0.7 Covalent bonding and electron pair sharing  (Page 4/8)

Octet Rule: For elements in Groups 4 through 8 in Periods 2 and 3 of the Periodic Table, the valence of each atom plus the number of valence electrons in each atom is equal to 8.

Observation 2: covalent bonding

As a way to think about this pattern, remember that for each of the atoms in these two periods, the valence shell can accommodate eight electrons. It turns out that the valence of each atom in Groups 4 through 8 is equal to 8 minus the number of valence electrons the atom has. For example, the valence of an N atom is 3, which is equal to 8 minus the number of valence electrons in N, which is 5. This tells us that, for each of these atoms, the number of bonds the atom typically forms is equal to the number of vacancies in its valence shell.

This suggests a model which would account for the Octet Rule. It appears that each atom in these Groups attempts to bond to other atoms so as to completely fill its valence shell with electrons. For elements in Groups 4 through 8, this means that each atom attempts to complete an “octet” of valence shell electrons. (Why atoms should behave this way is a question unanswered by this model.)

Consider, for example, the combination of two chlorine atoms to form Cl ₂ . Each chlorine atom has seven valence electrons and seeks to add a single electron to complete an octet. Hence, chlorine has a valence of 1. Either chlorine atom could satisfy its valence by “taking” an electron from the other atom, but this would leave the other atom now needing two electrons to complete its valence shell. The only way for both atoms to complete their valence shells simultaneously is to share two electrons. Each atom donates a single electron to a shared electron pair. This sharing of electrons is what we call a chemical bond. More specifically, we call this a “covalent bond,” so named because the bond acts to satisfy the valence of both atoms. The two atoms are thus held together by the need to share the electron pair.

Let’s apply this to a molecule with an H atom, HCl. H atoms are not expected to have an octet. In fact, the valence shell in H can accommodate just two electrons. Because an H atom has 1 valence electron, it thus has a single vacancy, or a valence of 1. Our model for electron pair sharing works for H atoms as well. When H and Cl bond, they share a pair of electrons forming a covalent bond so that the H atom has a full valence shell (two electrons) and the Cl atom has a full valence shell (eight electrons).

We now have two important pieces of our model. First, atoms form a covalent bond by sharing a pair of electrons. Second, for atoms in Groups 4 through 8, they tend to share enough electrons to fill the valence shell with 8 electrons, and this determines the number of covalent bonds they form. An H atom shares one pair of electrons to form a single covalent bond.

Observation 3: molecular structures in 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 CH ₄ , a primary component of household natural gas. Other simple common fuels 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 .