0.5 Covalent bonding and electron pair sharing  (Page 3/8)

Observation 2: compounds of carbon and hydrogen

Many of the most important chemical fuels are compounds composed entirely of carbon and hydrogen, i.e. hydrocarbons. The smallest of these is methane $C{H}_4$ , a primary component of household natural gas. Other simple commonfuels include ethane $C_2{H}_6$ , propane $C_3{H}_8$ , butane $C_4{H}_{10}$ , pentane $C_5{H}_{12}$ , hexane $C_6{H}_{14}$ , heptane $C_7{H}_{16}$ , and octane $C_8{H}_{18}$ . It is interesting to note that there is a consistency in thesemolecular formulae: in each case, the number of hydrogen atoms is two more than twice the number of carbon atoms, so that eachcompound has a molecular formula like $C_n{H}_{2n+2}$ . This suggests that there are strong similarities in the valences ofthe atoms involved which should be understandable in terms of our valence shell electron pair sharing model. In each molecule, thecarbon atoms must be directly bonded together, since they cannot be joined together with a hydrogen atom. In the easiest example ofethane, the two carbon atoms are bonded together, and each carbon atom is in turn bonded to three hydrogen atoms. Thus, in this case,it is relatively apparent that the valence of each carbon atom is 4, just as in methane, since each is bonded to four other atoms.Therefore, by sharing an electron pair with each of the four atoms to which it is bonded, each carbon atom has a valence shell ofeight electrons.

In most other cases, it is not so trivial to determine which atoms are bonded to which, as there may be multiplepossibilities which satisfy all atomic valences. Nor is it trivial, as the number of atoms and electrons increases, to determinewhether each atom has an octet of electrons in its valence shell. We need a system of electron accounting which permits us to seethese features more clearly. To this end, we adopt a standard notation for each atom which displays the number of valenceelectrons in the unbonded atom explicitly. In this notation, carbon and hydrogen look like , representing the single valence electron in hydrogen and the four valence electrons in carbon.

Using this notation, it is now relatively easy to represent the shared electron pairs and the carbon atom valenceshell octets in methane and ethane. Linking bonded atoms together and pairing the valence shell electrons from each gives .

Recall that each shared pair of electrons represents a chemical bond. These are examples of what are called Lewis structures , after G.N. Lewis who first invented this notation. These structures reveal, at a glance, which atomsare bonded to which, i.e. the structural formula of the molecule. We can also easily count the number of valence shell electrons aroundeach atom in the bonded molecule. Consistent with our model of the octet rule, each carbon atom has eight valence electrons and eachhydrogen has two in the molecule.

In a larger hydrocarbon, the structural formula of the molecule is generally not predictable from thenumber of carbon atoms and the number of hydrogen atoms, so the molecular structure must be given to deduce the Lewis structure andthus the arrangement of the electrons in the molecule. However, once given this information, it is straightforward to create aLewis structure for molecules with the general molecular formula $C_n{H}_{2n+2}$ such as propane, butane, etc. For example, the Lewis structure for "normal" butane (with all carbons linked one afteranother) is found here .