Lewis acid-base definition
/ LOO-iss /
Bronsted-Lowry pinned acidity to the proton — but plenty of reactions that smell exactly like acid-base chemistry involve no proton at all. When BF3 gas meets ammonia, they snap together into a solid, F3B-NH3, with a brand-new bond, yet not one H+ moved. To capture this, Gilbert Lewis (also in 1923) went one level deeper: forget the proton, look at the electron pair. A Lewis base is a species that DONATES a lone pair of electrons; a Lewis acid is a species that ACCEPTS that pair. They join by forming a covalent bond where both electrons come from the base.
This reframes acidity as the desire for electrons, basicity as the ability to give a lone pair. A Lewis acid needs somewhere to put the incoming pair — an empty low-lying orbital (the LUMO). BF3 qualifies because boron has only six valence electrons and an empty p orbital; metal cations like Al3+, Fe3+, or Cu2+ qualify because they have empty orbitals hungry for electron density. A Lewis base needs an available lone pair (a high-lying HOMO) — ammonia, water, fluoride, carbon monoxide, a phosphine. The Bronsted proton itself is just a special, very small Lewis acid: H+ has an empty 1s orbital, so when ammonia donates its lone pair to H+ you get NH4+. Every Bronsted base is a Lewis base; the Lewis idea simply does not require the acid to be a proton.
This is the most general and most useful definition in all of inorganic chemistry. The entire field of coordination chemistry is Lewis acid-base chemistry: a metal ion (Lewis acid) accepts lone pairs from ligands (Lewis bases) to build a complex. Catalysis by metal centers, the behavior of electron-deficient boron and aluminum compounds, solvent effects, and the hard-soft acid-base principle all sit on this foundation. The product of a Lewis acid and base is called an adduct, and the donated pair becomes a coordinate (dative) bond.
BF3 + NH3 gives F3B-NH3. Ammonia (Lewis base) donates its nitrogen lone pair into boron's empty p orbital (BF3, Lewis acid). A new B-N coordinate bond forms; no proton moves. The same logic explains Fe3+ accepting six water lone pairs to give [Fe(H2O)6]3+.
An acid-base reaction with no proton: base donates a lone pair, acid accepts it into an empty orbital.
Every Bronsted acid-base reaction can be recast in Lewis terms, but not the reverse — BF3 + NH3 is a Lewis acid-base reaction with no Bronsted description. Lewis is the broader category; the proton is just one very common Lewis acid.