Final answer:
Formal charges are assigned by calculating the difference between the number of valence electrons in the free atom and those in the Lewis structure. The most stable resonance structure of COCl₂ will have minimal formal charges and satisfy the octet rule, usually with the carbon double-bonded to oxygen and single-bonded to chlorines. Resonance is the concept that the actual structure is a hybrid of all valid Lewis structures.
Step-by-step explanation:
To assign formal charges to each atom in the two resonance forms of COCl₂, one would follow the standard procedure of calculating formal charge, which is the difference between the number of valence electrons in the free atom and the number assigned to it in the Lewis structure. The main resonance forms of COCl₂ will typically involve a central carbon atom double-bonded to one oxygen and single-bonded to two chlorines, or double-bonded to the oxygen and one chlorine, and single-bonded to the other chlorine.
The most stable resonance structure is generally the one with formal charges closest to zero. Considering that carbon prefers to have four bonds and oxygen prefers two, the structure with carbon double-bonded to oxygen and single-bonded to both chlorines (with no formal charges) generally contributes more to the overall structure of COCl₂, due to minimal formal charges and a satisfaction of the octet rule for all involved atoms.
Resonance is a concept used to describe molecules like COCl₂, which can be represented by more than one valid Lewis structure. The actual structure of the molecule, known as the resonance hybrid, is a blend of all valid resonance forms.