Lewis’ octet rule, purpose

Many chemical models are known to be incorrect, for example, the Lewis theory of bonding, or an incomplete description of chemical phenomena, for example, the octet rule (Chapter 4). For the purposes of teaching chemistry it is often preferable to use simple approximate bonding models that give correct predictions for the majority of cases, than to use a more accurate but complicated model, such as the quantum mechanical model (Chapter 12), which is based on the electron s wave properties. 

Formal charges correspond to the difference between the number of valence electrons in the neutral free atom and the number of valence electrons in its bonded state. The number of electrons in the neutral free atom is the same as the atom s group number in the periodic table. To determine the electron count of an atom in a Lewis formula, we add the total number of electrons in unshared pairs to one-half the number of electrons in bonded pairs. It s important to note that counting electrons for the purpose of assigning formal charge differs from counting electrons to see if the octet rule is satisfied. A second-row... 

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