TYPES OF BONDS

Covalent Bonds form when 2 atoms come very close together and share one or more of their electrons. In a single bond, one electron from each of the 2 atoms is shared; in a double bond a total of four electrons are shared. Each atom forms a fixed number of covalent bonds in a defined spatial arrangement. C, for example, forms 4 single bonds arranged tetrahedrally.

Often electrons are shared unequally which results in a polar covalent bond. For example, the covalent bond between O and H, or N and H, is polar whereas that between C and H has electrons attracted much more equally by both atoms and is relatively nonpolar. Sometimes the words "oxidation" where electrons are transferred from one atom to another and "reduction" where electrons are added are used to describe polarity. For example, when a C becomes covalently bonded to an atom with a strong affinity for electrons like O, CL, or S, the C gives up more than its equal share of electrons and forms a polar covalent bond. Polar bonds are very important because they create permanent dipoles that allow molecules to interact through electrical forces.

(1) Hydrocarbons: C and H form covalent bonds. These hydrocarbons are nonpolar, do not form hydrogen bonds and are usually insoluble in water.

(2) C- O bonds: play essential roles in carbohydrate  metabolism.

(3) C-N bonds (These also occur in nucleic acids)

(4) Phosphates

Noncovalent Attractions or Bonds

(1) Hydrogen Bonds: A hydrogen bond forms when a H atom is sandwiched between 2 electron attracting atoms (usually O or N). Hydrogen bonds are very important with respect to water. 2 atoms, connected by a covalent bond, may exert different attractions for the electrons of the bond. In such cases the bond is called "polar" with one end slightly negatively charged and the other end positively charged. This is exactly what happens with water. Although a water molecule has an overall neutral charge (the same # electrons and protons), the oxygen nucleus draws electrons away from the hydrogen nuclei, leaving the H nuclei with a small net positive charge. The small negative charge on the O atom can in turn hydrogen bond with a H atom on a neighboring water molecule.

        Molecules that contain nonpolar bonds (CH bonds) are usually insoluble in water and are called "hydrophobic." Substances which are polar can dissolve readily in water and are termed "hydrophilic." Hydrogen bonding is also used between nucleotide bases and is common between amino acids in polypeptide chains.

Hydrogen bonding is also very important with respect to stabilizing proteins. Such hydrogen bonds can form (1) between atoms of 2 peptide bonds, (2) between atoms of a peptide bond and an amino acid side chain and (3) between 2 amino acid side chains.

(2) Van der Waals attractions: The electron clud around any nonpolar atom will fluctuate producing a flickering dipole. Such dipoles will transiently induce and oppositely polarized flickering dipole in a nearby atom.  The 2 atoms will be attracted to each other in this way until the distance between their nuclei is about equal to the sum of their van der Waals radii. Although weak, these forces can become important when 2 macromolcecular sufaces fit very close together, because many atoms are involved.

(3) Ionic Bonds: occur when electrons are donated by one atom to another in contrast to covalent bonds which involve the sharing of electrons. Ionic bonds are more likely to be formed by atoms that have just 1 or 2 electrons in addition to a filled outer shell or are just one or 2 electrons short of acquiring a filled outer shell. They can often attain a completely filled outer electron shell more easily by transferring electrons to or from another atom than by sharing electrons. For example, Na, with atomic # 11, can strip itself down to a filled shell by giving up the single electron external to its second shell. By contrast, a Cl atom, with atomic number 17, can complete its outer shell by gaining just one electron. When an electron jumps from Na to cl, both atoms become electrically charged ions (Na+, Cl-). Because of their opposite charges they are attracted to each other and held together in an ionic bond.

(4) Hydrophobic Force: This force is caused by a pushing of nonpolar surfaces out of the hydrogen bounded water network. The force is rather nonspecific but central to the proper folding of protein molecules.

See also Isomers