# Why is SO3 a Lewis acid?

May 14, 2018

Well a $\text{Lewis-acid}$ is an electron-pair acceptor....

#### Explanation:

And certainly the electron-deficient sulfur atom in $S {O}_{3}$ is such an electron pair acceptor...i.e...

$S {O}_{3} \left(g\right) + {H}_{2} O \left(l\right) \rightarrow {H}_{2} S {O}_{4} \left(a q\right)$

$S {O}_{3} + {C}_{6} {H}_{5} N \rightarrow \left\{{C}_{6} {H}_{5} N \cdot S {O}_{3}\right\}$...

We would give the following Lewis structure for $S {O}_{3}$ as...

$O = \stackrel{2 +}{S} {\left(- {O}^{-}\right)}_{2}$

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

What is the expected geometry for this molecule?