Students should learn that all chemical bonding is a matter of degrees, from the very ionic to the completely covalent, with a range of polarity in between. In organic chemistry, we most often refer to covalent bonds, and the extent of polarity in many cases determines the chemical reactivity. In the SN2 reaction of hydroxide with chloromethane (Figure 2), one way ask the student where the hydroxide is most likely to attack. Being negatively charged, it should that atom that bears the most partial-positive charge. The C – H bond has negligible polarity, but the C – Cl bond is much more polarity, with the partial-negative charge residing on the more electronegative element, chlorine, and the partial-positive charge existing on the less electronegative element, carbon. The hydroxide, therefore, will attack the carbon, resulting in the breaking of the carbon-halogen bond while forming a carbon-oxygen bond. Because electronegativity is important in determining bond polarity, students can be reminded of how various key concepts affect each other. Biochemical example: the N – H bond in adenine and thymine (Figure 4) are very polar covalent bonds. They play an important role in the structure of the DNA double helix. For example, the hydrogen atoms in these N – H bonds bear a partial-positive charge (and the nitrogen atoms bear a partial-negative charge). Likewise, the C=O bond of thymine is polar, with the partial-negative charge on oxygen (only two of the many polar covalent bonds show the polarity symbols). The hydrogen atom is therefore strongly attracted to the adjacent, highly electronegative oxygen atom on the adjacent molecules in the opposite strand of the DNA double helix (The “base pair”). This type of intermolecular interaction is known as “hydrogen bonding” and is a result of the high polarity of the covalent bonds. This argument is the same for the other base-pairing sites (cytosine and guanine).