Is the Octet Rule Always Applicable in Chemistry?

[marc32123]

I read on chemwiki that "The Octet Rule requires all atoms in a molecule to have 8 valence electrons--either by sharing, losing or gaining electrons--to become stable"

Is it true that all atoms in a molecule have 8 electrons by sharing losing or gaining them?

 

[cgk]

It is true that this is what the octet rule says[1]. It is not true in reality. First, how to assign electrons to atoms is nowhere near clear. This cannot be done on a purely physical basis, but requires some empirical input. Second, various methods of assigning/counting the electrons based on first principles calculations show that most bonds are quite polar in practice and that even hydrogens and first row main group atoms and tend to have "shared electron numbers" which far deviate from the ideal octets one might think they should have. For higher main group elements and transition metals then all bets are off.

The octet rule is good for rationalizing many compounds, but do not take it too seriously. It is a reasonable model for a broad class of compounds, but it does not reflect reality. What one often sees in practice is that chemists think of very "interesting" ways of counting electrons with the sole purpose of fitting their compounds into the model.

[1] ...for main group compounds. Hydrogen is supposed to acquire two valence electrons, not eight, and d-metals 18