Copper atom. Diagram of an atom of the element copper, pulling back from the central nucleus to reveal the surrounding electron orbitals. Copper is the 29th element, and has 29 protons (red) in its nucleus, along with most commonly 35 neutrons (blue). Copper has 29 electrons, and each of its orbitals can hold a maximum of two. Orbitals are filled from the lowest energy (nearest the nucleus) first, so there are two electrons in the inner 1s orbital, and two in the higher energy 2s orbital (green spheres), and two each in the slightly higher energy 2p orbitals (blue, red and yellow). Outside the 2p orbitals is a spherical 3s shell, and three 3p shells (magenta, blue and brown). At a higher energy is a spherical 4s orbital, which contains only one electron, and then five 3d orbitals. The 3d orbitals are arranged as three X-shaped orbitals arranged in the x, y and z plane (red, green and yellow), one X-shaped orbital in the x plane but at 45 degrees to the other (blue), and one two-lobed orbital in the z plane, with a small ring around the nucleus (purple). For clarity, solid lines show the paths of the electrons. In reality, electrons can be found anywhere, and the shape of each orbital is based only on where it is most likely to be found.