The answer is (3) Cu2O. Copper (I) has an oxidation state of +1 (that's what the "I" indicates). You can also think of this as copper (I) having a charge of +1. Oxygen has an oxidation state of -2 (that's just a rule you have to know), and you can think of it as oxygen having a charge of -2. You need oxidation numbers in a neutral compound to add up to 0 (or charges in a neutral compond to add up to 0), so you need two Cu to balance the O, which is Cu2O.
In gamma decay, no change in proton number occurs, so the atom does not become a different element
4.0
i think it has something to do with molar ratios and finding the limiting reactant
4.0 mol NO * 2 mol NO2/2 mol NO = 4.0 moles of NO2
4.0 mol O2 * 2 mol NO2/1 mol O2 = 8.0 moles of NO2
so the limiting reactant (the reactant that runs out the quickest leaving an excess) is NO
once the limiting reactant is found, we can use that data for that substance to calculate the amount of product
4.0 mol NO * 2 mol NO2/2 mole NO = 4.0 moles of NO2
