Molecules and compounds
The atoms of most elements interact with one another. If the interaction is a sharing of electrons the interacting atoms are called a molecule. Molecules are two or more atoms held together by shared electrons and can be made up of atoms of the same element or atoms of different elements. Molecules made up of atoms of different elements are called compounds. (Compounds are also atoms of different elements held together by electrostatic attraction [ionic bonds]).
The oxygen in the air consists of molecules made up of pairs of atoms of the same element-oxygen. These pairs are represented by the chemical formula O2. A chemical formula is a type of "shorthand" used to describe a molecule. The atoms are represented by symbols, such as O for oxygen.
A subscript shows the number of these atoms present in the molecule. An example of a compound with which you are all familiar is water, H2O. In this chemical formula, the symbol H stands for the element hydrogen. The chemical formula H2O shows that water is made up of two atoms of hydrogen and one atom of oxygen. A space-filling model of one molecule of water is shown to the left.
Three factors influence whether an atom will interact with other atoms. In addition, these factors influence the type of interactions likely to take place. These factors are (1) the tendency of electrons to occur in pairs, (2) the tendency of atoms to balance positive and negative charges, and (3) the tendency of the outer shell, or energy level, of electrons to be full. This third factor is often called the octet rule.
The word octet means "eight objects" and refers to the fact that eight electrons is a stable number for an outer shell; the outer electron shell of many atoms contains a maximum of eight electrons. (The first energy level is an exception to this rule because it contains a maximum of two electrons.) The octet rule states that an atom with an unfilled outer shell has a tendency to interact with another atom or atoms in ways that will complete this outer shell. Although the octet rule does not apply to all atoms, it does apply to all biologically important ones-those involved in the structure, energy needs, and information systems of living things. These atoms and the molecules they make up are discussed later in this chapter.
Atoms of elements that have equal numbers of protons and electrons and have full outer-electron energy levels are the only ones that exist as single atoms. Atoms with these characteristics are called noble gases, or inert gases, because they do not react readily with other elements. Many stories explain why they are called noble, but they all center around the concept of nobility-those who have everything (in this case a full outer shell), need nothing, and interact little with others. Most of the noble gases-helium, neon, argon, krypton, xenon, and radon-are rare. In addition, they are relatively unreactive and unimportant in living systems. Helium and radon are probably the best known and the least rare noble gases. Radon, in fact, has gained more recognition in recent years. Formed in rock or soil particles from the radioactive decay of radium, radon gas can seep through cracks in basement walls and remain trapped in homes that are not well ventilated. Prolonged exposure to radioactive radon in levels greater than those normally found in the atmosphere is thought to lead to lung cancer. Figure 2 shows the areas of the United States in which radon gas is most prevalent.
Figure 2 Areas of the United States in which radon gas is most prevalent. Note: A picocurie is a unit used to describe the rate of disinitegration of the nucleus of a radioactive element.