Question

What is the difference in structure of $ S{{O}_{3}} $ and $ SO_{3}^{2-} $ ?

Answer 1

Hint :We have to know that the sulphur is an element which lies in period three of the modern periodic table. The significance of sulphur is that it can have an extended valency. It’s valency ranges from $ -1 $ to $ +6. $ Hence, the maximum valency of sulphur is $ 6. $ Thus, this extended valency enables it to bond with different atoms ions easily.

Complete Step By Step Answer:
We have to remember that the sulphur has 6 electrons in the outermost shell. But it can have a valency of six as well and that of two as well. Let’s look at molecules. Here, one atom of sulphur is bonded with two atoms of oxygen.
We know that oxygen atoms have a valency of two. It has six atoms in the outermost shell and it needs only two electrons from another atom in bonding to complete its octet. For two atoms of oxygen we need in total four electrons. Therefore, in this case in molecule the valency of sulphur is $ +4. $
 $ S{{O}_{3}} $ does not have the extra two electrons $ SO_{3}^{2-} $ has two electrons extra. Therefore, the Lewis dot structures will actually be very different. In $ S{{O}_{3}} $ there are three double bonds, and no lone pairs at sulfur. Adding two extra electrons changes that by giving two more electrons to sulfur. $ S{{O}_{3}} $ is sulfate trioxide, while $ SO_{3}^{2-} $ is called the sulfite ion.

Additional Information:
Gaseous sulfur trioxide is a trigonal planar molecule. Also it is an anhydride of sulfuric acid. It is a highly reactive substance and a strong oxidizing agent and also acts as a fire hazard. Sulfur trioxide behaves as the primary substance in an acid rain or we can say it is a pollutant in gaseous form.

Note :
We have to remember that elements such as sulphur and phosphorus have extended valances. This is because their valence shell has enough orbitals to accommodate more electrons. If we talk about sulphur, it has vacant d-orbitals to accommodate electrons. That is the reason why in sulphur dioxide it shows $ +4 $ valency and in sulphur trioxide it shows $ +6 $ valency.