what is the lowest possible energy level that an electron can occupy?

The lowest possible energy level that an electron can occupy is called the ground state. In this state, the electron is in the lowest energy configuration available to it, which corresponds to the first energy level in an atom, often noted as n=1n=1n=1 in quantum mechanics and the Bohr model of the atom. This ground state is the most stable and lowest energy level for an electron, and electrons can only move to higher energy levels (excited states) if they gain energy. The energy of the ground state electron in a hydrogen atom, for example, is about -13.6 electron volts (eV).