Q1.
Why are Bohr’s shells called K, L, M, N... and not, A, B, C, D?
Answer
Because the letters were borrowed from X-ray spectroscopy, where the naming had already started at K.
- The physicist Charles Barkla, studying X-rays given out by elements, found a series of X-ray lines and called the first one he identified the K series.
- He deliberately did not begin at A. He left room in case a series with even higher energy — earlier in the alphabet — was discovered later.
- No such series was ever found, so the letters below K were never used.
- Bohr simply adopted Barkla's existing notation for the atomic shells: K, L, M, N, …
Why it happens: the two namings match physically, not just by accident. The K X-ray line is produced when an electron drops into the innermost shell, so “K” already meant “innermost level” before Bohr used it. Keeping the letter avoided two competing names for the same thing.
Did you know? The shells also carry the number n = 1, 2, 3, 4 …, and it is n that goes into the formula for the maximum number of electrons a shell can hold, 2n². The letters are the older name; the numbers are the useful one.