How do you calculate the formal charge on atoms of an ion?

Question

Caleb Adams · Accepted Answer

With care!

Most of the time it is fairly straightforward to assign the formal charge on molecule or a radical ion, by considering the formal charge of the individual atoms. In the given example, we considered the neutral ammonia molecule, versus the ammonium cation, #NH_4^+#. Here I will consider the oxygen molecules, #O_2# versus the ozone, #O_3#, molecule. Now both species are neutral gases, and our Lewis structures should reflect this, nevertheless, in the ozone molecule there is formal charge separation. For #O#, #Z=8#, there are #6# valence electrons; the other #2# electrons are inner core and do not participate in bonding. For the #O_2 " molecule"#, there are #12# valence electrons, i.e. #6# electron pairs to distribute over #2# #O# atoms, and a #O=O# molecule results. Why is each oxygen atom neutral here? Each oxygen atom has #2# lone pairs of electrons, and shares the electrons involved in the double bond. Thus each #O# atom claims #4# electrons from the lone pairs (lone pairs devolve to the parent atom) plus it is conceived to share #1/2# the electrons that form each covalent bond. Thus each oxygen atom is associated with #2+4+1+1# electrons. These #8# electrons electrostatically balance the #8# positive charges in the #O# nucleus, and so each #O# is depicted as neutral. Now contrast this with the neutral ozone molecule, #O_3#, with #18# valence electrons. The typical VESPER structure of ozone is as bent #O=O^(+)-O^-# molecule, with formal charge separation. Going from right to left in the #O=O^(+)-O^-# structure, there are #8#, #7#, and #9# electrons on each individual oxygen atom, and the resultant charges are #0,+1, -1# as required. Do you see the formalism? If you re happy with this, try formal charge assignment with the neutral sulfuric acid molecule. We can represent this as either #(HO-)_2S(=O)_2# or better, #(HO-)_2S^(2+)(-O^-)_2#. How?

Natalie Anton · Answer

undefined To calculate the formal charge on an atom in an ion, subtract the number of nonbonding electrons (lone pairs) and half the number of bonding electrons (shared in covalent bonds) from the atom's valence electrons. The formula is:

Formal Charge = Valence Electrons - (Nonbonding Electrons + 1/2 * Bonding Electrons)

Compare the result with the atom's usual valence electron count to determine the formal charge.