Oxidation Numbers and other animals

    Every element has a number (sometimes more than one) assigned to it that represents the number of electrons it has to share, give away, or can take when that element combines with another.  The ideal number of electrons for an element to have around it in the outermost shell or orbital is 8, and atoms work very hard to achieve this magical number of atom satisfaction.  They will beg, borrow, steal from, or donate electrons to any other atom in the general area that they can catch.  The number of electrons it gives away or takes on is called its oxidation number.  [You can find this number on your periodic table] The well behaved elements follow these general rules:

Elements in Group 1 (H down) have one electron in their outer shell and are quite happy to give it away, resulting in an oxidation number of +1.
Group 2 (Be down) have two electrons in their outer shell and are anxious to give both of them away, resulting in an oxidation number of +2.
Group 3(B down) have three electrons in their outer shell and ditch all three at the slightest provocation.  This makes them +3.
Group 4 (C down) have 4 electrons so they can go either way-they can take or give 4 electrons.  This makes their oxidation number either +4 or -4.
Group 5 (N down) have 5 electrons, so they only need 3 to total 8.  This gives them an oxidation number of -3.
Group 6 (O down) have 6, so they only need 2 to achieve atomic karmic bliss.  This makes their number -2.
Group 7 (F down) have 7, so they are only 1 away from perfection.  This makes their number -1.
Group 8 (He down) are the snobs of the atomic world and already have 8 electrons in their outer orbital, so they do not need any electrons to make them complete.  Their oxidation number is 0.

The transition metals (Groups 3 through 13) are completely unpredictable.  These crazy atoms follow no rules of any kind.  So for this wild bunch, you must MEMORIZE.  Fortunately for you,  they usually pick one oxidation state and stick with it.
Some of them are total rule-breakers however.  Some transition metals not only have an unpredictable oxidation number, but they also change that number on a whim.  Some of these outlaws include:

Fe [ +2, +3] ; Pb [+2, +4] ; Cu [+1, +2] ;  Sn [+2, +4] ; Hg [+1, +2]  plus many more-consult your local periodic table for listings.  These rebels even have special names.

Now That You Know What an Oxidation Number Is...

    What the heck do you do with it?  When two atoms meet across a crowded erlenmeyer, what do they do?  They tell each other their number, and the rest is chemistry.  But seriously, when you have to combine two elements, you need to know their oxidation numbers.  You can find this out by digging in your own vast repertoire of knowledge or, failing that, by looking on your periodic table.  In order for two elements  to come together, opposites must attract.  Two positives will not normally bond, nor two negatives. But again, there are exceptions to every rule.
Example:
Write the formula for the compound created when Mg and Cl combine.
First, look at their oxidation number:  Mg  +2     Cl -1
Okay, their signs are opposite.  They can bond.  But Mg has two electrons to give, while Cl can only accept one.  Their union seemed to be doomed until...
Take the oxidation number of Mg and the oxidation number of Cl, and  "cross-multiply"- to get the subscripts.
i.e. Mg +2 Cl-1 becomes Mg1Cl2 .  Now just like in Math, ditch the "1" next to the Mg-it's understood.  Your final answer should read  MgCl2 .
Example:
Write the formula for the compound that comes from Ca and O.
Oxidation numbers:  Ca +2   O -2  [Their signs are opposite, so they can bond]
Ca +2 O-2  gives you Ca2O2.  And just like in Math again, if you can divide both subscripts by a common denominator, do so.  The final answer should be  CaO [after dividing by 2, both subscripts are 1, so you can leave them both out]

Polyatomic Ions
See, I told you some of this stuff sounded like early Swahili.  This term just means compounds that act like single atoms - they bond like single atoms, they stay together and have an oxidation number.  Kind of like jocks-several people hanging out together, but sharing one brain cell. These you will have to memorize.  Sorry!  Ya win some, ya lose some..

Families of Polyatomic Ions
Some polyatomic ions are grouped into families with specific rules of naming. These rules of naming are not based on logic or any other predictable reason.  Rather, they are based on the most common member of the family. For example,  NO3 is more common than NO2, so it gets the name of nitrate.  Similarly, the most common member of the phosphorus family is PO4, so it gets the -ate ending.  The most common member of each family is listed below in bold.  Changing the number of oxygen atoms does not change the oxidation number. The names follow this rule:  The most common member of the family gets the ending -ate.  Adding 1 oxygen changes the name to per ______-ate.  One less oxygen gets -ite, and (if it exists), two less oxygen get the name hypo_____ite.  The chlor- family where ClO3 is the most commonly found member has the structure Perchlorate ClO4, Chlorate ClO3, Chlorite ClO2, and Hypochlorite ClO. All of them have an oxidation number of -1.
 

There are more polyatomic ions, but I am sure you have a table somewhere in the back of that text, so crack it open!

Example:
Write the formula for the compound that results when sodium combines with acetate.

Sodium      Na    +1
Acetate      CH3COO  -1
The acetate is treated as if it were a single atom.
The formula is Na 1 (CH3COO)1.  Given that you can drop the "1" as a subscript, your answer should read NaCH3COO

Go to examples of this type of question

Exceptions to the positive/negative bonding rule:
CO2, CO, NO2, NO3, SiO2, NaH, SO2, SO3, SO4 and a few others come to mind.