# Why is the mass defect of nuclei used while calculating the $Q$ value of decay reactions?

i just came across two questions today

the first one goes like:

Here the answer is calculated with (mass defect of nuclei)*C^2,why do we use the mass of nuclei to calculate Q value in this case

but for the questions like

we use the mass defect of atomic masses why is this happening?

• Because the 'mass defect' is the binding energy of all the protons and neutrons coming together from isolation (fully separated) to form a particular nuclei. So, to figure out the energy difference of one configuration of protons and neutrons with another, you conceptually separate them all apart to infinity, then put the new configuration together and see how much energy is left over. – Jon Custer Jan 26 at 14:56