Electron shells: These are concentric circles or spherical rings, with each shell representing a fixed distance from the nucleus. Shells are also labeled with a quantum number (n), starting from n = 1 for the innermost shell.

Electron subshells: Each electron shell is divided into subshells, also known as orbitals. These subshells have different shapes and energy levels. Common subshells include s, p, d, f, and g orbitals, denoted by the quantum number (l).

Atomic orbitals: An atomic orbital is a specific three-dimensional mathematical function that describes the wave behavior of an electron and predicts the probable region where an electron may be found around the nucleus. Different orbitals differ in their spatial orientations and energy levels.