The electron shells are named KLMN rather than ABCD primarily due to historical reasons related to the investigation of X-ray spectra.
Here's a breakdown:
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Charles Barkla's X-Ray Research: The naming convention originates with Charles G. Barkla, a spectroscopist who was researching X-ray scattering and absorption. He observed that X-rays emitted from elements came in two distinct types, which he initially labeled "A" and "B."
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Higher and Lower Energy X-rays: These "A" and "B" types represented X-rays with different energies. "A" represented the higher energy X-rays and "B" represented lower energy X-rays.
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Changing the Nomenclature: Barkla eventually decided to use the letters K and L instead of A and B. It is widely believed that he chose K because it was the next available letter proceeding alphabetically from what he had used before. The exact reason for this change isn't entirely documented, but the K and L designations stuck.
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Continuing the Sequence: As more electron shells were discovered further from the nucleus, the alphabetical sequence continued from K and L to M, N, and so on.
In essence, the naming convention is not based on any particular property of the shells themselves (such as their energy level or radius), but rather on a historical development in the study of X-ray spectra by Charles Barkla. He just happened to start with K. The naming also left space to name the shells closer to the nucleus to the K-shell with letters such as J, I, H and so on if these are discovered.
