An atom has three main particles: the proton, the neutron and the electron. The first two particles are contained in the center of the atom, called the nucleus. Electrons, however, are found outside the nucleus in levels called energy levels. Electrons at higher energy levels are located farther from the nucleus. These energy levels are orbitals, regions of space where electrons are most likely to be found. Electrons are not in orbits, defined paths that track the direction an object goes and where it finds itself. Since Heisenberg's Uncertainty Principle states that it is impossible to know the direction and position of an electron at the same time, plotting an orbit for an electron is impossible ("Discussion of Electron Structure"). All stable isotopes of copper are atoms of the same element but with different masses: this must be taken into account before establishing an average atomic mass for the element. There are two such isotopes, copper-63 and copper-65 (“Copper Isotopes”). Copper has an atomic number of 29. This means it has 29 protons. All stable isotopes are electrically neutral. Therefore, the number of electrons and protons must be equal. In the case of copper, there are 29 electrons. However, to take into account the fact that they are isotopes, they have a different number of neutrons. Protons have a relative mass (on the carbon-12 scale) of about one, and electrons 1/1836 (almost no mass). Neutrons, even with a mass of about one, explain the difference in the masses of different isotopes (“Discussion of the Structure of Electrons”). Therefore, copper-63 has 34 neutrons and copper-65 has 36 neutrons. The natural abundance of copper-63 is 69.17% and the abundance of copper-65 is 30.83% (“Copper Isotopes”). As for the electrons themselves, they completely fill the first, second and third levels. Furthermore, an electron enters the fourth level. Within the sublevels they are distributed in the electronic configuration 1s2 2s2 2p6 3s2 3p6 4s1 3d10. The “s”, “p” and “d” are subshell type labels. S subshells can hold up to two electrons, p subshells can hold up to six, and d subshells can hold up to ten (“Discussion of Electron Structure”). Thus, the electronic configuration indicates that the sublevel s in the first, second and third levels are completely full. Furthermore, the p subshells in shells two and three and the d subshell in shell three are completely filled. However, the sublevel s in level four is only half full. However, only two electrons can fit into an orbital (“Discussion of the structure of electrons).