By posing that initially in the K shell there is a single vacancy (and, hence, a single electron is already there), as well as that the L shell is not entirely empty in the final state of the transition, this definition limits the minimal number of electrons in the atom to three, i.e., to lithium (or a lithium-like ion). In the case of two- or one-electron atoms, one talks instead about He-alpha and Lyman-alpha, respectively. In a more formal definition, the L shell is initially fully occupied. In this case, the lighter species with K-alpha is neon.NIST X-Ray Transition Energies Database This choice also places K-alpha firmly in the X-ray energy range.
