Advanced Placement (AP) Chemistry Practice Exam

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What is formal charge used for in Lewis structures?

To determine the molecular geometry
To find the most likely structure
To identify molecular polarity
To calculate the molecular mass

The correct answer is: To find the most likely structure

Formal charge is a valuable tool in drawing Lewis structures as it helps determine the most stable or likely structure for a molecule based on electron distribution. Each atom in a molecule can be assigned a formal charge by applying the formula: formal charge = (valence electrons) - (non-bonding electrons + 1/2 bonding electrons). In the context of Lewis structures, the most favorable arrangement of atoms is one where the formal charges on each atom are minimized and ideally close to zero. When there are multiple possible structures for a given molecule, the one with the lowest formal charges or the one where any negative formal charges reside on the more electronegative atoms is often considered the most probable representation of that molecule. While formal charge does help provide insights into other aspects such as molecular geometry or polarity indirectly, its primary role is to assess the viability of different Lewis structures and ascertain which is the most stable or likely form of a molecule.