2.3: Chemical Bonds

Atoms form bonds to make molecules, and there are three main classes of chemical bonds. There are two subsets of covalent bonds, both of which are strong bonds. They involve unequal or equal sharing of a pair of electrons. Unequal sharing of electrons results in polar covalent bonds. Equal sharing forms nonpolar covalent bonds. Ionic bonds are created by electrostatic interactions between elements after they gain or lose electrons, and these bonds are weaker than covalent bonds. Hydrogen bonds (H-bonds) are in a class by themselves. Their electrostatic interactions account for the physical and chemical properties of water. They are also involved in interactions between and within other molecules. Note that while atoms can share, gain, or lose electrons in chemical reactions, they will neither gain nor lose protons or neutrons. Let’s look more closely at chemical bonds and how even the “weak” bonds are essential to life.

2.3.1. Covalent Bonds

Electrons are shared in covalent bonds. Hydrogen gas (\(\rm H_2\)) is a molecule, not an atom! The two H atoms in the \(\rm H_2\) molecule share their electrons equally. Likewise, the carbon atom in methane (\(\rm CH_4\)) shares electrons equally with four hydrogen atoms. A single pair of electrons forms the covalent bond between two H atoms in the hydrogen molecule (\(\rm H_2\)). In methane, the carbon (C) atom has four electrons in its outer shell that it can share. Each H atom has one electron to share.

If a C atom shares each of its four electrons with the electron in each of four H atoms, there will be eight (four paired) electrons moving in filled orbitals around the nucleus of the C atom some of the time and one pair moving around each of the H atomic nuclei some of the time. Thus, the outer shells of the C atom and the H atoms are filled at least some of the time. This stabilizes the molecule. Remember that atoms are most stable when their outer shells are filled and when each electron orbital is filled (i.e., with a pair of electrons). The equal sharing of electrons in nonpolar covalent bonds in \(\rm H_2\) and \(\rm CH_4\) is shown in Figure 2.4.

Figure 2.4: Nonpolar covalent bonds in hydrogen gas (left) and methane (right).

Polar covalent bonds form when electrons in a molecule are shared unequally. This happens if the atomic nuclei in a molecule are very different in size, as is the case with water (Figure 2.5).

Figure 2.5: Electrons on the H atoms of water molecules are drawn close to the large, positively charged nucleus of the O atom (molecule at left). As a result, H atoms “lose” electrons and acquire a partial positive charge (δ+) while oxygen atoms “gain” those electrons and have a partial negative charge (δ-). The polar covalent water molecules then attract and bind to other water molecules (molecules at the right)

The larger nucleus of the oxygen atom in \(\rm H_2O\) attracts electrons more strongly than does either of the two H atoms. As a result, the shared electrons spend more of their time orbiting the O atom, such that the O atom carries a partial negative charge while each of the H atoms carries a partial positive charge. The Greek letter delta (δ) indicates partial charges in polar covalent bonds. In Figures 2.4 and 2.5, compare the position of the paired electrons in water with those illustrated for hydrogen gas or methane.

Water’s polar covalent bonds allow it to attract and to interact with other polar covalent molecules, including other water molecules. The polar covalent nature of water also goes along way to explaining its physical and chemical properties and the reason why water is essential to life on this planet!

124 Covalent Bonds

Both polar and nonpolar covalent bonds play a major role in the structure of macromolecules, as in the protein hormone insulin, modeled in Figure 2.6.

Figure 2.6: Computer-generated space-filling (left) and “ribbon” models (right) of insulin.

The X-ray image of a space-filling model of the hexameric form of stored insulin (Figure 2.6, left) emphasizes its tertiary structure in detail. The ribbon diagram (Figure 2.6, right) highlights regions of internal secondary structure. When secreted from the Islets of Langerhans cells of the pancreas, active insulin is a dimer of A and B polypeptides (blue and cyan in the ribbon diagram, respectively). The subunit structure and the interactions holding the subunits together result from many electrostatic interactions (including H-bonds) and other weak interactions. The disulfide bonds or bridges (seen as yellow Vs in the ribbon diagram) stabilize the associated A and B monomers. We will look at protein structure in more detail in an upcoming chapter.

2.3.2. Ionic Bonds

Atoms that gain or lose electrons to achieve a filled outer shell acquire a negative or a positive charge (respectively) to form ions. Despite their electrical charge, ions are stable because their outer electron shells are filled. Common table salt (Figure 2.7) is a good example.

Figure 2.7: Ionic bonds in table-salt (NaCl) crystals result from the attraction of oppositely charged small \(\rm Na^{+}\) and large \(\rm Cl^{-}\) ions that bind them together in a regular, crystalline array

Na (sodium) atoms can donate a single electron to Cl (chlorine) atoms, generating partially charged \(\rm Na^{+}\) (sodium) and \(\rm Cl^{-}\) (chloride) ions. The oppositely charged ions then come together to form an ionic bond, an electrostatic interaction of opposite charges that holds the \(\rm Na^{+}\) and \(\rm Cl^{-}\) ions together in crystal salt. Look up the Bohr models of these two elements,and see how ionization of each leaves filled outer shells (energy levels) in the ions.

2.3.3. Hydrogen Bonds

The hydrogen bond is a subcategory of electrostatic interactions formed by the attraction of opposite charges. As noted above, water molecules attract one another (cohere) because of strong electrostatic interactions that form the H-bonds. Water’s polar covalent structure enables it to attract positively and negatively charged groups of molecules, making it a good solvent. Solutes (soluble molecules) or polar (charged) molecular surfaces that are attracted to water are hydrophilic. Lipids, like fats and oils, are not polar molecules and therefore do not dissolve in water; they are hydrophobic (from hydro: “water”; phobic: “fearing”). Next, we’ll take a closer look at the chemistry and properties of water.