The naming of electron shells with letters, starting from the letter "K," is based on the historical development of atomic theory and the understanding of electron configurations.
The practice of naming electron shells using letters originates from the Bohr model of the atom, proposed by Danish physicist Niels Bohr in 1913. According to Bohr's model, electrons occupy specific energy levels or shells around the atomic nucleus. These energy levels were labeled using letters of the alphabet.
The choice of starting with the letter "K" was arbitrary, and it was simply the first letter available after the letter "J" (which was already used to represent other quantum numbers). The letters "K," "L," "M," and so on were chosen as a convenient and systematic way to label the different energy levels.
In modern atomic theory, the electron shells are labeled using the principal quantum number "n." The shells are numbered starting from n = 1, which corresponds to the innermost or closest energy level to the nucleus. Each subsequent shell has a higher principal quantum number (n = 2, 3, 4, and so on) and represents a higher energy level farther from the nucleus.
While the original letter-based labeling system is still used in certain contexts, such as X-ray spectroscopy and electron configurations, the principal quantum number (n) is the more widely used and accepted method for identifying electron shells in modern atomic theory.