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AP®ChemistryCollege BoardStudy GuidesUnit 2: Compound Structure & PropertiesLewis DiagramsFormal Charge

Formal Charge (College Board AP® Chemistry)

Study Guide

Martín

Written by: Martín

Reviewed by: Stewart Hird

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The Octet Rule & Formal Charge

  • Atoms form chemical bonds to get the electron configuration of their closest noble gas

  • The octet rule explains why atoms usually want eight electrons in their valence shell in order to have more stability

  • In some cases, it is possible to have multiple Lewis structures while still following the octet rule

  • Each of these variations is a resonance structure and they contribute to the formation of the hybrid of resonance

  • In order to figure out the resonance structure that contributes the most, the formal charge (FC) of all the atoms in the molecule is determined

Formal Charge

  • Formal charge is a charge assigned to all the atoms in a molecule by assuming that all the bonding electrons are shared equally between the atoms, regardless of differences in electronegativity

  • The formula of FC is

FC space equals space straight V minus 1 half straight B minus straight N

  • V is the number of valence electrons

  • B is the number of bonding electrons

  • N is the number of nonbonding electrons

Worked Example

What is the formal charge of boron in the BH4- ion?

Answer:

  • Boron is a Group 13 element, by using the periodic table information it can be deduced that it has 3 valence electrons.

  • Hydrogen is in Group 1, therefore it has one valence electron.

  • Since the BH4- ion has lost one electron, the total number of valence electrons in the structure is 8.

  • By following the steps to draw a Lewis diagram, the structure should look like this:

resonance-and-formal-charge-5

Lewis formula of  BH4– 

  • The number of bonding electrons in B is 8 and the number of nonbonding electrons in B is 0

FC space equals space straight V minus 1 half straight B minus straight N

FC space equals space 3 minus 1 half cross times 8 minus 0

FC space equals space minus 1

Choosing the preferred resonance structure

  • The concept of formal charge can help select competing resonance structures

  • The following rules should be applied when analyzing which of the resonance structures contributes the most

    • The sum of the formal in neutral molecules is zero

    • The sum of the formal charges in ions is the charge of the ion

    • The smaller the formal charge (or 0), the better it is. Large formal charges are not common in atoms

    • Negative formal charges should be assigned to the most electronegative atom

Worked Example

What is the formal charge on the two resonance structures shown?

resonance-structures-of-hydrogen-cyanide

Resonance structures of hydrogen cyanide

Deduce which is the preferred structure

Answer:

Structure 1

begin mathsize 14px style FC equals straight V minus 1 half straight B minus straight N end style

FC space on space hydrogen equals 1 minus 1 half stretchy left parenthesis 2 stretchy right parenthesis minus 0 equals 0

FC space on space carbon equals 4 minus 1 half stretchy left parenthesis 8 stretchy right parenthesis minus 0 equals 0

FC space on space nitrogen equals 5 minus 1 half stretchy left parenthesis 6 stretchy right parenthesis minus 2 equals 0

Structure 2

begin mathsize 14px style FC equals straight V minus 1 half straight B minus straight N end style

FC space on space hydrogen equals 1 minus 1 half stretchy left parenthesis 2 stretchy right parenthesis minus 0 equals 0

FC space on space carbon equals 4 minus 1 half stretchy left parenthesis 6 stretchy right parenthesis minus 2 equals negative 1

FC space on space nitrogen equals 5 minus 1 half stretchy left parenthesis 8 stretchy right parenthesis minus 0 equals plus 1

  • Structure 1 is the preferred structure because

    • The sum of the formal charges is zero since its a neutral molecule

    • All the formal charges on the atoms are 0

    • It has the least electronegative atom (the carbon atom) in the central position

  • Structure 2 has +1 on N and -1 on C

    • The negative formal charge in Structure 2 is not on the most electronegative ion, therefore, the structure is unstable

Examiner Tips and Tricks

The terms Lewis Diagrams, Lewis Structures or Lewis Formulas mean the same thing

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    Martín

    Author: Martín

    Expertise: Chemistry Content Creator

    Martín, a dedicated chemistry teacher and tutor, excels in guiding students through IB, AP, and IGCSE Chemistry. As an IB Chemistry student, he came from hands-on preparation, focusing on practical exam techniques and rigorous practice. While at Universidad San Francisco de Quito, his academic journey sparked a passion for computational and physical chemistry. Martín specializes in chemistry, and he knows that SaveMyExams is the right place if he wants to have a positive impact all around the world.

    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.