Electron Shell
Definition
A group of electrons at roughly the same distance from the nucleus, like floors of a building where electrons live.
Technical definition
An electron shell, also called a principal energy level (n = 1, 2, 3, ...), is a set of orbitals sharing the same principal quantum number, with a maximum capacity of 2n^2 electrons. The outermost shell's electrons, called valence electrons, govern chemical behavior.
Example
Sodium's 11 electrons fill shell 1 with 2 electrons, shell 2 with 8 electrons, and leave 1 electron in shell 3. That lone outer electron is why sodium is so reactive.
Common confusion
Shells are a simplified model. Real electrons occupy orbitals with specific shapes (s, p, d, f), and higher shells can start filling before lower ones finish: the 4s orbital fills before 3d.