Electron configurations and valence electrons

An electron configuration describes how an atom's electrons are arranged in energy levels and sublevels. Electrons fill the lowest available energy sublevels first, in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on, and each orbital holds at most two electrons, so an s sublevel holds 2 electrons and a p sublevel holds 6. Sodium, with 11 electrons, has the configuration 1s2 2s2 2p6 3s1. The electrons in the outermost energy level are the valence electrons, which are the ones involved in bonding. For main-group elements, a Lewis dot structure shows them as dots around the element's symbol; sodium has one valence electron and one dot, while chlorine has seven.

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