Question

What is the oxidation number of $S{O}_3$?

Answer 1

Hint: We need to know that the oxidation number and oxidation state are two different things. Oxidation state tells us how many numbers of electrons are being added to or removed from a species. Oxidation state can be positive or negative integers and also it can be zero.

Complete answer:
Now we will learn what oxidation number tells us? Oxidation number in simple terms can be termed as the number that is defined to elements in a chemical combination. The oxidation number is basically the count of electrons that atoms in a molecule can share, lose or gain while forming chemical bonds with other atoms of a different element.
In sulphur trioxide, there are three oxygen and one sulphur atom. The oxygen atom carries $-2$ charge. Since we have three oxygen atoms, thus the total charge on oxygen added up to $-6$. Now we know that sulphur trioxide is a neutral compound, it does not carry any charge so we add up the values of charges to zero in order to find the oxidation number for sulphur in sulphur trioxide.
Let oxidation number be x for sulphur, thus
$x+\left(-6\right)=0$
$x=6$
Thus the oxidation number of sulphur in sulphur trioxide is $6$ .

Note:
We need to remember that the free elements have an oxidation number zero for example: $Na$ , $Fe$. In an ion the oxidation number represents the charge on an ion and for neutral compounds all oxidation numbers are added to zero. Like for alkali metals oxidation number will be one since they belong to group one thus oxidation number will be $+1$ .