Charge-Balancing Ionic Compounds
Once an atom "ionizes" (gives up or captures valence electrons) the ion has a (+) or (-) charge. When two oppositely charged ions come together they bond "electrovalently," meaning they are electrostatically attracted to each other (remember - magnets?). In order to write the correct chemical formula we need to make sure all the (+) and (-) charges cancel each other out.
The math here is very straightforward - every negative charge has to be balanced out by a positive charge so the final compound (write this down) has a net charge of zero. This is what makes compounds charge neutral!
The easiest compounds to charge are the ones with a 1:1 charge ratio.
- Lithium's ion Li1+ bonds with chlorine's ion Cl1- to form Lithium Chloride, LiCl.
- Magnesium's ion Mg2+ bonds with oxygen's ion O2- to form Magnesium oxide, MgO.
- Aluminum's ion Al3+ bonds with nitrogen's ion N3- to form Aluminum nitride, AlN
Notice that you don't see any subscripts in the chemical formulas above because we don't write subscripts for 1s.
When you don't have a 1:1 ratio you have to find the combination that balances all of the charges
- Lithium's ion Li1+bonds with oxygen's ion O2- to form Lithium oxide, Li2O
- Magnesium's ion Mg2+ bonds with nitrogen's ion N 3- to form Magnesium nitride, Mg3N2.
- Aluminum's ion Al3+ bonds with chlorine's ion Cl1-to form Aluminum chloride, AlCl3
How did I determine these subscripts? Easy! It's just like finding the lowest common denominator in fractions! Look for a common number both charges will divide evenly into. When one ion has a charge of 2 and the other has a charge of 3, for example, a good "target" number they will both divide into is 6.
- 2 x (1+) = 2 and 1 x (2-) = -2 so Li2O has a net charge of 0
- 3 x (2+) = 6 and 2 x (3-) = -6 so Mg3N2 has a net charge of 0
- 1 x (3+) = 3 and 3 x (1-) = -3 so AlCl3has a net charge of 0
Let's walk through one visually
Ask yourself (self?) what
number will both charges
divide evenly into?
Here 2+ and 3- will both divide evenly into 6 so your target numbers are 6 and -6.
Since 6 and -6 will cancel to 0 divide each target by the charge to get each subscript.
Tada! The final formula. Wasn't that easy?
Monoatomic vs. polyatomic ions and the compounds they form
All of the examples above contained monoatomic ions (ions with ONE element). But nature is full of polyatomic ions, too. Polyatomic ions are made up of more than one element. Polyatomic ions include ions like NO31- (the nitrate ion) and CO32- (the carbonate ion). Here is a list of common polyatomic ions and their charges.
The idea of charge balancing compounds with polyatomic ions is the same. You just need to be careful about using parentheses as needed. The general rule is: If you only need 1 of the polyatomic ions then you don't need to surround them with a parentheses. If you need 2 or more of a polyatomic ion then you do need to put them in parentheses so it's clear which subscript belongs to the polyatomic ion and which subscript is there as the charge balancing "inventory" number. Here are some example:
- Lithium's ion Li1+bonds with the nitrate ion NO31- to form Lithium nitrate LiNO3
- Magnesium's ion Mg2+ bonds with the nitrate ion NO31- to form Magnesium nitrate Mg(NO3)2
- Aluminum's ion Al3+ bonds with the carbonate ion CO32- to form Aluminum carbonate Al2(CO3)3
Notice you still start by looking for that lowest common multiple to find your target number. Once you have your targets, divide each target by the charge on the associated ion to find the subscript (sounds more complicated than it is).
This video is a little lengthy but he does a nice job showing how to charge balance compounds containing polyatomic ions.
Need more help?
These websites/videos will give you more information (and lots of examples!) on how to charge balance and write chemical formulas. While we haven't looked at naming compounds just yet, they are still good practice resources:
A video tutorial from Mr. Kent's Chemistry class
A video tutorial from Papapodcast
How much do you need to know about charge-balancing?
- Be able to charge balance simple compounds containing monoatomic ions with 1:1 charge ratios.
- Be able to charge balance simple compounds containing monoatomic ions with uneven charge ratios.
- Be able to charge balance compounds containing polyatomic ions.