In a certain chemical reaction Compound combines with Compound to produce Compound (and no other products). Measurements were taken of the amounts of and present before and after a reaction that produced some :

2.1 g

Explanation:

The question is incomplete and the complete question is shown in the image attached to this answer.

Now look at that image closely; you will notice that:

i) Reactant A is the limiting reactant because it was used up in the reaction.

ii) Reactant B is the reactant in excess because some mass of it was left behind after the reaction.

The mass of the product C formed is the same as the mass of the limiting reactant used up. Hence the theoretical yield of C is 2.5 g

% yield = actual yield/theoretical yield * 100

since = 83%

Actual yield = %yield * theoretical yield/ 100

Actual yield =  83 * 2.5/100

Actual yield = 2.075 g

the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons