Think About It

Do you think ionic bonds are common in the human body? Explain your answer.

Covalent bonds form by the sharing of electrons. For example, two hydrogen atoms share valence electrons to form a hydrogen molecule, H2 (fig. 2.6a). The two electrons, one donated by each atom, swarm around both nuclei in a dumbbell-shaped cloud. A single covalent bond is the sharing of a single pair of electrons. It is symbolized by a single line between atomic symbols, for example H—H. A double covalent bond is the sharing of two pairs of electrons. In carbon dioxide, for example, a central carbon atom shares two electron pairs with each oxygen atom. Such bonds are symbolized by two lines, for example O—C—O (fig. 2.6b).

Nonpolar covalent C—C bond

Polar covalent O — H bond

Figure 2.7 Nonpolar and Polar Covalent Bonds. (a) A nonpolar covalent bond between two carbon atoms, formed by electrons that spend an equal amount of time around each nucleus, as represented by the symmetric blue cloud. (b) A polar covalent bond, in which electrons orbit one nucleus significantly more than the other, as represented by the asymmetric cloud. This results in a slight negative charge (8 — ) in the region where the electrons spend most of their time, and a slight positive charge (8 + ) at the other pole.

Hydrogen atom Hydrogen atom (a)

HH Hydrogen molecule (H2)

Hydrogen atom Hydrogen atom (a)

HH Hydrogen molecule (H2)

Oxygen atom Carbon atom Oxygen atom

Oxygen atom Carbon atom Oxygen atom

O C O Carbon dioxide molecule (CO2)

O C O Carbon dioxide molecule (CO2)

Figure 2.6 Covalent Bonding. (a) Two hydrogen atoms share a single pair of electrons to form a hydrogen molecule. (b) A carbon dioxide molecule, in which a carbon atom shares two pairs of electrons with each oxygen atom, forming double covalent bonds. How is the octet rule illustrated by the CO2 molecule?

When shared electrons spend approximately equal time around each nucleus, they form a nonpolar covalent bond (fig. 2.7a), the strongest of all chemical bonds. Carbon atoms bond to each other with nonpolar covalent bonds. If shared electrons spend significantly more time orbiting one nucleus than they do the other, they lend their negative charge to the region where they spend the most time, and they form a polar covalent bond (fig. 2.7b). When hydrogen bonds with oxygen, for example, the electrons are more attracted to the oxygen nucleus and orbit it more than they do the hydrogen. This makes the oxygen region of the molecule slightly negative and the hydrogen regions slightly positive. The Greek delta (8) is used to symbolize a charge less than that of one electron or proton. A slightly negative region of a molecule is represented 8— and a slightly positive region is represented 8 + .

A hydrogen bond is a weak attraction between a slightly positive hydrogen atom in one molecule and a slightly negative oxygen or nitrogen atom in another. Water molecules, for example, are weakly attracted to each other by hydrogen bonds (fig. 2.8). Hydrogen bonds also form between different regions of the same molecule, especially in very large molecules such as proteins and DNA. They cause such molecules to fold or coil into pre-

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 2 The Chemistry of Life 63

Covalent bond -

Covalent bond -

Hydrogen bond -

—Water molecule

Figure 2.8 Hydrogen Bonding in Water. The polar covalent bonds of water molecules enable each oxygen to form a hydrogen bond with a hydrogen of a neighboring molecule. Thus, the water molecules are weakly attracted to each other.

Why would this behavior raise the boiling point of water above that of a nonpolar liquid?

cise three-dimensional shapes. Hydrogen bonds are represented by dotted or broken lines between atoms: —C=O- ■ -H—N—. Hydrogen bonds are the weakest of all the bond types we have considered, but they are enormously important to physiology.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Consider iron (Fe), hydrogen gas (H2), and ammonia (NH3). Which of these is or are atoms? Which of them is or are molecules? Which of them is or are compounds? Explain each answer.
  2. Why is the biological half-life of a radioisotope shorter than its physical half-life?
  3. Where do free radicals come from? What harm do they do? What protections from free radicals exist?
  4. How does an ionic bond differ from a covalent bond?
  5. What is a hydrogen bond? Why do hydrogen bonds depend on the existence of polar covalent bonds?

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Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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