Question

Why the energy of electron inside the atom is -ve while at infinity is +ve or 0

Answer 1

Energy values are always given relative to some reference. In the case of atoms, the typical reference (set to zero) is the state where one electron is removed from the atom and taken infinitely far away. The atom with its electron is more stable than this ionized reference state, hence its energy is lower than the reference state energy (taken as zero) hence its energy is negative. Note that the kinetic part of the energy can never be negative, it can only go as low as zero if the electron stops moving. However, the electron never stops because of the uncertainty principle, thus the kinetic energy is always positive. The potential energy (the binding energy) is even more negative, so that the total is still negative and the electron is stably bound and the entire atom is stable relative to the reference state.

Answer 2

Because inside the Atom two charges, one charge is electrone and second is nucleus, electrone charge is negative otherwise nucleus charge is positive, with this charge electrones are attached with nucleus due to attraction.