Major electronic transitions in atoms are determined by the selection rules. If the atom is in an excited state and there are no lower energy empty states to which it can transition and satisfy the selection rules, then it is stuck in the excited state. Such states are called metastable states. One such example is the 2s state in Hydrogen atom.
Then why does such an excited state eventually decay? Because there are other processes (apart from those caused by the dipole moment operator) which can cause electronic transitions, such as collisions.
Major electronic transitions in atoms are determined by the selection rules. If the atom is in an excited state and there are no lower energy empty states to which it can transition and satisfy the selection rules, then it is stuck in the excited state. Such states are called metastable states. One such example is the 2s state in Hydrogen atom.
Then why does such an excited state eventually decay? Because there are other processes (apart from those caused by the dipole moment operator) which can cause electronic transitions, such as collisions.