For an atom that becomes an ion, does it have a formal charge, or net charge? What is the difference between formal charge, oxidation state, and net charge?

1 Answer
Jul 21, 2016

For an atom that just became an ion, like "O"^(2-), that is not its formal charge, but it is its net charge.


Formal charge is the hypothetical charge for an atom when assuming that electrons are perfectly evenly shared in a chemical bond.

It's not the same as an oxidation state; it's pretty much the complete opposite.

  • Strictly speaking, "O"^(2-) doesn't have a formal charge because it's not within the context of a molecule.
  • In "CO"_2, we have :stackrel(..)"O"="C"=stackrel(..)"O":. The formal charge on "C" is 0, and the formal charge on each "O" is 0. We can say that because the atoms are in the context of a molecule.

Oxidation state is the hypothetical charge for an atom when assuming that electrons would be completely transferred from one atom to another in a chemical bond.

  • If you look at a lone atom, its charge is its oxidation state. The oxidation state of the "O"^(2-) anion is color(blue)(-2), the same as its charge, 2-.
  • The oxidation state of "O" in "CO"_2 would be -2, so the oxidation state of "C" in "CO"_2 would be +4. We write that as stackrel(+4)("C")stackrel(-2)("O"_2). It does not mean that carbon has given away all of its n = 2 valence electrons.

Net charge is simply the total charge on an atom or molecule or compound.

  • For "O"^(2-), it's only one atom, so its net charge is its actual charge, color(blue)(2-).
  • For :stackrel(delta^(-))("C")-=stackrel(delta^(+))("O"):, the net charge is 0, because the formal charge on "C" is -1, and the formal charge on "O" is +1, so -1+1 = 0.