Good question, as this involves the interesting
properties of electrons, bonds and chemical
reactions and compounds.
SO4 is bonded covalently,
the electrons are shared between the Sulfur and
Oxygen atoms. This means that the Sulfur atom
does not lose or gain any electrons. Since a
normal Sulfur atom has six valence electrons, we
know by Octet’s Rule that the atom would
like two more electrons to fill its outer shell.
However, the atom will not gain two electrons, but
it will share two electrons with two
neighboring oxygen atoms. These oxygen atoms form
a covalent double bond with the sulfur atom.
In the formation of SO4, the
compound will gain two electrons from an outside
source. These electrons go to the remaining two
Oxygen atoms. These Oxygen atoms have a single
bond, meaning the atoms will have six valence
electrons and one shared valence electron with
Sulfur. These Oxygen atoms would like one more
electron to fill its outer most shell. An
electron will come in to the Oxygen atom and give
it a net -1 charge. This happens twice: one extra
electron for each remaining Oxygen atom.
In summary, Sulfur is neutral in
SO4, and has six valence electrons. The
sulfur atom shares four valence electrons with two
oxygen atoms as two covalent double bonds, and
shares its remaining two electrons with two oxygen
atoms, forming two covalent single bonds.
These two oxygen atoms each have seven valence
electrons between their atom and the sulfur atom,
so would like one more to fill its outer most
shell. These two oxygen atoms then get an extra
electron each from an outside source in a chemical
reaction. The 2 electrons gained by these Oxygen
atoms give SO4 a net -2 charge.
Hope this helps to show how bonds are formed in
this chemical compound!