Lewis Theory
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eg. 1:
In this reaction, BF3 accepts a pair of electrons from F− to form a new covalent bond and, therefore, acts as an acid; F− contributes a pair of electrons toward a new covalent bond with BF3 and, therefore, acts as a base.
In a reaction a species that acts as an acid according to Lewis theory is called a Lewis acid and a species that acts as a base a Lewis base.
eg. 2: