Question

Why shell near nucleus have minimum energy whereas farther away shell has maximum energy?

Answer 1

Basically, you can think of the nucleus as sitting at the bottom of a potential energy well. Because the electron and nucleus have opposite charge, the force is attractive, meaning that for an electron to exit away from the nucleus it needs to "climb up" the potential energy well. The further away from the nucleus, the more energy it must have. If an electron gains energy (say by absorbing a photon) it can climb up further (ie more distance away). Conversely, if it drops back down the energy well (ie moves closer to the nucleus) it gives out energy (as a photon).However, the energy well is actually quantized, not continuous. The electron can only take certain values of energy. Those values are determined by solutions of the Schrodinger equation and are defined by the quantum numbers of that electron (n, m, l and s). Each combination of quantum numbers give rise to a particular "orbital" (e.g. the 5d orbital has $n=5,m=2,l=\frac{1}{2}and-\frac{1}{2}$).