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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).

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    $\begingroup$ Search in Google for their molecular structures. You'll understand the difference. $\endgroup$ May 16 '17 at 18:50
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    $\begingroup$ For one thing, $\ce{SO3^{2-}}$ has two extra electrons, so it's an ion. $\ce{SO3}$ is a neutral molecule. There are a different number of electrons, so they're just different. $\endgroup$
    – Zhe
    May 16 '17 at 18:57
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    $\begingroup$ The key is that they actually have different molecular formulae and are thus different compounds. Remember that charge counts ;) $\endgroup$
    – airhuff
    May 16 '17 at 18:58
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    $\begingroup$ It's pretty hard to explain this to a 10th grader. Just wait for a while, 11th grade teaches you all about different types of bonds. :) $\endgroup$ May 17 '17 at 2:40
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I'll give you the structures. Then you'll understand everything:

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

enter image description here

Structure of Sulfur Trioxide:

enter image description here

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

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    $\begingroup$ I don't think that is the structure of $\ce{SO3}$. The real structure has one double bond and 2 dative bonds. $\endgroup$ May 17 '17 at 2:38

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