Answer:
-) Lewis structure: See the figure 1
-) Oxidation number of As = +5
-) Oxidation number of O = -2
Explanation:
The first step is to calculate the total amount of electrons, for As we will have 5 electrons, for all the oxygens we will have 24 electrons (6 electrons for each atom) and finally we will have 3 more due to the charge (-3), so in total we will have: 5+24+3= 32 electrons.
In the lewis structure, we will have as central atom "As" then we have to put the oxygens around the central atom. In this case, As is an exception of the octet rule, in this structure As would have 10 electrons and 8 electrons for each oxygen. So, in the total distribution of the electrons in the structure, we will have the 32 electrons that we calculate in the first step (figure 1).
Finally, in the oxidation number calculation, we have to start with the oxidation number of oxygen. The oxidation number of O is -2 then when we multiply by the number of atoms we will get -8, so the As must have an oxidation number of +5 in order to obtain a total charge of -3 (figure 2).
