ROGER HARRIS / SCIENCE PHOTO LIBRARY ROGER HARRIS / SCIENCE PHOTO LIBRARY
Aluminium atom. Diagram of an atom of the element aluminium, pulling back from the central nucleus to reveal the surrounding electron orbitals. Aluminium is the 13th element, and has 13 protons (red) in its nucleus, along with, usually, 14 neutrons (blue), although it has other isotopes. Aluminium has 13 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, also containing two, then an outer 3p shell (blue). 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.
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