Lewis Structures (also called Electron Dot Structures)

Purpose:

• Good for finding the shapes of molecules which is important for understanding stuff like:
  • Why our tastebuds think Nutrasweet tastes sweet but the rest of our body doesn't treat it like sugar
  • How ants and bees communicate with chemicals
  • Finding drugs that will trigger certain reactions in the body
• Good for understanding how electrons move in organic synthesis reactions. This helps chemists figure out how to make synthetic versions of important but hard to get natural substances (e.g. makes life-saving drugs easier and cheaper to get)

Basic Rules:

1. Add up total number of valence electrons. Do not worry about keeping tarck of which electrons come frm which atoms. It is the total number of electrons that is important.
2. Find the least electronegative atom and put that one in the middle. (Hydrogen, though, is never in the center)
3. Arrange other atoms around central atom
4. Put a pair of electrons between the central atom and each atom surrounding it by drawing a line between the atoms. (each of these lines represents a bond. Each of these bonds includes two electrons)
5. Subtract the number of bonds * 2 from the total number of valence electrons
6. Put dots around the remaining atoms in the structure trying to satisfy the duet rule for hydrogen and the octet rule for the rest
7. For each dot placed, subtract one from the unused valence electrons
8. Check your work:
   • Add up the total number of electrons used (bonds are worth 2 and each dot is worth 1) and compare to total number of valence electrons calculated in step 1
   • Count the number of electrons around each atom and decide if the octet and duet rules are satisfied (lone pairs count as 2, shared pairs count as 2)

Advanced Rules:

1. Revision of rule 6: you may need to include double and triple bonds to satisfy octet rule with given number of electrons
2. Some atoms exceed the octet rule (3rd period and higher on the periodic table)
3. Some atoms have fewer electrons than an octet: Beryllium and Boron
4. If electrons are left over after satisfying the octet rule, put the extra on the central atom.
5. When calculating number of valence electrons for ions:
   • a. Anions: add electrons to the total according to the size of the charge
     • For example: -1, add 1 electron to total
     • -2, add 2 electrons to total
   • b. Cations: subtract electrons from the total according to the size of the charge
     • For example: +1, subtract 1 electron to total
     • +2, subtract 2 electrons to total

Resonance Structures:

Compounds with double and triple bonds may have more than one Lewis structure that satisfies the rules above. These are called resonance structures. None of them are exactly correct (does not agree with experimental observation for bond length) - its more correct to think of them as all correct. The compounds actual structure is sort-of an average of these structures. This is a limitation of Lewis structures.

To deal with this limitation, we draw all the structures that are possible and choose the best set of equivalent structures depending on formal charge rules (see below).

• Equivalent lewis structures mean they are basically the same - as in the resonance for NO₃^-
• Non-equivalent lewis structures are structures that are different. In the sulfate ion, one set of structures has 2 double bonds while another set has no double bonds.

Formal Charge

The goal is to figure out the "charge" on an atom in a covalent bond. It is not a real charge as in ionic bonds but is useful for figuring out if one Lewis structure is better than another .

1. You need to calculate a formal charge for each atom in the structure
2. Write the formal charge of each atom as a superscript of the atom it belongs to on the Lewis diagram.
3. The equation for finding formal charge: V-(n+b)
   • V = number of valence electrons
   • n = number of the elctrons owned in lone pairs, each pair counts as 2
   • b = number of bonds around an atom (double bonds count as 2 and triple bonds count as 3)
4. To decide how good the Lewis structure is
   • consider which atom has the most negative charge. If it is the most electronegative atom, that is good.
   • the best structure will have formal charges closest to zero

Geometry

• Once you have determined the correct structure, you can figure out the shape of the molecule
• Count the number of bonds around the central atom. Each bond counts as 1.
• Add the number of lone pairs. Each pair counts as 1.
• Geometry of Molecules