There are two stable isotopes of bromine . their masses are 78.9183 amu and 80.9163 amu. if the average atomic mass of bromine is 79.90 amu, what is the natural abundance of the lighter isotope?

There is more of the lighter isotope, because the average is close to the lighter isotope. When doing the calculation for the average atomic mass, it considers the fraction of the isotopes available in a sample. Therefore samples of the element that have a values that do not deviate far from the sample are more abundant.

Exothermic,  physical change after it is already baked

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.