Which of the following statements best describes the SN for the sulfur atom in SO3

Question

Sulfur trioxide, SO3, reacts with water to yield sulfuric acid. Sulfur trioxide has the following three resonance structures and others (not shown) in which the octet is expanded. Which of the following statements best describes the SN for the sulfur atom in SO3

Image: http://my.thinkwell.com/questionbank/3001-4000/3913/img/46359.gif

Options:
The SN is different for each resonance structure for SO3.
The SN in all three structures is the same. It is equal to 3.
The SN varies between 1 and 3.
OR
The SN changes when resonance structures with expanded octets are considered.

Answer 1

Steric Number is the number of atoms and lone electron pairs on a central atom. In your image, sulfur exhibits no lone pairs and is always bonded to three oxygen atoms (doesn't matter if it is single, double, or triple bond). Therefore: The SN in all three structures is the same. It is equal to 3.