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How are electron shells arranged around an atomic nucleus?

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Bohr model diagram of a hydrogen atom showing a green nucleus labeled H, a single electron orbit, and text with atomic mass 1.008 and electron configuration 1. — atom The Bohr model describes the structure of atoms, in this case hydrogen.

In the shell atomic model, electrons are arranged in diffuse regions called electron shells that surround the atomic nucleus. These shells are organized by a principal quantum number n, where n = 1 represents the shell closest to the nucleus, and higher values extend outward.12

The shells are sometimes designated by capital letters, starting with K for the first shell, followed by L, M, N, and so on. Each shell has a specific capacity for electrons:23
* n = 1 (K): 2 electrons
* n = 2 (L): 8 electrons
* n = 3 (M): 18 electrons
* n = 4 (N): 32 electrons12

Within these shells, electrons occupy orbitals grouped into subshells, which are determined by their energy levels and quantum states. Electrons fill these orbitals according to the Aufbau principle, which dictates that lower-energy orbitals are filled before higher-energy ones. This arrangement, known as electronic configuration, is fundamental to understanding an atom's chemical properties and its position in the periodic table.1324