Why is the first shell called K shell?

The first shell in an atom, known as the K shell, is named after the German word "Kugel" (meaning "sphere" or "ball"), which was used by the Danish physicist Niels Bohr to describe the first energy level or electron shell in an atom. Niels Bohr developed the Bohr model of the atom in the early 20th century, which was one of the earliest successful attempts to describe the structure of atoms based on quantum principles.

In the Bohr model, electrons are arranged in discrete energy levels or shells, with the K shell being the innermost shell closest to the nucleus. The subsequent energy levels are named L, M, N, and so on. Each shell can hold a specific number of electrons, with the K shell holding the fewest, typically a maximum of 2 electrons.

The naming convention using letters (K, L, M, etc.) was adopted to denote these electron shells and is still widely used in atomic physics and chemistry to describe the arrangement of electrons in atoms. The reason for starting with "K" is historical and likely a choice made by Bohr to label the first energy level in his atomic model.