Distinguish between the ground state and the excited state of electron in an atom what has to happen for an electron to move from ground state to an excited state.
Well, if you forget just for a moment that we are talking about electrons, which are super-tiny subatomic particles, to get anything to shift from what is considered a "relaxed" or ground state to a more excited state, it has to absorb energy of some kind. In the electrons case, that would be a photon, which are tiny energy packets on the subatomic level. If an electron absorbs a photon or two, it basically absorbs more energy and operates in a higher energy plane. This same excited proton can also radiate, or give off, photons of energy, and return to its more relaxed, ground state. Electrons follow both the wave theory of light and the particle theory of light, so, the absorption or radiation of photons of energy makes perfect sense when you think about it.