1.2 Molecular structure and physical properties (ticket #3493)  (Page 3/5)

We can now draw modified Lewis structures to account for ionic bonding, but these are very different from ourprevious drawings. Sodium chloride can be represented as shown in .

This indicates explicitly that the bonding is due to positive-negative ion attraction, and not due to sharing ofan electron pair. The only sense in which the ${\mathrm{Na}}^{+}$ ion has obeyed an octet rule is perhaps that, in having emptied its valence shell of electrons, the remaining outer shell of electronsin the ion has the same octet as does a neon atom. We must keep in mind, however, that the positive sodium ion is attracted to manynegative chloride ions, and not just the single chloride ion depicted in the Lewis structure.

Observation 2: molecular dipole moments

Our Lewis model of bonding, as currently developed, incorporates two extreme views of the distribution ofelectrons in a bond. In a covalent bond, we have assumed up to this point that the electron pair is shared perfectly. In completecontrast, in ionic bonding we have assumed that the electrons are not shared at all. Rather, one of the atoms is assumed to entirelyextract one or more electrons from the other. We might expect that a more accurate description of the reality of chemical bonds fallsin general somewhere between these two extremes. To observe this intermediate behavior, we can examine molecular dipolemoments.

An electric dipole is a spatial separation of positive and negative charges. In the simplest case, a positivecharge $Q$ and a negative charge $-Q$ separated by a distance $R$ produce a measurable dipole moment , $\mu$ equal to $\times (Q, R)$ . An electric field can interact with an electric dipole and can evenorient the dipole in the direction of the field.

We might initially expect that molecules do not in general have dipole moments. Each atom entering into achemical bond is electrically neutral, with equal numbers of positive and negative charges. Consequently, a molecule formed fromneutral atoms must also be electrically neutral. Although electron pairs are shared between bonded nuclei, this does not affect thetotal number of negative charges. We might from these simple statements that molecules would be unaffected by electric ormagnetic fields, each molecule behaving as a single uncharged particle.

This prediction is incorrect, however. To illustrate, a stream of water can be deflected by an electricallycharged object near the stream, indicating that individual water molecules exhibit a dipole moment. A water molecule is rather morecomplicated than a simple separation of a positive and negative charges, however. Recall though that a water molecule has equaltotal numbers of positive and negative charges, consisting of three positively charged nuclei surrounded by ten electrons.Nevertheless, measurements reveal that water has a dipole moment of $6.17E-30Cm=1.85\mathrm{debye}$ . (The debye is a unit used to measure dipole moments: $1\mathrm{debye}=3.33E-30Cm$ .) Water is not unique: the molecules of most substances have dipolemoments. A sampling of molecules and their dipole moments is given in .