Using formal charge to determine the most stable structure (extension)

Using formal charge to determine the most stable structure (extension)

1. When drawing Lewis structures of molecules containing elements that do not obey the octet rule (such as S in SO3), it is possible to draw alternate structures

2. To decide which is the most likely structure, assign 'formal charge' to each atom

   • The formal charge is the difference between the number of valence electrons in a single atom of that element (6 for sulfur and 6 for oxygen) and the number of valence electrons that the atom has in the molecule

3. (O-)2S=O

   • In this structure, the S is assigned 4 electrons (half share of 4 covalent bonds), which means the S has 2 fewer than the normal S atom
     • Formal charge is +2
   • The double-bonded O atom has a formal charge of zero, while the 2 singularly bonded atoms have a formal charge of -1 (since they have 6 non-bonded electrons and a half share of a single covalent bond (1 more than the O atom))

4. O-S(=O)2

   • Formal charge on S is +1 (assigned 5 electrons rather than 6 found in sulfur atom), and formal charge on singly bonded O atom is -1

5. S(=O)3

   • Formal charge on all atoms is zero
     • This structure therefore has the smallest separation of formal charge, and is therefore the most stable structure