Molecules and Chemical Bonds

Molecules are chemical particles composed of two or more atoms united by a covalent chemical bond (the sharing of electrons). The atoms may be identical, as in nitrogen (N2), or different, as in glucose (C6H12O6). Molecules composed of two or more different elements are called compounds. Oxygen (O2) and carbon dioxide (CO2) are both molecules because both consist of at least two atoms, but only CO2 is a compound, because it has atoms of two different elements.

Molecules can be represented by molecular formulae, as shown here, that identify their constituent elements and show how many atoms of each are present. Molecules with identical molecular formulae but different arrangements of their atoms are called isomers4 of each other. For example, both ethanol (grain alcohol) and ethyl ether have the molecular formula C2H6O, but they are certainly not interchangeable! To show the difference between them, we use structural formulae that show the location of each atom (fig. 2.5).

The molecular weight (MW) of a compound is the sum of the atomic weights of its atoms. Rounding the atomic mass units (amu) to whole numbers, we can calculate the approximate MW of glucose (C6H12O6), for example, as

6 C atoms X 12 amu each = 72 amu 12 H atoms X 1 amu each = 12 amu 6 O atoms X 16 amu each = 96 amu

Figure 2.5 Structural Isomers, Ethanol and Ethyl Ether. The molecular formulae are identical, but the structures and chemical properties are different.

Molecular weight (MW) = 180 amu

Molecular weight is needed to compute some measures of concentration, as we shall see later.

A molecule is held together, and molecules are attracted to each other, by forces called chemical bonds. The three bonds of greatest physiological interest are ionic bonds, covalent bonds, and hydrogen bonds (table 2.3).

An ionic bond is the attraction of a cation to an anion. Sodium (Na+) and chloride (CP) ions, for exam-

Figure 2.5 Structural Isomers, Ethanol and Ethyl Ether. The molecular formulae are identical, but the structures and chemical properties are different.

Table 2.3

Types of Chemical Bonds

Bond Type

Definition and Remarks

Ionic bond

Relatively weak attraction between an anion and a cation. Easily disrupted in water, as when salt dissolves.

Covalent bond

Sharing of one or more pairs of electrons between nuclei.

Single covalent

Sharing of one electron pair.

Double covalent

Sharing of two electron pairs. Often occurs between carbon atoms, between carbon and oxygen, and between carbon and nitrogen.

Nonpolar covalent

Covalent bond in which electrons are equally attracted to both nuclei. May be single or double. Strongest type of chemical bond.

Polar covalent

Covalent bond in which electrons are more attracted to one nucleus than to the other, resulting in slightly positive and negative regions in one molecule. May be single or double.

Hydrogen bond

Weak attraction between polarized molecules or between polarized regions of the same molecule. Important in the three-dimensional folding and coiling of large molecules. Weakest of all bonds; easily disrupted by temperature and pH changes.

ple, are attracted to each other and form the compound sodium chloride (NaCl), common table salt. Ionic compounds can be composed of more than two ions. Calcium has two valence electrons. It can become stable by donating one electron to one chlorine atom and the other electron to another chlorine, thus producing a calcium ion

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(Ca2+) and two chloride ions. The result is calcium chloride, CaCl2. Ionic bonds are weak and easily dissociate (break up) in the presence of something more attractive, such as water. The ionic bonds of NaCl break down easily as salt dissolves in water, because both Na+ and CP are more attracted to water molecules than they are to each other.

_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

Carbon Covalent Bonding

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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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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Responses

  • kaytee
    do you think ionic bonds are common in the human body? explain
    8 years ago
  • juan
    How many alcohols and ethers share the molecular formula c2h6o?
    8 years ago

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