: a charge assigned to each atom in a structure by assuming that the shared electrons are divided equally between the bonded atoms. Most Lewis structures do not require any formal charges. tructures that require formal charges can be recognized by the number of bonds made by the elements. Carbon forms 4 bonds, Nitrogen 3, xygen 2 and F forms 1 bond. ow do we know which structural formula is the best one to use? Find the Formal charge for the atoms in each structure. If the Lewis structure requires MRE than these, the atom will have a PITIVE formal charge. If the Lewis structure requires LE than these, the atom will have a NEGATIVE formal charge.
The Formal charge on an atom can be calculated from this equation: Calculate the formal charge on each atom in the molecule, C F.C. = (# of valence electrons) (# lone pair electrons) (! # shared electrons) F.C. of C = 4 2!(6) = 1 F.C. of = 6 2!(6) = +1 Draw the Lewis structure of the ammonium cation, showing formal charges as needed. + 1. keleton structure of N 4 is 2. Total # Valence electrons: 4() 4 x 1 = 4 e 1(N) 1 x 5 = 5 e +1 charge = 1 e 8 e 3. : 5 0!(8) = +1! Which of these structures for N2 - is correct?! left = 6-6 -!(2) = 1-! N = 5-2 -!(6) = 0! right = 6-4 -!(4) = 0! left = 6-4 -!(4) = 0! N = 5-2 -!(6) = 0! right = 6-6 -!(2) = 1- Both structure have the same formal charge. Therefore either structure is equally correct.
(A)! = 1-0 -!(2) = 0! C = 4-0 -!(8) = 0! = 6-4 -!(4) = 0 (B)! = 1-0 -!(2) = 0! C = 4-4 -!(4) = 2-! = 6-0 -!(8) = 2+ Which of these structures for 2C is the most favorable? Using to predict the most stable structure. 1 tructures with the smallest formal charge are favored. 2 3 tructures that have adjacent atoms with formal charges of the same sign are much less favorable. tructures that place negative formal charges on the more electronegative element are favored. 4 Formal charges of opposite signs are usually on adjacent atoms. tructure A has the smaller formal charge. It is therefore the more favorable arrangement. Now it s time to Practice! ow do we know which structural formula is the best one to use? & Resonance The one on the RIGT, it has the smallest formal charge (0).
xidation Numbers xidation Numbers x # s are assigned or calculated based on known fixed positive and negative charges, and can be determined by examination of the formula for the species. xidation numbers are useful to identify how charges change in a redox (oxidation-reduction) reaction. Known ox # s per atom um of all charges in compound = 0 xidation Numbers xidation Numbers xidation numbers Lewis tructure s
xidation Numbers xidation Numbers Finally, both oxidation numbers and formal charges must add up the same way: For compounds, which are always electrically neutral, the sum of all oxidation numbers or the sum of all formal charges must equal zero. For polyatomic ions, which always have a specific charge, the sum of all oxidation numbers or the sum of all formal charges must equal the charge on the ion. xidation Numbers xidation Numbers Now it s time to Practice! xidation Number & xidation Numbers