Electrons are located on the shells of an atom because of the attractive electrostatic forces between the negatively charged electrons and the positively charged atomic nucleus. According to the Rutherford atomic model, an atom consists of a tiny nucleus surrounded by a much larger volume of space where the electrons are located. The distribution of electrons in an atom is determined by their quantum mechanical properties, particularly their wave-particle duality.

Each electron in an atom occupies a specific energy level or electron shell. The shells are arranged concentrically around the nucleus, with the innermost shell being closest to the nucleus and the outermost shell being furthest away. The energy levels increase as the distance from the nucleus increases. Electrons can only occupy discrete energy levels, and the allowed energy levels are determined by the quantum numbers associated with electron orbitals.

The principal quantum number (n) determines the main energy shell an electron occupies, with n = 1 corresponding to the innermost shell, n = 2 to the second shell, and so on. The angular momentum quantum number (l) determines the shape of the electron orbital within a given shell. Each main energy shell can have multiple subshells with different angular momentum quantum numbers. The magnetic quantum number (ml) describes the orientation of the electron orbital in space. Finally, the spin quantum number (ms) specifies the two possible spin states of an electron.

In summary, electrons are located on the shells of an atom due to the electrostatic forces between electrons and the nucleus. Their distribution is governed by quantum mechanics, with electrons occupying discrete energy levels and orbitals determined by quantum numbers. This shell structure plays a crucial role in determining the chemical properties and behavior of atoms and molecules.