1. Chemistry XXI Unit 2 How do we determine structure? M4. Inferring Charge Distribution Analyzing the distribution of electrons in molecules. M3. Predicting Geometry Predicting the three dimensional geometry of molecules. M2. Looking for Patterns Deducing atom connectivity based on atomic structure. M1. Analyzing Light-Matter Interactions Using spectroscopy to derive structural information. The central goal of this unit is to help you develop ways of thinking that can be used to predict the atomic and molecular structure of substances.

2. Chemistry XXI Unit 2 How do we determine structure? Module 3: Predicting Geometry Central goal: To deduce the Lewis structure of molecules and predict their three dimensional geometry based on the analysis of the number and type of valence electron pairs surrounding each atom.

3. Chemistry XXI The Challenge The properties of a substance are determined by the structure of its molecules. How can we predict molecular geometry given information about atomic composition and atom connectivity ? Molecular structure depends on: Atomic Composition Atom Connectivity Molecular geometry Aspirin C9H8O4C9H8O4 Modeling How do I predict it?

4. Chemistry XXI We have seen that when two atoms of nonmetallic elements combine, their valence electrons are reorganized. The number of covalent bonds that are formed are determined by the most stable electron configurations (full valence shell). Electron Distribution We can use the octet rule (or full valence shell rule) to make predictions about how electrons will distribute among the different atoms in a molecule. O2O2 N2N2 O N Useful Tool: Lewis Electron-dot Structures

5. Chemistry XXI 2.Count valence electrons: H = 1 and O = 6 Total = (2 x 1) + 6 = 8 valence electrons This electrons will organize in 4 pairs (spin pairing to minimize energy) There are some simple rules that facilitate the creation of Lewis structures. Let’s illustrate them with the molecule of water H 2 O. Lewis Structures 1.Choose the central atom; never H (it forms only one bond). The central atom tends to be the one with the lowest ionization potential. O is central in this case

6. Chemistry XXI Lewis Structures 3.Use as many pairs as needed to form single bonds between the central atom and the surrounding atoms. Each bond line represents a pair of electrons 4.Use the remaining pairs to satisfy the full valence shell rule in each atom as needed. Start with terminal or outside atoms, but not if H; place any leftover electrons on the central atom. 8 valence e - Lone e - pairs Bond e - pairs

7. Chemistry XXI Lewis Structures Let’s consider another case: Carbon dioxide CO 2. 1.What is the central atom? 2.How many valence e - ? How many pairs? 4 + 2 x 6 = 16 valence e -  8 e - pairs 3.What is the backbone? 4.How do we distribute the e - pairs left? 5.How do we satisfy the octet rule for all atoms? Form double bonds

8. Chemistry XXI Let’s Think A variety of substances contribute to indoor air pollution. Among the most common we find: Build the Lewis structures of the following greenhouse gases: CH 4, CO,NH 3, CH 2 O 1.What is the central atom? 2.How many valence e - ? How many pairs? 3.What is the backbone? 4.How do we distribute the e - pairs left? 5.How do we satisfy the octet rule for all atoms? STRATEGY

9. Chemistry XXI Experimental data indicates that both bonds in the O 3 molecule have the same length, but the value is intermediate between those of single and double bonds. Interesting Cases Bond Length (pm) 148 O3O3 127.8 121 O For some molecules, the derivation of their actual Lewis structure is not so straightforward. Consider for example the ozone molecule, O 3, which plays a central role in our atmosphere. How do we explain it?

10. Chemistry XXI Let’s Think Build the Lewis structure of O 3. This molecule illustrates a structural feature that we need to take into account when deciding how to distribute electrons among atoms in a molecule. What is it?

11. Chemistry XXI Molecular Hybrids The structure is a hybrid of: Resonance Structures Resonance structures are drawn when a single Lewis structure cannot represent the actual electron distribution in a molecule. 3 e - pairs / 2 bonds Intermediate between single and double

12. Chemistry XXI Resonance In molecules that exhibit resonance the electrons are “delocalized” over the entire system. This delocalization tends to stabilize the molecule (reduces its potential energy). Resonance Hybrid Benzene C 6 H 6

13. Chemistry XXI Let’s Think SO 3 CH 2 O Which of these pollutants exhibits resonance stabilization? How many resonance structures do they have?

