balancing charges

[Olivia Suhy 2K]

hi! could someone explain a little bit more about balancing charges? for example, if you have like H with a -2 charge and Pb with a -3 charge, why would the combination of these two things be H₃Pb₂? like i know we are supposed to do this but i don't really understand why? and also are there exceptions?

[Ingrid Wu 2H]

Hi,

In order to have a neutrally charged ionic compound, the sum of the charges of each ion must be zero, so you need to have balanced charges for each ion. (Covalent molecules don't have full charges on their atoms so we won't have to go over them here.) When you have atoms with the same magnitude of charge, like Mg (+2) and O (-2), the charges (+2 and -2) sum to 0 and cancel each other out. Therefore, balancing charges is easy because you just need one of each ion, and your compound is MgO.

When you have ions with magnitudes of charge that are different, like Al (+3) and O (-2), you can't just have one of each ion because the resulting compound, AlO, would have a charge of +1 because the charges (+3 and -2) would sum to +1 and not 0. Therefore, in order to have balanced charges in the compound, you need to find the greatest common multiple of both charges. In this case, the greatest common multiple of 3 and 2 is 6. Therefore, you need 2 Al ions (total charge is +3 * 2= +6) and 3 oxygen ions (total charge is -2 * 3 = -6). Now, the charges +6 and -6 sum to 0 and the charge on the compound is balanced, and you know that the compound is Al2O3. Hope this helps