Kaplan MCAT General Chemistry Review

Of course nobody really likes to share anything. We only say we do because we know that’s what expected of us. When nobody’s looking, a child will grab a few extra candies for herself, and even adults are known to fight over property lines, the last slice of pizza, or whether someone paid his fair share of the dinner tab. Atoms of elements that differ moderately in their electronegativities will share their electrons unevenly, resulting in polar covalent bonds. While the difference in their electronegativities (typically between 0.4 and 1.7 Pauling units) is not enough to result in the formation of an ionic bond, it is sufficient to cause a separation of charge across the bond, with the more electronegative element acquiring a greater portion of the electron pair(s) and taking on a partial negative charge, , and the less electronegative element acquiring a smaller portion of the electron pair(s) and taking on a partial positive charge, . For instance, the covalent bond in HCl is polar because the two atoms have a moderate difference in electronegativity (approximately 0.9). The chlorine atom gains a partial negative charge, and the hydrogen atom gains a partial positive charge. The difference in charge between the atoms is indicated by an arrow crossed at its tail end (giving the appearance of a “plus” sign) pointing toward the negative end, as shown in Figure 3.1.

Figure 3.1

A molecule that has such a separation of positive and negative charges is called a polar molecule. The dipole moment of the polar bond or polar molecule is a vector quantity, µ, defined as the product of the charge magnitude (q) and the distance between the two partial charges (r):

µ = qr

The dipole moment vector, represented by an arrow pointing from the positive to the negative charge, is measured in Debye units (coulomb-meter). Please note that the convention used by chemists for designating the direction of the dipole moment from positive to negative is the opposite of the convention used by physicists, who designate the direction of a dipole moment from negative to positive.

COORDINATE COVALENT BOND

In a coordinate covalent bond, the shared electron pair comes from the lone pair of one of the atoms in the molecule, while the other atom involved in the bond contributes nothing to the relationship. (This works great for atoms but might not be the best way to form a lasting marriage!) Once such a bond forms, however, it is indistinguishable from any other covalent bond. The distinction is only helpful for keeping track of the valence electrons and formal charges (see Figure 3.2). Coordinate covalent bonds are typically found in Lewis acid-base compounds (see Chapter 10, Acids and Bases). A Lewis acid is any compound that will accept a lone pair of electrons, while a Lewis base is any compound that will donate a pair of electrons to form a covalent bond; for example, as in the reaction between borontrifluoride (BF₃) and ammonia (NH₃) shown in Figure 3.2.

Figure 3.2

NH₃ donates a pair of electrons to form a coordinate covalent bond; thus, it acts as a Lewis base. BF₃ accepts this pair of electrons to form the coordinate covalent bond; thus, it acts as a Lewis acid. Lewis acids, incidentally, are commonly encountered in some often-tested organic chemistry reactions, such as anti addition across double bonds, and as catalysts in electrophilic aromatic substitutions (EAS).

COVALENT BOND NOTATION