Lewis Dot Structures

Transcription

1 Lewis Dot Structures Lecture 21 Reading: Zumdahl Recommended Problems: 13.51, 13.53, 13.57, Outline Lewis Dot Structure Basics Resonance Formal Charge

2 Localized Bond Models Consider our energy diagram for H 2 bonding:

3 Localized Model Limitations It is important to keep in mind that the models we are discussing are just that..models. We are operating under the assumption that when forming bonds, atoms share electrons using atomic orbitals (AO). Electrons involved in bonding: bonding pairs. Electrons not involved in bonding: lone pairs. In reality, once atoms come together in a molecule, the electrons are no longer localized in AOs...they now occupy delocalized molecular orbitals. But don t fret...molecular orbitals appear in Chem 162! Just remember that electrons don t necessarily stay with the atom they came in with.

4 Background J.J. Thomson (1914): Proposed that electrons (which he discovered) are arranged in shells within atoms. Outermost shell contains valence electrons which may not exceed 8. Valence electrons are transferred between atoms in ionic solids to achieve noble gas configurations. Gilbert N. Lewis (1916): Concerned with bonding in organic compounds. Originated idea of shared electron pairs between atomic pairs. The chemical bond is at all times and in all molecules merely a pair of electrons jointly held by two atoms. Proposed the cubic atom, now shown as a dot structure and rule of eight. Irving Langmuir (1919): Popularized and extended Lewis theory. Originated term Octet Rule.

5 Lewis Dot Structures (cont.) 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]3s 1 Na + : [Ne] Cl: [Ne]3s 2 3p 5 Cl - : [Ne]3s 2 3p 6 = [Ar]

6 G.N. Lewis Structures: Early Concepts (see text figure 13.14)

7 Irving Langmuir GE scientist Re-designed incandescent light bulb Invented vacuum pump Pioneer in surface science First non-academic to win Chemistry Nobel Prize Gifted speaker: popularized/extended Lewis Cubic Atom Model, and minted expression octet rule.

8 Langmuir s Octet Equation ν=#covalent bonds=h+(8n-σe)/2 Electroneutrality Principle: Atoms share electrons in a molecule in such a way as to make the charge on each atom as close to zero as possible

9 Some Cubic Molecules by Langmuir Problems: Static cubic atomic model did not agree with dynamic electron atomic models of Bohr and Schroedinger No way to show a triple bond

10 Another Spokesman for GE Radio and T.V

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

12 Lewis Dot Mechanics (cont.) Three steps for basic Lewis structures: 1. Sum the valence electrons for all atoms to determine total number of electrons. 2. Use pairs of electrons to form a bond between each pair of atoms (bonding pairs). 3. Arrange remaining electrons around atoms (lone pairs) to satisfy the octet rule ( duet rule for hydrogen).

13 LDS Mechanics (cont.) An example: Cl 2 O 20 e - 16 e - left

14 LDS Mechanics (cont.) An example: CH 4 8 e - 0 e - left Done!

15 LDS Mechanics (cont.) An example: CO 2 16 e - 12 e - left Octet Violation 0 e - left CO double bond

16 Multiple bonds Sometimes atoms have to share more than one pair of electrons in order to fulfill the octet rule: O 2 O O O O Each O only has 7: Still not happy O O O O

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

18 LDS Rules of Thumb 1. In a polyatomic molecule, the atom that can make the most number of bonds typically goes in the center. Another statement of this is that the least electronegative atom typically goes in the center...these rules are pretty much the same. 2. H can only form one bond, so it goes on the outside of the molecule...h is a terminal atom. 3. If O and H both appear in a chemical formula, they are probably bonded to each other. 4. When several C atoms appear in the same formula, they are probably bonded to each other in a chain. In other situations the C atoms can form a closed loop, or branching structures, but we will not consider such cases here.

19 Which is the correct Lewis dot structure for NO 2-? A B C D

20 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

21 Resonance Structures (cont.) The classic example: O 3. Both structures are correct!

22 Resonance Structures (cont.) In this example, O 3 has two resonance structures: Conceptually, we think of the bonding as 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. Bond lengths O-O pm O=O pm Ozone pm

23 LDS Mechanics (cont.) NO 3-24 e -

24 Resonance Structures (cont.) NO 3 - is a classic example of resonance:

25 Benzene: C6H6 The structure of the organic molecule benzene was a mystery until the German chemist August Kekule (1865) proposed that it is a hexagonal ring of carbon atoms. The Lewis structure has alternating double bonds as shown on the left below. Experimentally, benzene does not display chemistry expected of a molecule with alternating double bonds. Instead, benezene exists as a resonance structure with each bond of order 1.5.

26 Structural Isomers What if different sets of atomic linkages can be used to construct correct LDSs: Both are correct, but which is more correct?

27 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 Z Formal C Structure with less F. C. is more correct.

28 Example: CO 2 Formal Charge e Z FC More Correct

29 Triatomic Bonding Patterns Compare CO 2 with N 3- both 16 e - systems: FC FC

30 What is the most likely structure of N 2 O? FC FC

31 Practice drawing Lewis Structures BrO - CO CH 2 F 2 COCl 2 CH 3 CH 2 OH

32 Lewis Structure of Carbon Monoxide: CO C: 4 valence electrons O: 6 valence electrons Three valid Lewis Structures

33 Lewis Structure of Carbon Monoxide cont. In the structure with three covalent bonds, the octet rule is satisfied, but the electropositive carbon has a negative formal charge. The structure with two covalent bonds would be consistent with the very low dipole moment of the molecule if the bonds were nonpolar. The structure with one covalent bond expresses the greater electronegativity of oxygen and the calculated net atomic charges. None of them do exactly meet the real electronic structure. Calculations with natural bond orbitals show that the structure with a triple bond is the most important Lewis structure (for the free molecule).

34 The Diatomic Oxygen Anamoly It is easy to write a Lewis Structure for Diatomic Oxygen O2: This valid Lewis Structure is NOT born out by experiment. Liquid O2 sticks to magnet poles. This means it is Paramagnetic and has at least two unpaired electrons. The unpaired electrons contribute to the double bond.

35 Molecules without Complete Octet Lewis Structures

36 Nitrogen Dioxide

37 Lewis Structure Summary