Question

BF3 is electron deficient so that act as lewis acid.

Answer 1

Exactly ... BF3 is electron deficient and so it acts as lewis acid.

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Answer 2

The boron in BF3 is electron poor and has an empty orbital, so it can accept a pair of electrons, making it a 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.
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