Why does the octet rule have exceptions? Give the three major categories of exceptions and an example of each.
Solution 29E Step 1 of 4 Sometimes , a molecule in which atoms respect the octet rule is not so stable. We can distinguish three cases in which octet rule isn’t respected: 1- When we have an odd number of electrons of valence 2- When an atom has less than eight electrons 3- When an atom has too much electron of valence Step 2 of 4 When we have an odd number of electrons of valence Example: That’s what happens in Nitrogen monoxide. 2 3 Nitrogen has this electronic configuration: 1s2, 2s , 2p , thus it has 5 valence electrons electrons ( two electrons in 2s orbital, three electrons in 2p orbital). 2 4 Oxygen has this electronic configuration: 1s2, 2s , 2p , thus it has 6 valence electrons electrons ( two electrons in 2s orbital, four electrons in 2p orbital). In total we have 5 +6 = 11 valence electrons. Scriviamo la struttura di Lewis iniziano a porre 2 elettroni tra i due atomi: N ·· O After that, 9 electrons are left to use to form other bonds. Let’s complete the octet on Nitrogen atom; in order to do that we have to locate three electros on Nitrogen: two in form of lone pair and one who stands alone. Let’s complete the octet on Oxygen; we have six electron: a couple of them can form a bond with Nitrogen, the other four complete the Oxygen shell by forming to lone pairs. We obtain Nitrogen monoxide, in which Nitrogen doesn’t have a complete octet In any other hypothesis we can obtain both atoms with a complete octet for this molecule. This confirms that if we have an odd number of valence electrons, the atoms that build up the molecule become an exception to the octet rule.