Rydberg atoms have one or more electrons in states of large principal quantum number n, making them extremely large, with loosely bound valence electrons easily perturbed or ionized.
The orbital radius scales as n² — the n = 137 state of hydrogen has an atomic radius of about 1 µm — and the geometric cross-section as n⁴.
The only truly stable state of a hydrogen-like atom is the ground state with n = 1.