aBetterChemText: Lewis Dot Structures of Molecules

Lewis took the Octet Rule and applied it to molecules. For instance, we already know that a chlorine atom is unstable because it has only 7 electrons, and that it can become stable if it can steal an electron from another atom to become an -1 ion. We also know that chlorine appears in nature as Cl₂ not as single atoms of chlorine and that the molecule has a single bond.

Lewis deduced that if electrons involved in the tug-o-war each counted for both atoms, then each atom would have an octet and would be stable.

In other words, the chlorine atom on the left "owns" 7 valence electrons and is pulling on another, for a total of 8. The chlorine on the right has the same situation. As a result, each atom "feels" like it has 8 electrons - an octet.

Using this logic, we can draw the LDS's for other molecules, like OCl₂:

Notice that the oxygen started with 6 valence electrons, so it needed to make 2 different bonds in order to "have" 8 valence electrons.

What all of this means is that every "single" electron that an atom has wants to find a "partner" through bonding. So nitrogen, with 5 valence electrons (1 pair and three singles) tends to make 3 bonds, while carbon, with 4 valence electrons, tends to make 4 bonds.

Lewis also determined that hydrogen does not obey the octet rule, but rather follows its own “duet rule.” Of course this makes sense when we remember that hydrogen only has one orbital (the 1s) which can only hold two electrons. So LDS's that involve hydrogen (H₂, CH₄) look like this:

Remember that Lewis didn't care how the electron dots were placed around the atomic symbol. That means that all three of the following are correct LDS's for water: