Scientists identify electron shells using the principal quantum number. This foundational number is always represented by the lowercase letter n. It takes on positive integer values starting from one.
Older scientific models identify these shells using the letters K, L, M, and N. Shells located further away from the nucleus possess higher baseline energy levels. These distant shells can also theoretically hold a larger total number of electrons. Each main electron shell is physically divided into smaller distinct regions called subshells.
Chemists identify these specific subshells using the lowercase letters s, p, d, and f. The subshells contain specific three-dimensional spaces that scientists call orbitals. An orbital is a mathematical region where an electron is most likely located.
Many students incorrectly confuse these fuzzy electron shells with rigid planetary orbits. Electrons do not travel in perfect predictable circles around the central atomic nucleus. Atoms naturally fill their innermost electron shells first because they offer lower energy states.
A completely full outermost shell makes any given atom chemically very stable. Chemists refer to this critical outer layer as the valence shell. Elements sitting in the same periodic table column share similar valence shells.