Potassium atom. Diagram of an atom of the element potassium, pulling back from the central nucleus to reveal the surrounding electron orbitals. Potassium is the 19th element, and has 19 protons (red) in its nucleus, along with most commonly 20 neutrons (blue). Potassium has 19 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 (bright green), containing only one electron. 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.