Yes, the ground state of an atom can be different from the lowest energy level of its electrons. This occurs in cases where there are additional factors or interactions that affect the overall energy of the atom beyond the individual energy levels of the electrons.
One example of this is the phenomenon of electron-electron interactions in multi-electron atoms. In atoms with more than one electron, the presence of multiple negatively charged electrons introduces electron-electron repulsion. This repulsion affects the energy levels of the electrons and leads to deviations from the simple energy level structure of a single-electron atom.
The electron-electron repulsion can cause the energy levels of the electrons to shift, and as a result, the lowest energy level of the electrons may not be the ground state of the atom. The ground state represents the overall lowest energy state of the entire atom, which includes the combined energy of all the electrons and the nucleus.
For example, in helium, which has two electrons, the lowest energy level of the electrons would be the 1s orbital, but the ground state of the helium atom is actually the singlet state, where both electrons occupy separate 1s orbitals. This configuration results in a lower overall energy for the atom due to the electron-electron repulsion being minimized.
Similarly, in more complex atoms, electron-electron interactions and other factors can lead to variations between the lowest electron energy level and the ground state. These effects are described by the field of quantum mechanics and are typically analyzed using methods such as the Hartree-Fock approximation or more advanced computational techniques.
In summary, deviations between the lowest energy level of the electrons and the ground state of an atom arise due to electron-electron interactions and other factors that influence the overall energy of the atom beyond the individual electron energy levels.