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why does bf3 behave as lewis acid

A Lewis acid is defined as an electron-pair acceptor. So for something to act as a Lewis acid, it needs to want electrons. Prime examples are $ce{H+}$, the hardest Lewis acid around (zero polarisability, very high charge per volume ratio) and practically every metal cation out there: $ce{Al^3+, Zn^2+, Fe^3+, Ag+}$ just to name a few. Consider boron, a rather electropositive element Б it counts as a metalloid so it is somewhere between non-metals and metals. We are binding it to fluorine, the most electronegative element, and weБre doing that three times. It should be evident that there is hardly any electron density left on boron.

How happy would it be, if some other atom gladly donated their electron pair to share? Now what are we going to do if there is no Lewis base around? Well, initially boron will still be there, depleted of all its valence electrons by fluorine (or nearly at least). This is where fluorine discovers its charity side: All three fluorines donate just a tad of electron density so that the baby boron in the middle will stop crying. This is what you referred to as Бback bondingБ and Ivan calls Бmesomeric stabilisationБ. But the point is: That doesnБt help against the electron deficiency in any way, itБs more like boronБs final counter-measure against loosing electrons.
No. vii.

Why BF behaves as a Lewis acid? Answer. A substance, molecule or ion which can accept a pair of electrons is called Lewis acid. BF is an electro deficient compound and can accept pair of electrons so it behaves as Lewis acid. For exampe No. viii. Water is amphoteric specie according to Bronsted-Lowry concept. What is its nature according to Lewis concept? Answer. Water is amphoteric specie according to Bronsted-Lowry concept because it behaves as an acid as well as a base.

But as water molecule has lone pair of electrons, it behaves as a base according to Lewis concept. No. i. When acids react with carbonates and bicarbonates, which gas evolves out? Answer. When acids react with carbonates and bicarbonates, they form salts and carbon dioxide gas evolves out. For example, reaction of Calcium carbonate with hydrochloric acid forms calcium chloride and carbon dioxide evolves out. Similarly, when sulphuric acid reacts with sodium bicarbonate, sodium sulphate forms along with evolution of carbondioxcide gas. P

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