My book here says that ' Despite the shielding of the outer electrons from the nucleus by the inner shell electrons, the attractive force experienced by the outer shell electrons increases with increase of nuclear charge. In other words, the energy of interaction between the nucleus and electron (that is orbital energy) decreases (that is more negative) with the increase of atomic number. ' Now i understood the first part. But whatever is said after 'in other words', i am not able to understand what is said here. How is more attractive force synonymous to decrease of energy of interaction b/w nucleus and electron?