Q.All Bronsted bases are also Lewis bases, but all Bronsted acids are not Lewis acids. Explain.

Step 1. A Bronsted base is defined as a proton acceptor. To accept a proton (H+), a species must have a lone pair of electrons available to form the new bond to that proton.

Step 2. But 'having a lone pair to donate/share' is EXACTLY the Lewis definition of a base (a Lewis base donates a share in an electron pair). So any Bronsted base necessarily also qualifies as a Lewis base -- the two definitions coincide for bases.

Step 3. A Bronsted acid, by contrast, is defined narrowly as a proton donor -- it must actually possess an ionizable H+ to give up.

Step 4. The Lewis definition of an acid is broader: any species that can ACCEPT a share in an electron pair, whether or not it has a proton at all. Species like BF3 or AlCl3 (incomplete octet, no acidic proton) or a bare metal cation (positively charged, can accept a lone pair) are Lewis acids by this broader definition, but they have no proton to donate, so they fail the Bronsted-Lowry test for an acid. …

Compare what each theory actually requires: Bronsted needs a proton to move; Lewis only needs an electron pair to be shared, which is …

• Assuming the two theories are simply 'the same idea in different words' -- they coincide for bases but not for acids. …