i'm not exactly sure how to phrase my question but here goes.

 

how do you identify where the charge on a molecule comes from? example OH-. if its covalently bonded, where does the negative charge comes from. is the negative charge due to O having one more electron than the protons? and what about NO2+?

 

Thanks!

Try drawing the lewis structures and see if that helps. That will let you see the formal charges on each atom.

read up on formal charges. There are a few examples where the charge doesnt exactly lie on any particular atom. That's called delocalisation or resonance, so make sure you include those. If you have a textbook, it will be sure to have a section on these.

look at redox reactions

 

in OH-

 

generally...

hyrogen always has a plus 1 charge

oxygen has a -2 charge (unless it is in a peroxide in which case it is a minus 1 charge)

 

there are just some rules that you must memorize, but, again, the lewis structures will explain it better

Lewis structures will partially explain the bonding. It is extremely useful, but does not work all the time. This does not mean that it should be cast aside though. It is adequate for most molecules.

 

Fortunately for ions, or an ionic compound, the oxidation number is just the actual ionic charge (for that element - remember a compound has an oxidation stae of zero unless staed).

Oxxidation numbers can also be used for covalently bonded molecules - group 1 elements will always have an oxidation state of 1, and flourine will always have an oidation number of -1, etc, just a few exceptions.

 

A knowledge of molecular orbital theory accounts for bonding in more detail.

However, hybridisation crushes the problem with a lewis structure for certain molecules. For example, a lewis structure of the cyanate ion CNO- is shown as a stucture with either a negaive nitrogen, or a negative oxygen - this NEVER OCCURRS. The molecule will be more simply a mixture of the two, but never one or the other. This is why hybridisation is extremely useful, as it shows how electrons are distributed between the bonding orbitals orbitals of the cyanate ion, with no distinct "contributor" structure.