Cracking the Code of Polyatomic Ions: A Chemistry Student's Guide

[MichaelXY]

Homework Statement

I have discovered that in order to suceed in Chemistry, one must learn the polyatomic Ions. My instructor said the best way is to just memorize them, but here is the thing. Ok I can memorize that peroxide is O2, but memorizing the charges is kicking my behind. I was thinking that I should be able to figure it out using Lewis structure or something but that is not working. For example O2 means One oxygen with 6 electrons bonds with another oxygen with 6 electrons, so oxy1 gives up to electrons to fill oxy2 shell, so now oxy 1 is minus 2 electrons, so is that why O2 has a minus 2 charge? Doing a lewis structure for say Citrate loses me. Any suggestions?

Homework Equations

The Attempt at a Solution

 

[Kushal]

if you practice your chemistry you will remember the charge for most polyatomic ions, believe me... it will come automatically after a while... just read your chemistry often...

 

[Gannon]

I agree; over time you'll memorize them. If you really want a systematic approach, then you could draw the Lewis structures and assign each atom a formal charge (count lone electrons as one and bonds as one). If the number matches the valence number for the atom, it is neutral.

For example, NH3 is a netural molecule because N has two lone electons + 3 bonds = 5. The valence number for nitrogen is 5, so it is neutral.

NH4+ has a positive charge because it has 4 bonds and no lone electrons = 4.

This gets harder and takes more time for resonance structures like NO2 or SO3.