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#.