Draw a lewis structure for the resonance form of aso4^-3, with the lowest possible formal charges. include any nonzero formal charges and lone pair electrons in the structure.
-what is the oxidation number of as
-what is the oxidation number of o

-) 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).

explanation:

metals have excess of electrons whereas non-metals have deficiency of electrons. thus, in order to gain stability metals easily donate their electrons to the non-metals.

for example, electronic configuration of sodium is and electronic configuration of phosphorous is .

when a phosphorous atom and sodium atom combine together then sodium will donate is extra one electron to the phosphorous atom. as a result there will be formation of ionic bond between sodium and phosphorous.

the reaction will be as follows.

there, are three sodium sodium atoms and each of them will donate its one extra electron to phosphorous atom. thus, the sub-shell of phosphorous gets completely filled.  

thus, we can conclude that electrons are transferred from atoms of sodium to atoms of phosphorous. this transfer makes the sodium atoms donate one electron and the phosphorous atoms to gain one electron. as a result, the sodium and phosphorus atoms strongly attract each other.