14. Chemistry XXI Electron Repulsion Once the Lewis structure of a molecule is derived, its geometry can be predicted applying a simple principle: Regions of high electron density around any single atom will be located as far as possible due to electron repulsions. Valence Shell Electron Pair Repulsion (VSEPR) Theory Minimizing repulsions allows us to find the most stable shape (lower energy).

15. Chemistry XXI Let’s Think Consider the following Lewis structures for these molecules in our atmosphere: F F Cl How many regions of high electron density do you identify around each central atom? How will these regions be located in space due to electron repulsions?

16. Chemistry XXI Molecular Geometry 2 Molecular geometry e - pair geometry Example# e - regions 3 3 Linear (180 o ) Bent or Angular Trigonal Planar (~120 o ) Trigonal Planar (< 120 o ) Linear Trigonal planar 118 o

17. Chemistry XXI 4 Molecular geometry e - pair geometry Example# e- regions F F Cl Tetrahedral Tetrahedral (109 o ) Molecular Geometry Tetrahedral (< 109 o ) Bent or Angular 104.5 o 4 4 Tetrahedral (< 109 o ) Trigonal Pyramid 107.8 o

18. Chemistry XXI Let’s Think Apply VSEPR theory to derive the molecular geometry of the following atmospheric molecules: SO 2, SO 3, CH 4, N 2 O Estimate the bond angles in these molecules. Follow the sequence STEP 1STEP 2 STEP 3

19. Chemistry XXI 4 bond pairs, 0 lone pairs 3 bond pairs, 1 lone pair Larger Molecules The same ideas can be applied to deduce the molecular geometry of larger molecules. The task is simplified by recognizing the following patterns for some of the most common central atoms: 2 bond pairs, 2 lone pairs C Tetrahedral C Trigonal planar Linear O Bent N Trigonal Pyramid Trigonal planar N

20. Chemistry XXI Larger Molecules Consider the molecule of ethanol C 2 H 6 O: The molecule has three main “centers”: The overall geometry is determined by the geometry around each of these centers. Tetrahedral 109 o Bent ~105 o

21. Chemistry XXI Let’s Think Consider the molecule of acetone C 3 H 6 O: How many centers are in this molecule? What is the geometry around each of these centers? What bond angle characterizes each center?

22. Chemistry XXI Assess what you know Let′s apply!

23. Chemistry XXI Functionality A central idea in chemistry is that the chemical properties of many molecules are determined by the presence of “distinctive arrangements of atoms” that tend to behave as a single chemical entity during a reaction. This distinctive arrangements of atoms are called “functional groups” and their properties are determined by their atomic composition, connectivity and geometry. R Hydroxyl group

24. Chemistry XXI Functional Groups Let′s apply! Determine the geometry around the atomic centers of the following “functional groups”: Chemical Class Functional group Structural formulaMolecular geometry Alcoholhydroxyl Ketonecarbonyl Carboxylic acid carboxyl AminePrimary amine Aromaticphenyl R R R R1R1 R2R2 R

25. Chemistry XXI Let′s apply! Phenylalanine is an essential aminoacid needed by our body to biochemically synthesize a wide variety of proteins Predict What functional groups are present in this molecule? Estimate the value of the marked bond angles and make an sketch of the geometry of this molecule.

26. Chemistry XXI Summarize in once sentence the basic principle that determines molecular geometry.

27. Chemistry XXI Summary Predicting Geometry The octet rule can be used to deduce the distribution of valence electrons among the different atoms in a molecule. The distribution of electrons is represented through the Lewis structure of the molecule. 8 valence e - Lone e - pairs Bond e - pairs

28. Chemistry XXI Once the Lewis structure of a molecule is derived, its geometry can be predicted applying a simple principle: Regions of high electron density around any single atom will be located as far as possible due to electron repulsions (VSEPR Theory). Summary Predicting Geometry We can deduce the entire molecular geometry of a complex molecule by analyzing the electron pair distribution around each of its atoms.

29. Chemistry XXI For next class, Investigate how molecular composition and geometry affect the distribution of electrons within a molecule. What is the difference between a polar and a non-polar molecule?