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The diagrams of the atoms and ions that are used in the Visual Chemistry tutorial are based on the Bohr model of the hydrogen atom (Niels Bohr: Nobel Prize in Physics 1922). Bohr described “a shell” or energy level around the central nucleus of the hydrogen atom. Nuclei are made from protons, each carrying one positive charge, and uncharged particles called neutrons. The diagrams used in this website show electrons in "shells" or energy levels. Moving away from the nucleus, each new shell contains electrons at a higher energy level than the previous shell.
The paths that the electrons take as they move around the nucleus are more complex than this model suggests. Each electron shell is sub-divided into orbitals. The first electron shell (EL1) has only one orbital. All the other shells are made up of more than one orbital. It is the paths of the electrons as they move around the nucleus that determines the shapes of the orbitals.
The different types of orbitals have different energy levels. The prefix given to the different types of orbitals comes from the appearance of their lines shown by a spectroscope.
s = sharp, p = principal, d = diffuse, f = fundamental
The diagram below indicates the total number of electrons to be found in each orbital. Strangely enough, with increasing atomic number, sometimes the additional electron does not automatically occupy the next energy level. In the diagram, the number shown near each of the electrons indicates the order in which they are added to the orbitals.
Each orbital holds a maximum of 2 electrons. The diagram below shows the shape of the orbitals in the neon atom. The first shell has only one orbital (1s) made from the movement of two electrons. The second shell has 8 electrons separated into 4 pairs (2s, 2px, 2py, 2pz).
The Visual Chemistry tutorial keeps to the simple model showing shell energy levels. However, you should bear in mind that present theory indicates a much more complex explanation of atomic structure.