Charge v. Oxidation Number

The difference between an atom or compound’s charge and oxidation number is a tricky concept to grasp because they are very similar, and actually the same, in many cases. A charge is when an element gains or loses an electron, causing it to be positive or negative. I’m sure by this point we all know that certain groups on the periodic table tend to have certain charges when they form a compound. However, charges can only exist within ionically-bonded compunds.

We really don’t need to worry too much about the oxidation numbers of ionic compounds because they are actually the same as the ion’s charge. For atoms that are bonded covalently, oxidation numbers are a sort of imaginary charge, as if the molecule was bonded ionically. Let’s use H2O as an example.Though the electrons are shared, they are shared unequally because oxygen has a greater attraction for electrons than does hydrogen. For the purpose of assigning oxidation numbers, we act as if the atom that has a greater pull or attraction actually possesses all of the electrons, just as one atom in an ionic bond would.

Here are some critical rules for assigning oxidation numbers that are listed in our book:

• The oxidation number of an atom in an element is 0. E.g. Na, O2, F2, Pb
• The oxidation number of a monatomic ion is the same as its charge. E.g. Na+, Al+3, S-2
• Fluorine’s oxidation number is always -1 in its compounds. E.g. In NaF, fluorine’s oxidation number is -1.
• Oxygen is usually -2 in its compounds (Exception: peroxides, containing O2 2- : in this case oxygen is -1). E.g. In H2O, oxygen is -2.
• Hydrogen is +1 in its compounds. E.g. H is +1 in HCl.

For all other atoms in a covalent compound, their oxidation numbers may vary. When looking to assign oxidation numbers to atoms in covalent compounds, use the presence of oxygen, hydrogen, and/or fluorine as a guide because their oxidation numbers don’t change.

Let’s try some:

• CO2 — We know oxygen is -2, and since there are 2 O atoms, we are now at -4. The single carbon atom is therefore responsible for putting this compound back up to 0, so to compensate, we can deduce that it is +4.
• SF6 — We know that F is -1, so multiplied by 6 we have -6. Therefore the sulfur atom is in this case +6 (this is an example where an atom’s oxidation number differs from its charge).
• NO3- –Though this has a charge, polyatomic ions are actually formed through covalent bonding, so we have to treat it as we would any other molecule, with one exception. Instead of trying to attain 0, we want to reach -1 to compensate for the ion’s charge. Because we know that O is -2 and multiplied by 3 is -6, we find that N is +5, which together adds to -1.