The most preferred structure among the following with the lowest energy for $SO_{3}$ is:

According to the principles of chemical bonding found in the sources, the preference for a particular Lewis structure is determined by formal charges . The sources state that 'Generally the lowest energy structure is the one with the smallest formal charges on the atoms' .

1. Sulphur's Valence: Sulphur belongs to the third period and can expand its octet to accommodate more than eight valence electrons because it has 3d orbitals available for bonding .
2. Formal Charge Calculation: In a structure where Sulphur forms three double bonds with three Oxygen atoms:

• Formal Charge on S: $6$ (valence) $- 0$ (lone pairs) $- \frac{1}{2}(12)$ (bonding electrons) $= 0$.
• Formal Charge on each O: $6$ (valence) $- 4$ (lone pairs) $- \frac{1}{2}(4)$ (bonding electrons) $= 0$. Since all atoms in this structure have a formal charge of zero, it is the most stable, lowest energy, and thus the most preferred structure compared to those where Sulphur maintains an octet but carries a high positive formal charge (like +2) .