The oxidation number is = group # of the element - # of electrons surrounding the element in the ionic limit.
In the ionic limit the atom with the larger electronegativity in a pair gets all of the bonding electrons. For example, consider H-F. The Lewis structure is
and the EN of F is 4.0 and for H is 2.1 so we give F the two electrons involved in the bonding so that F has 8 electrons surrounding the element in the ionic limit and H has 0 electrons in the ionic limit.
Therefore the oxidation number of F is 7-8 = -1 and that of H is 1-0 = +1.
For SCl2, S would have an oxidation number of +2 and Cl would have -1. Check to be sure you get the same answer. Remember that these are not the real charges that exist on the atoms. These are ionic limits and serve to tell us where the delta+ and delta- charges are and they help us to understand redox reactions.
Formal Atom Charge
For covalent bonds it is better to have the formal atom charge which is equal to the group# - the number of non-bonding electrons - 1/2 the number of bonding electrons (this assumes an equal sharing of the bonding electrons between the two bonding atoms.
For ClO4- the formal charge on C would be -1 and for Cl would be +3. Now we can sum these to get 3 -1 -1 -1 -1 = -1 which is the correct charge for this species. Be sure that you can obtain these numbers yourself.
One use of the formal charges is that when you have several choices of Lewis structures for a molecule, the most favorable structure is the one with the most negative formal charge residing on the atom with the highest electronegativity. Here is an example problem for you to work using this concept. NCO- can be drawn to have three resonance structures. Using formal charges, determine which structure might be the most favorable.
Go to this great site for help on formal charges
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