The structure of the atom consists of a cloud of electrons around a nucleus. The nucleus, consisting of protons and neutrons, contains over 99% of the mass of the atom. According to the quantum theory, the electrons (shown as red dots) surround the nucleus in shells, with their precise individual positions and momenta being only statistically determined. You can simulate the electron "motions" using the checkbox. The size of the nucleus is greatly magnified in the graphic—it is actually only about times the radius of the innermost electron shell. In accordance with the Pauli exclusion principle, the innermost shell has a capacity of two electrons, while the second shell can hold a maximum of eight electrons.