Why is SO3 a Lewis acid?

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Answer 1 · 2018-05-14T07:49:49

Well a #"Lewis-acid"# is an electron-pair acceptor....

And certainly the electron-deficient sulfur atom in #SO_3# is such an electron pair acceptor...i.e...

#SO_3(g) + H_2O(l) rarr H_2SO_4(aq)#

#SO_3 +C_6H_5Nrarr{C_6H_5N*SO_3}#...

We would give the following Lewis structure for #SO_3# as...

#O=stackrel(2+)S(-O^(-))_2#

...which illustrates the electron paucity of the sulfur centre.

What is the expected geometry for this molecule?