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For questions about determining Lewis structures (or their validity) for different chemical species. Do not use this tag if your question is about determining [oxidation-state] or [formal-charges], use the other respective tags instead.

A Lewis structure (usually referred to as a Lewis dot structure) is a simple model or approximation used to determine the bonding behavior of atoms within molecules. Dots represent valence electrons for each atom while solid lines represent shared pair of electrons between atoms (i.e. covalent bonds).

For example, this is the Lewis structure of the water molecule $\ce{H2O}$ (See also simple Wikipedia):

Lewis structure of water

Notice the two lone pairs (= 4 electrons) on the oxygen atom shown by dots and the two bond pairs (O-H) shown by solid lines.

Lewis structures may also be drawn for charged species. Here is the Lewis structure for the ammonium ion:

Lewis structure of the ammonium ion

Notice that a solid line with an arrow indicates a coordinate covalent bond, with the arrow pointing from the donor atom to the receiver atom.