Lone pairs for Lewis acid/base

[Chem_Mod]

Question: Dr. Lavelle, when identifying a reaction compound as a Lewis Acid or Lewis Base do we base it on the electrons they have available or their charge? For example, SO2 is a lewis acid because it has an extra pair of electrons which can then bond to a Cl-? So why is NH3 base if it has one lone pair of electron? thanks

[Chem_Mod]

Answer: Lewis acid is a species that can accept an extra pair of electrons. In SO2, S only has 6 valence electrons. It needs one more pair of electrons to satisfy octet rule. Therefore, it acts as an Lewis acid to accept a pair of electrons from Cl-. Lewis base is a species that can donate a pair of electrons. NH3 has one lone pair of electrons on N, therefore, it can act as a Lewis base.

[304310808]

Just for clarification,
Can we use this rule of thumb for all Lewis acids or bases: If it has a lone pair, it is a Lewis Base. If it doesn't fill the octet rule and needs a lone pair, it is a Lewis acid?

[Chem_Mod]

That is often a good rule of thumb but remember not to constrict yourself to that definition. For instance, both water and I₂ are examples of Lewis acids, even though they fulfill the octet rule. Ethyne (HCCH) or benzene could also be electron-donating Lewis bases.