Science, Tech, Math › Science Formal Charge Example Problem Share Flipboard Email Print Daviewales / Wikimedia Commons / CC BY 4.0 Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated February 04, 2020 Resonance structures are all the possible Lewis structures for a molecule. Formal charge is a technique to identify which resonance structure is the more correct structure. The most correct Lewis structure will be the structure where the formal charges are evenly distributed throughout the molecule. The sum of all the formal charges should equal the total charge of the molecule.Formal charge is the difference between the number of valence electrons of each atom and the number of electrons the atom is associated with. The equation takes the form: FC = eV - eN - eB/2 where eV = number of valence electrons of the atom as if it were isolated from the moleculeeN = number of unbound valence electrons on the atom in the moleculeeB = number of electrons shared by the bonds to other atoms in the molecule The two resonance structures in the above picture are for carbon dioxide, CO2. To determine which diagram is the correct one, the formal charges for each atom must be calculated. For Structure A: eV for oxygen = 6eV for carbon = 4 To find eN, count the number of electron dots around the atom. eN for O1 = 4eN for C = 0eN for O2 = 4 To find eB, count the bonds to the atom. Each bond is formed by two electrons, one donated from each atom involved in the bond. Multiply each bond by two to get the total number of electrons. eB for O1 = 2 bonds = 4 electronseB for C = 4 bonds = 8 electronseB for O2 = 2 bonds = 4 electrons Use these three values to calculate the formal charge on each atom. Formal charge of O1 = eV - eN - eB/2Formal charge of O1 = 6 - 4 - 4/2Formal charge of O1 = 6 - 4 - 2Formal charge of O1 = 0Formal charge of C = eV - eN - eB/2Formal charge of C1 = 4 - 0 - 4/2Formal charge of O1 = 4 - 0 - 2Formal charge of O1 = 0Formal charge of O2 = eV - eN - eB/2Formal charge of O2 = 6 - 4 - 4/2Formal charge of O2 = 6 - 4 - 2Formal charge of O2 = 0 For Structure B: eN for O1 = 2eN for C = 0eN for O2 = 6Formal charge of O1 = eV - eN - eB/2Formal charge of O1 = 6 - 2 - 6/2Formal charge of O1 = 6 - 2 - 3Formal charge of O1 = +1Formal charge of C = eV - eN - eB/2Formal charge of C1 = 4 - 0 - 4/2Formal charge of O1 = 4 - 0 - 2Formal charge of O1 = 0Formal charge of O2 = eV - eN - eB/2Formal charge of O2 = 6 - 6 - 2/2Formal charge of O2 = 6 - 6 - 1Formal charge of O2 = -1 All the formal charges on Structure A equal zero, where the formal charges on Structure B show one end is positively charged and the other is negatively charged. Since the overall distribution of Structure A is zero, Structure A is the most correct Lewis structure for CO2.