I am thinking the answer is B. I think this because Bohr had a model similar, but his was lacking the cloud of electrons. His just had electrons orbiting the nucleus.
I hope this is right!<span />
In order to determine if a reactant is in excess or limiting, we must first know the required amount of reactants. This is referred to as the stoichometric amount of reactant, and it is obtained from the chemical equation.
From the equation, we form a ratio of the reactants. The reactant supplied in excess of that ratio is the excess reactant, while the other is the limiting reactant.
For 60.0 g sample, if it remains 60-52.5=7.5 g, it will go through 60->30->15->7.5, 3 half-life. So the time being taken is 269*3=807 years.
