What are Lewis acids?

Which of the following are Lewis acids ?

Which of the following are Lewis acids? H_2O, BF_3, H^+ and NH_4^+

What is the shape of an Ex_(5) molecules ? Why does it behave a a Lewis acid?

Why does BF_3 behave as Lewis acid?

Why does boron triflouride behave as a Lewis acid?

Why BF_3 acts as a Lewis acid?

Identify the interhalogen compound with which ClO^(-) ion is isoelectronic. Does it behave as a Lewis acid?

What are the acids present in acid rain?

Explain why the trihalides of boron act as Lewis acid.