Covalent bonds result when atoms share their valence electrons. Covalent bonds that are formed between identical atoms, as in oxygen gas (O2) and hydrogen gas (H2), are the strongest because their electrons are equally shared. Since the electrons are equally distributed between the two atoms, these molecules are said to be nonpolar and the bonds between them are non-
■ Figure 2.2 A hydrogen molecule showing the covalent bonds between hydrogen atoms. These bonds are formed by the equal sharing of electrons.
polar covalent bonds. Such bonds are also important in living organisms. The unique nature of carbon atoms and the organic molecules formed through covalent bonds between carbon atoms provides the chemical foundation of life.
When covalent bonds are formed between two different atoms, the electrons may be pulled more toward one atom than the other. The end of the molecule toward which the electrons are pulled is electrically negative compared to the other end. Such a molecule is said to be polar (has a positive and negative "pole"). Atoms of oxygen, nitrogen, and phosphorus have a particularly strong tendency to pull electrons toward themselves when they bond with other atoms; thus, they tend to form polar molecules.
Water is the most abundant molecule in the body and serves as the solvent for body fluids. Water is a good solvent because it is polar; the oxygen atom pulls electrons from the two hydrogens toward its side of the water molecule, so that the oxygen side is more negatively charged than the hydrogen side of the molecule (fig. 2.4). The significance of the polar nature of water in its function as a solvent is discussed in the next section.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.