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

This concept also applies to elements just below carbon, nitrogen, oxygen, and fluorine. Silicon will form SiH ₄ , so an Si atom has a valence of 4. Phosphorous forms PH ₃ , so P has a valence of 3, and Sulfur forms H ₂ S, so S has a valence of 2. Each halogen atom (Cl, Br, I) prefers to form molecules by combining with a single hydrogen atom (e.g. HCl, HBr, HI), so each halogen has a valence of 1.

We can make further progress using the valence of the halogens. Lithium, sodium, potassium, and rubidium each bind with a single Cl atom to form LiCl, NaCl, KCl, and RbCl. Therefore, they also have a valence of 1. Because we also find that, for example, the combination of two potassium atoms with a single oxygen atom forms a stable molecule, our assignments are all still consistent, since oxygen’s valence of 2 can be satisfied by the two K atoms, each with a valence of 1. We can proceed in this manner to assign a valence to each element by simply determining the number of atoms to which this element’s atoms prefer to bind.

If we arrange the valences according to Periodic Table as in [link] , we discover that there is a pattern. Just as we would expect from the Periodic Law, elements in 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 is immediately preceded in the table by one of the halogens: fluorine precedes neon, chlorine precedes argon. And each halogen has a valence of one. This “one step away, valence of one” pattern can be extended. The elements just prior to the halogens (oxygen, sulfur, selenium, tellurium) are each two steps away from the inert gases in 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. NH ₃ , PH ₃ ), and the elements before that (carbon and silicon most notably) have valences of four (CH ₄ , SiH ₄ ). The two groups of elements immediately after the inert gases, the alkali 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. This systemization is quite remarkable and is very useful for remembering what molecules may be easily formed by a particular element.

Next we discover that there is an additional very interesting aspect to the pattern of 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 instance, carbon has a valence of 4 and has 4 valence electrons; nitrogen has a valence of 3 and has 5 valence electrons; oxygen has a valence of 2 and has 6 valence electrons. We have made one of the most important observations in Chemistry, the “Octet Rule”: