Writing Lewis Formulas The Octet Rule

A water molecule can be represented by either of the following diagrams. 

An H2O molecule has two shared electron pairs, that is, two single covalent bonds. The O atom also has two unshared pairs. 

In dash formulas, a shared pair of electrons is indicated by a dash. The following diagrams show the two double bonds in carbon dioxide and its Lewis formula. 

A CO2 molecule has four shared electron pairs in two double bonds. The central atom (C) has no unshared pairs. 

The covalent bonds in a polyatomic ion can be represented in the same way. The Lewis formula for the ammonium ion, NILj, shows only eight electrons, even though the N atom has five electrons in its valence shell and each H atom has one, for a total of 5 + 4(1) = 9 electrons.The NH4 ion, with a charge of 1+, has one fewer electron than the original atoms. 

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