Lewis Dot Structures Developed by G. N. Lewis to serve as a way to describe bonding in polyatomic systems. Central idea: the most stable arrangement of electrons is one in which all atoms have a “noble” gas configuration. Example: NaCl versus Na+Cl- Na: [Ne]3s1 Cl: [Ne]3s23p5 Na+: [Ne] Cl-: [Ne]3s23p6 = [Ar]

LDS Mechanics Atoms are represented by atomic symbols surrounded by valence electrons. Lone Pair (6 x) Electron pairs between atoms indicate bond formation. Bonding Pair

LDS Mechanics (cont.) Three steps for “basic” Lewis structures: Sum the valence electrons for all atoms to determine total number of electrons. Use pairs of electrons to form a bond between each pair of atoms (bonding pairs). Arrange remaining electrons around atoms (lone pairs) to satisfy the “octet rule” (“duet” rule for hydrogen).

LDS Mechanics (cont.) An example: Cl2O 20 e- 16 e- left

LDS Mechanics (cont.) An example: CH4 8 e- 0 e- left Done!

LDS Mechanics (cont.) An example: CO2 16 e- 12 e- left 0 e- left Octet Violation 0 e- left CO double bond more than one pair of electrons is shared

Lewis Diagram for CO2

LDS Mechanics (cont.) An example: NO+ + + 10 e- 8 e- left +

Whiteboarding! In a group of 3 try the following Lewis Structures!

CCl4 NF3 H2O H2Se NH3 OF2

Resonance Structures We have assumed up to this point that there is one correct Lewis structure. There are systems for which more than one Lewis structure is possible: Different atomic linkages: Structural Isomers Same atomic linkages, different bonding: Resonance

Resonance Structures (cont.) The classic example: O3 When a molecule or ion contains double bonds next to single bonds, there are often several different and equally correct Lewis structures that we can draw Both structures are correct!

Resonance Structures (cont.) In this example, O3 has two resonance structures: Conceptually, we think of the bonding being an average of these two structures. Electrons are delocalized between the oxygens such that on average the bond strength is equivalent to 1.5 O-O bonds.

Resonance Structures for CO3-2 The double bond in each of the possible locations in a series of diagrams and connecting each diagram with a set of double arrows. It must be emphasized, however, that the pair of electrons does not move around between pairs of atoms as the diagrams might suggest.

Structure with less F. C. is more correct. Formal Charge Formal Charge: Compare the nuclear charge (+Z) to the number of electrons (dividing bonding electron pairs by 2). Difference is known as the “formal charge”. #e- 7 6 7 7 6 7 Z+ 7 6 7 7 7 6 Formal C. 0 0 0 0 +1 -1 Structure with less F. C. is more correct.

Formal Charge Example: CO2 e- 6 4 6 6 4 6 7 4 5 Z+ 6 4 6 6 6 4 6 6 4 FC 0 0 0 0 +2 -2 -1 +2 -1 More Correct

Beyond the Octet Rule There are numerous exceptions to the octet rule. We will deal with two classes of violation here: Sub-octet systems Valence shell expansion

Beyond the Octet Rule (cont.) Some atoms (Be and B in particular) undergo bonding, but will form stable molecules that do not fulfill the octet rule. Experiments demonstrate that the B-F bond strength is consistent with single bonds only.

Beyond the Octet Rule (cont.) For third-row elements (“Period 3”), the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding. When this occurs, more than 8 electrons can surround a third-row element. Example: ClF3 (a 28 e- system) F obey octet rule Cl has 10e-

In the end, you have to practice…..a lot! Summary Remember the following: C, N, O, and F obey the octet rule B and Be are often sub-octet Second row (Period 2) elements never exceed the octet rule Third Row elements and beyond can use valence shell expansion to exceed the octet rule. In the end, you have to practice…..a lot!