoot around the nucleus, it will be helpful to know that the orbital shells are tilted at various angles.
It also helps if you know that electrons travel at different distances from the nucleus. And crowding is further controlled because each shell has a maximum population of electrons. None of the atoms shown there has more than two electrons in the first (innermost) shell. Note too, that once the second shell is filled with eight electrons, a third shell must be formed to accommodate additional electrons. Although it's not illustrated here, a fourth shell is required once the third shell contains eight electrons.
The most accurate way to describe electron shells is in terms of energy levels. Shels dose to the nucleus are of a lower energy level than are shells located farther away. This is because the closer an electron is to the nucleus, the more strongly it is attracted to the oppositely charged protons located there. Thus a close-in, tightly held 'electron has less energy than one located farther from the nucleus. Since the outermost-shell electrons are higher in energy than other electrons, it is not surprising that the outermost electrons are most involved in the chemical reactions that occur between atoms.
From Atoms to Molecules. The chemical characteristics of an element are determined largely by the number of electrons in the outermost shell of its atoms. Studies of the chemically inert-that is, stable-elements has shown that the atoms of these elements have full outer shells. For example, helium, with lust two electrons in its only shell, does not react with other atoms. Nor does neon, which, with ten electrons, has two electron shells and a full complement of eight electrons in the second shell.
