0.4 Covalent bonding and electron pair sharing  (Page 6/8)

This Lewis structure reveals not only that each carbon and oxygen atom has a completed octet of valence shellelectrons but also that, in the stable molecule, there are four non-bonded electrons on the oxygen atom. Ethanol is an example ofan alcohol . Alcohols can be easily recognized in Lewis structures by the C-O-H group. The Lewis structures of all alcoholsobey the octet rule.

In dimethyl ether, the two carbons are each bonded to the oxygen, in the middle, shown here .

Ethers can be recognized in Lewis structures by the C-O-C arrangement. Note that, in both ethanol and dimethyl ether,the octet rule is obeyed for all carbon and oxygen atoms. Therefore, it is not usually possible to predict the structuralformula of a molecule from Lewis structures. We must know the molecular structure prior to determining the Lewisstructure.

Ethanol and dimethyl ether are examples of isomers , molecules with the same molecular formula but different structural formulae. In general, isomers have ratherdifferent chemical and physical properties arising from their differences in molecular structures.

A group of compounds called amines contain hydrogen, carbon, and nitrogen. The simplest amine is methyl amine, whose Lewis structure is here .

"Halogenated" hydrocarbons have been used extensively as refrigerants in air conditioning systemsand refrigerators. These are the notorious "chlorofluorocarbons" or "CFCs" which havebeen implicated in the destruction of stratospheric ozone. Two of the more important CFCs include Freon 11, $CF{\mathrm{Cl}}_3$ , and Freon 114, $C_2{F}_4{\mathrm{Cl}}_2$ , for which we can easily construct appropriate Lewis structures,shown here .

Finally, Lewis structures account for the stability of the diatomic form of the elementalhalogens, $F_2$ , ${\mathrm{Cl}}_2$ , ${\mathrm{Br}}_2$ , and $I_2$ . The single example of $F_2$ is sufficient, shown here .

We can conclude from these examples that molecules containing oxygen, nitrogen, and the halogens areexpected to be stable when these atoms all have octets of electrons in their valence shells. The Lewis structure of each moleculereveals this character explicitly.

On the other hand, there are many examples of common molecules with apparently unusual valences, including:carbon dioxide $C{O}_2$ , in which the carbon is bonded to only two atoms and each oxygen isonly bonded to one; formaldehyde $H_{2}CO$ ; and hydrogen cyanide $HCN$ . Perhaps most conspicuously, we have yet to understand the bondingin two very important elemental diatomic molecules, $O_2$ and $N_2$ , each of which has fewer atoms than the valence of eitheratom.

We first analyze $C{O}_2$ , noting that the bond strength of one of the $CO$ bonds in carbon dioxide is 532 kJ, which is significantly greater than the bond strength of the $CO$ bond in ethanol, 358 kJ. By analogy to the comparison of bonds strengths in ethane to ethene, we can imagine that this differencein bond strengths results from double bonding in $C{O}_2$ . Indeed, a Lewis structure of $C{O}_2$ in which only single electron pairs are shared ( ) does not obey the octet rule, but one in which we pair and share the extra electrons reveals that double bondingpermits the octet rule to be obeyed ( ).