0.4 Covalent bonding and electron pair sharing  (Page 2/8)

In doing so, we discover that the periodic table is a representation of the valences of the elements: elementsin the same group all share a common valence. The inert gases with a valence of 0 sit to one side of the table. Each inert gas isimmediately preceded in the table by one of the halogens: fluorine precedes neon, chlorine precedes argon, bromine precedes krypton,and iodine precedes xenon. And each halogen has a valence of one. This "one step away, valence of one" pattern can beextended. The elements just prior to the halogens (oxygen, sulfur, selenium, tellurium) are each two steps away from the inert gasesin the table, and each of these elements has a valence of two ( e.g. $H_{2}O$ , $H_{2}S$ ). The elements just preceding these (nitrogen, phosphorus, antimony,arsenic) have valences of three ( e.g. $N{H}_3$ , $P{H}_3$ ), and the elements before that (carbon and silicon most notably) havevalences of four ( $C{H}_4$ , $\mathrm{Si}{H}_4$ ). The two groups of elements immediately after the inert gases, thealkali metals and the alkaline earths, have valences of one and two, respectively. Hence, for many elements in the periodic table,the valence of its atoms can be predicted from the number of steps the element is away from the nearest inert gas in the table. Thissystemization is quite remarkable and is very useful for remembering what molecules may be easily formed by a particularelement.

Next we discover that there is a pattern to the valences: for elements in groups 4 through 8 ( e.g. carbon through neon), the valence of each atom plus the number of electrons in the valence shell in that atom always equals eight . For examples, carbon has a valence of 4 and has 4 valence electrons, nitrogen has a valence of 3 and has 5valence electrons, and oxygen has a valence of 2 and has 6 valenceelectrons. Hydrogen is an important special case with a single valence electron and a valence of 1. Interestingly, for each ofthese atoms, the number of bonds the atom forms is equal to the number of vacancies in its valence shell.

To account for this pattern, we develop a model assuming that each atom attempts to bond to other atoms so asto completely fill its valence shell with electrons. For elements in groups 4 through 8, this means that each atom attempts tocomplete an "octet" of valence shell electrons. (Why atoms should behave this way is a question unanswered by thismodel.) Consider, for example, the combination of hydrogen and chlorine to form hydrogen chloride, $H\mathrm{Cl}$ . The chlorine atom has seven valence electrons and seeks to add a single electron tocomplete an octet. Hence, chlorine has a valence of 1. Either hydrogen or chlorine could satisfy its valence by"taking" an electron from the other atom, but this would leave the second atom now needing two electrons to completeits 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 the electron pair which is shared. It is this sharingof electrons that we refer to as a chemical bond, or more specifically, as a covalent bond , so named because the bond acts to satisfy the valence of both atoms. The two atoms are thus heldtogether by the need to share the electron pair.