Is SO3 an ion or a molecule? Earlier I thought that $\ce{SO3^{2-}}$ (sulfite) is an ion with -2 charge but today I read on internet $\ce{SO3}$ (sulfur trioxide) is a molecule with no charge. How can this be possible? Please explain knowing that I am only a student of 10th standard (use normal chemistry concepts as far as possible).
I'll give you the structures. Then you'll understand everything:

Resonance structures of $\ce{SO3^{2-}}$ ion:



Structure of Sulfur Trioxide:



The absence of a double bond between two oxygen atoms in sulfite ion shows why it is bivalent.