Formal Charge Example Problem

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/2whereeV 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 moleculeThe 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 fo r oxygen 6eV for carbon 4To find eN, count the number of electron dots around the atom.br/>eN for O1 4eN for C 0eN for O2 4To 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 electronsUse 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 0For 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 -1All 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 th e most correct Lewis structure for CO2.