subshell

The electronic configuration describes how electrons are arranged in orbitals around an atom's nucleus. Electrons occupy specific regions called electron shells, each assigned a quantum number n starting from 1 for the shell closest to the nucleus. Within each shell are subshells, which vary based on electron rotation, orbital orientation, and spin.12

Electrons fill these orbitals starting with the lowest energy levels, following the Aufbau principle. For example, the first shell (n=1) has an s subshell and can hold a maximum of two electrons, as seen in helium (1s²). The second shell (n=2) includes s and p subshells and can hold up to eight electrons. This sequential filling of shells and subshells explains the structure of the periodic table of the elements.12345