Question

why we use K L M for shell?
can we use A B C for shell?
if can't then why

Answer 1

K L M is a conventional method

The names of the electron shells come from a fellow named Charles G. Barkla, a spectroscopist who studied the X-rays that are emitted by atoms when they are hit with high energy electrons. ... This innermost shell is now called the K-shell, after the label used for the X-ray.