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.
This Demonstration considers the first 10 atoms of the periodic table—hydrogen through neon. Only the most stable isotopes are shown. When the number of electrons equals the nuclear charge , the atom is electrically neutral. Otherwise it gives a positive or negative ion, which is labeled on the graphic.
Remember to change all three selectors when you consider a new atom or ion, otherwise the output can be erratic.