Question

Where and Why the electron's energy is regarded as zero?

Answer 1

The energy never becomes 0 not even at absolute zero as electron has both wave and particle like nature it always has that jiggle in it cause if the kinetic energy becomes zero this means it is not moving and that is not possible cause it is like a wave. But the kinetic energy can be made approximately 0 at or near absolute zero ie - 273 k temperature . A temp below that is perhaps not even possible to create.