How does #SO_4# have a charge of 2-?

1 Answer
Jul 3, 2016

Answer:

We are agreed that sulfate anion derives from #H_2SO_4#, sulfuric acid, or from sodium sulfate, #Na_2SO_4#, which are both manifestly neutral entities.

Explanation:

A typical (if outdated) Lewis structure of sulfuric acid is:

#(HO-)_2S(=O)_2#

This Lewis structure is equivalent to:

#(HO-)_2S^(2+)(-O^-)_2#

For a neutral chalcogen atom (#"chalcogen = S or O"#), there must be 6 valence electrons. In the representation #(HO-)_2S(=O)_2# there are certainly 6 electrons associated with each sulfur or oxygen. Lone pairs are owned by the atom, and thus on neutral oxygen there are 2 electrons from the double bond, and 4 electrons in the lone pairs).

Now of course both #H_2SO_4# and #HSO_4^-# are strong acids, and undergoes almost complete ionization in water:

#H_2SO_4(aq) +2H_2O(l)rarr SO_4^(2-) + 2H_3O^+#

Conservation of charge demands that the sulfate ion has 2 formal negative charges.

Nitric acid has an even more problematic representation: #(O=)N^(+)(-O^(-))(-OH)#, where there is formal charge separation in even the neutral acid (6 electrons around nitrogen rather than 7; 9 electrons around oxygen rather than 8 ).

See here for another example that assigns formal charge.