Video Transcript
What is the oxidation number of
sulfur in SO4 2−?
In this question, we need to
determine the oxidation number of the element sulfur in the sulfate ion.
Oxidation number is also referred
to as the oxidation state. The oxidation number or oxidation
state indicates the number of electrons lost or gained by an atom. In other words, this value is a
measure of how oxidized or reduced an atom in a chemical species is. We must note that the sulfate anion
is a polyatomic ion since it has a net charge. And it contains multiple covalent
bonds. This is a two-element system. It contains the element sulfur and
the element oxygen.
We can work out the oxidation
number of the elements by first looking at their electronegativity values. Electronegativity is the ability of
an atom in a bond to attract or pull the bonding electrons towards itself. Sulfur has an accepted
electronegativity value of 2.58, and oxygen has a value of 3.44. Oxygen is highly
electron-withdrawing and has one of the highest electronegativity numbers of all
chemical elements, and certainly higher than sulfur’s value. Sulfur has a much lower
electronegativity number. Now we can use the
electronegativity values to start assigning oxidation numbers.
Step one, in a two-element system
such as this, we assign a negative oxidation number to the element with the higher
electronegativity and a positive one to the element of lower electronegativity. So we can write a negative sign
above the oxygen atom and a positive sign above the sulfur atom.
Step two, to the more
electronegative element, assign its usual change value that it typically forms as an
ion. When oxygen is charged, it
typically gains two electrons, and so we write two next to the negative sign. Now we need to scale with the
subscript, which is four. This gives us a total of minus
eight for the four oxygen atoms.
Step three, use the overall charge
to determine the oxidation number of the less electronegative element, which in this
case is sulfur. The overall charge of this
polyatomic ion is two minus. We can make sulfur’s oxidation
number 𝑥 and solve for 𝑥 by summing the oxidation number of sulfur and the four
oxygen atoms to get a total charge of negative two. Solving for 𝑥, we see that sulfur
must have an oxidation number of plus six.
Finally, what is the oxidation
number of sulfur in SO4 2−? The answer is plus six.
