Download O`s in O are assigned ZERO

"Rules" For Assigning Oxidation Numbers
Oxidation Numbers can be assigned to any “Atom” in any chemical species using or applying
the following rules/principles (See other handout for detailed description of the meaning and
purpose of oxidation numbers)
1. Atoms in elements are assigned an oxidation number of ZERO. (Note: Ca2+ is not
the same as Ca! Ca2+ is an ion, not an element!)
O’s in O2 are assigned ZERO; O’s in O3 are assigned ZERO; Ca in Ca is assigned ZERO.
2. In compounds OR polyatomic ions, O atoms are typically assigned an oxidation
number of –2, and H atoms are typically assigned an oxidation number of +1. I will
not require you to know the exceptions to this rule (peroxides are a common exception).
(Note: Don’t forget about Rule 1 before assigning oxidation numbers to O and H!!!)
 You can figure out the oxidation numbers of other “atoms” in a species by using Rule
number 3:
3. The sum of the oxidation numbers of EACH “ATOM” in a chemical species must
equal the net charge on the species. (Don’t forget that if there is more than one atom
of the same type in a species, you must sum up the oxidation numbers on ALL of them.
BUT that sum is not the oxidation number! Please ask me if you are confused about
this.)
Example: in NO3-, each O is assigned an oxidation number of –2. Since there are 3
O atoms, the sum total of their oxidation numbers is 3 x –2 = -6. The oxidation
number of the single N atom must therefore be the number which when added to
–6 yields the overall charge of –1. Thus the oxidation number of N in NO 3- is +5.
 The oxidation number of O in the above example is –2; it is NOT –6.
=> For monatomic species, this rule (Rule 3) reduces to the following “trivial” rule: the
oxidation number of any monatomic species is equal to the actual charge on the
species. E.g., the oxidation number of Ca2+ is +2; for H in H+ the oxidation number
is +1 (not zero!!!).
Helpful hint: If a compound is ionic, rewrite the formula as the SEPARATED IONS
(whose charges you should either know or can figure out). Then assign the
oxidation numbers according to rules 2 and 3.
Example: Fe2(CO3)3. You need to rewrite as Fe3+ and CO32- to get all ox. numbers.
------------------------------------------------------------------------------------------------------------------------Solubility Rules Comments
NOTE: You are responsible for knowing/memorizing the following solubility “rules” (patterns):
1. All “nitrates” are (water) SOLUBLE
(i.e., all ionic compounds that have NO 3- as the anion are soluble in water)
2. All ionic compounds having a Group I metal cation (e.g., Li+, Na+, K+, Cs+, etc.) OR
NH4+ as the cation are (water) SOLUBLE.
The above rules will not be given to you on an exam because I'll assume you've memorized
them. However, OTHER solubility rules (similar to the ones in the table in the text, and possibly
additional ones, depending on the question asked) will be provided to you, but you will be
responsible for knowing how to interpret and apply them (i.e., I will not explain their meaning to
you during an exam).