A Lewis dot structure (named for American chemist G.N. Lewis) is a representation the basic chemical structure of a covalent or polar molecule with letters (representing atoms) and dots (representing valence electrons).

    H                    H
   ..         :          ..
H : O :   : O : O :   H : C : H
    ..     ..   ..        ..
                          H
  water    oxygen      methane

There are as many total dots in a Lewis dot structure as there are valence electrons in the atoms represented. Light elements (hydrogen, helium, lithium, and beryllium) want to be next to two dots; other elements generally want eight dots (except silicon and sulfur, which can often take 10 or more dots). Atoms tend to share pairs of electrons.

In ionic compounds, such as aluminium fluoride (AlF3), the Lewis dot structure is composed as such:

      x.
Al3+[:F :]-3
      ..

The pairs of dots in the diagram represent lone pairs, while the single dot represent a bond pair, and the "x" represents the electron taken from the aluminium. The negative ion in the compound is always the one inside the brackets, while the positive ion is the one outside.

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