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Using Formal Charge to Predict Lewis Structures Sometimes when we draw a Lewis structure, there are multiple arrangements possible, so you may have to decide which structure is most important. We refer to this as the preferable or dominant structure (the "BEST" representation) of the molecule or polyatomic ion. If there is more than one structure with different bonding arrangements, you can use formal charge to distinguish which is the preferable (or dominant) structure. Below are some guidelines that can help to determine the preferable structure for a particular molecule or polyatomic ion: . A Lewis structure in which all formal charges are zero (FC = 0) is preferable. . If the Lewis structure must have nonzero formal charges (FC0), the . arrangement with the smallest nonzero formal charges is preferable. When we must choose among several Lewis structures with similar distributions of formal charges, the structure with the negative formal charge(s) on the more electronegative atoms is preferable. Example: Use the formal charge to determine which of the following possible Lewis structures for carbon dioxide, CO2, is preferable. ww 0=c=0 :0=C-Ö: 0=0=C Structure 000 +1 0-1 0 +2 2 Formal charge Comparing the formal charges of the possible structures, we can identify the structure on the left as preferable because FC 0 for all its atoms.

7 1 point On a separate piece of paper, calculate the formal charge for each of the atoms in the Lewis structures for the cyanate anion, NCO, shown below: Structure A Structure B Structure C [:N-C=O:] ( N-c-8) A (N N-C-ö]- [:N=C-Ö:]- Based on formal charge, which structure is preferable (the BEST representation) for the cyanate anion, NCO? Structure A Structure B Structure C

8 1 polet Preferable Lewis Structure for a Molecule (or polyatomic ion) that has Multiple Possible Arrangements When you are asked to determine a preferable Lewis structure for a molecule or polyatomic lon that has multiple possible arrangements, pay attention to the question Sometimes you might be asked to choose the best representation that DOES NOT VIOLATE the OCTET RULE and other times the preferred structure might be the arrangement that minimizes FORMAL CHARGE. When asked: "What is the best structure that DOES NOT VIOLATE THE OCTET RULE? Make sure you follow the Steps for Drawing Lewis Structures (discussed earlier). All atoms should have an OCTET (with exceptions such as Hand B "What is the best structure based on FORMAL CHARGE? Choose the arrangement that minimizes formal charge. When required to use an expanded octet arrangement to minimize formal charge, follow any allowed expanded octet rules to create the best structure(s) N

Are the best structures Consider the following possible Lewis Structures for the sulfur dioxide molecule, 50. Each of these structures is a VALID Lewis structure. Option 1 S ← Study the options provided and answer the questions below. Which is the preferred structure based on formal charge? Choose the arrangement that minimizes formal charge Which is the preferred structure that follows the Steps for Drawing Lewis Structures and satisfies the octet rule for all its atoms! S: 0 :0: 6: Option 2 S