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Electron Arrangement

The electron arrangement or electron configuration is how the electrons are arranged in the energy levels or shells around the nucleus.

The general rule is that the first energy level (or the innermost energy level) holds a maximum of two electrons and the following shells each hold up to eight electrons. The outermost energy level is called the valence shell.

Electrons first fill up the inner energy lever, then the second energy level, then the third, and so on, so shells closest to the nucleus are filled first.

For example, a sodium atom has an atomic number of 11. This means it has 11 protons and therefore 11 electrons. The electron configuration is 2,8,1. This is because the first energy level is filled with two electrons. There are now nine electrons remaining. The second energy level is filled with eight electrons and the one remaining electron (or valence electron) is in the third (valence) shell.

The electron configuration of a sodium atom is shown in the diagram below: