Lewis Dot Structures of Atoms and Ions

To understand how Lewis Dot Structures (LDS's) are drawn, we can start with single atoms and ions. 

For any atom, Lewis starts with the symbol of the element and then imagines that there are four places around the symbol into which we can put electrons, shown here with empty ellipses. (We don't draw the ellipses in the Lewis Dot Structure--they are just here in this picture to show you the four places that dots can be placed)

Each valence electron is then represented by a single dot in one of those four spaces. 

However, they must be placed into those spaces carefully.

Lewis, drawing his understanding from the way atoms bonded, stumbled across his own version of Hund's Rule. In other words, when placing electrons in the spaces around an atom's symbol, Lewis placed one electron in each space before placing a second in any space. It does NOT matter what order the electrons are placed in the spaces. Therefore ALL of the following are correct Lewis Dot Structures for the nitrogen atom:

 

The LDS's for several other atoms are shown here:

The LDS can also be drawn for ions, recognizing that a negative charge means that extra electrons have been acquired and that a positive charge means that electrons have been lost. Lewis Dot Structures for ions are always enclosed in brackets with the charge indicated outside.

 

Lewis noted that stable elements, like neon and argon, as well as stable ions, like Cl^-1 and O^-2, have eight electrons and took from that the idea that all elements “want” to have 8 electrons. he called this idea the Octet Rule. 

Of course we recognize now that this is really just an application of Schrödinger's theory, where a full shell is stable, but Lewis was working prior to the development of that theory and arrived at his Octet Rule without the prior knowledge that we have.

Lewis Dot Structures of Molecules