Formal Charge Sums with Polyatomic Ions

[SarahOMalley1D]

Hi everyone! For polyatomic ions, will the formal charge of the most stable Lewis structure always add up to the charge of that polyatomic ion? For example, in class, we saw that the overall formal charge of sulfate is -2, and sulfate is SO4 2-. I'm not sure how important this question will be in class, but I'm curious because it might help me figure out if my polyatomic ion Lewis structures are correct.

[KyleNagasawaDisc3C_Chem 14B2022W_]

Hi Sarah, you are correct.

The sum of formal charges for Lewis Structures ALWAYS matches up to the net charge of a compound. A common example of this phenomenon is seen with zwitterionic species, such as ylides. Even though there is a dipolar phenomenon, there is no net charge, so we treat ylides as electrically neutral. This general notion will also help you to draw polyatomic species. If you know the charge of the polyatomic ion, you can use this reasoning to determine the structure based on the sum of formal charges on the species summing to the species' net charge.

[Ainsley McCabe 2D]

The sum of the formal charges of atoms in a molecule should always add up to the overall charge of the ion. These formal charges also give insights into which lewis structures are more stable than others.

[Jericho Ansay 1C]

Yes, formal charges of Lewis structures should add up to the overall charge of the ion. If the formal charge does not add up to the overall charge, then you would need to modify your structure to make sure that you have the correct bonds and lone pairs on each element.

[005683006]

Find the formal charge of each atom within the polyatomic ion. The closer the value is to 0, the more stable the compound is, and hence the most desirable.

[Jiayin Yola Yan 1G]

The most stable (aka usually the correct Lewis structure) will have FC of elements equals or close to 0. The sum of the FC of elements will also equal to the charge of the compound itself. It is very useful when you determine which is the most accurate Lewis Structure of the compound.