Molecular structure and covalent bonding

1. Molecular structure and covalent bonding Chapter 8

2. Key concepts • Understand the difference between Lewis structures and molecular geometries. • Know the basic shapes of several molecule types. • Know how to predict molecular shapes using valence-shell electron repulsion (VSEPR) model. • Understand how overall molecular shape can affect the dipole moment of the molecule. • Understand the use of valence-bond theory as an explanation for the VSEPR model. • Know different hybrid orbitals that function in molecular geometries.

3. Molecular geometries • Lewis structures help us understand the type of bonds between atoms in a molecule • But, they do NOT indicate the geometry in 3-D space. • VSEPR theory helps describe the actual geometry of molecules based on their covalent bonding.

4. Molecular geometries • two main parameters to know for molecular geometry: • bond length(s) – • bond angle(s) – • (there is a third parameter, the dihedral angle, important in molecules with more than one central atom, which we will occasionally encounter)

5. Fundamental geometry • five fundamental geometries behind all ABn molecular shapes • Linear • Trigonal planar • Tetrahedral • Trigonal bipyramidal • Octahedral • Table 8.1 (p. 305) illustrates fundamental geometries

6. valence-shell electron repulsion (VSEPR) model • used to predict geometries of ABn molecules where A is a p-block element. • Electron domains (“regions of high electron density”): areas where electrons are most likely found in a molecule. • two types of electron domains: • bonding domain - • non-bonding domain –

7. Basic principles of VSEPR • Electron domains repel each other; will force as far away from each other as possible. •  the best electronic geometry is the one that minimizes all electron repulsions in the molecule.

8. The electronic geometry is NOT the molecular geometry. • Electronic geometry – • molecular geometry –

9. Steps for predicting molecular geometries

10. Steps for predicting molecular geometries • Write Lewis formula and identify a central atom. • Examples: H2O, NH3, CH4, CO2, SO2, SO32-, COCl2, SF6, XeF4 • Count regions of high electron density (electron domains) on that central atom. • Single bonds, multiple bonds, lone pairs all count as ONE region.

11. Steps for predicting molecular geometries • Determine electronic geometry around central atom. • Determine molecular geometry around central atom. • If there is ≥ 1 non-bonding electron pair in the molecule, the electronic geometry is never the same as the molecular geometry.

12. Non-bonding pairs and multiple bonds affect the ideal molecular geometry. • Adjust molecular geometry for any lone pairs (or multiple bonds). • Non-bonding domains take up more space than bonding domains, and have a greater repulsive force that will compress bond angles in the molecule. • Multiple bonds have higher electron density than single bonds. •  multiple bonds compress bond angles between single bonds. • the amount of compression is Lone pair >> multiple bonds > single bonds

13. Steps for predicting molecular geometries • Determine hybrid orbitals, describe bonding. • Hybrid orbitals: How to explain both bonding between atoms and molecular geometries. Atomic orbitals cannot explain this. • Different atomic orbitals of the central atom mix to form hybrid orbitals that have proper shapes to produce observed molecular geometries.

14. Types of hybrids • What atomic orbitals combine to form the hybrid orbitals on the central atom? • sp – • sp2 – • sp3 – • sp3d – • sp3d2 – • The number of hybrid orbitals ________ the number of atomic orbitals

15. sp hybrids

16. sp2 hybrids

17. sp3hybrids

18. sp3d hybrids suppose I had SF4 instead of PF5. Where would the lone pair be located? Lone pairs of trig. bipyrimidal electronic geometries first occupy ____________ positions.

19. sp3d2hybrids What if we had XeF4 instead of SF6? Where would the lone pairs be located?

20. Steps for predicting molecular geometries • Can another central atom be identified? • Not in current examples, but how about C2H6?

21. Multiple bonds in the molecule • consider C2H4, which has a double bond (H2C=CH2). • What is the electronic geometry on each C? molecular geometry? What hybrid orbitals are used? • of the 2 bonds between C and C, one is between sp2 orbitals on each C. Where is the other bond?

22.  and  bonds • Bond between sp2 orbitals of carbons called a  (sigma) bond. Orbitals overlap head-to-head. • other bond forms between unhybridized 2p orbitals of carbons. This is called a  (pi) bond. Orbitals overlap side-by-side. •  bond extends above and below the plane of the  bond.

23. Triple bonds • Let’s use our molecular geometry process on acetylene (HCCH). What do we get for the molecular geometry? • What hybrid orbitals are used? • How are the multiple bonds formed?

24. Steps for predicting molecular geometries • Determine if molecule is polar or non- polar • We must examine the dipole moment along each bond in context of the molecular geometry • Dipoles are vectors, so they add like vectors.

25. To be or not to be…polar… Two conditions for polarity: • There must be at least one polar bond (or lone pair) on the central atom. • a. the polar bonds (if more than one) must not cancel each other out, or b. lone pairs (if more than one) must not have a geometry that cancels out. of our examples, which are polar? which are non-polar?