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Chapter 9

Chapter 9. Molecular Geometry Bonding Theories. Molecular Shape. A bond angle is the angle defined by lines joining the centers of two atoms to a third atom to which they are covalently bonded

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Chapter 9

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  1. Chapter9 Molecular Geometry Bonding Theories

  2. Molecular Shape • A bond angle is the angle defined by lines joining the centers of two atoms to a third atom to which they are covalently bonded • The molecular geometry or shape is defined by the lowest energy arrangement of its atoms in three-dimensional space.

  3. VSEPR Valence-Shell Electron-Pair Repulsion Theory The geometric arrangement of atoms bonded to a given atom is determined principally by minimizing electron pair repulsions of bonding and non-bonding electrons.

  4. Central Atoms without Lone Pairs Steric number (SN) is the number of volumes of space occupied by electrons surrounding a central atom

  5. Geometric Forms

  6. Predicting a VSEPR Structure • Draw Lewis structure. • Determine the steric number of the central atom. • Use the SN to determine the geometry around the central atom. • The name for molecular structure is determined by the number of volumes of space occupied by bonding electrons.

  7. Examples • What is the molecular geometry of BF3? • What is the molecular geometry of CH4

  8. Examples Lewis Structure (exception to Law of Octaves) F • What is the molecular geometry of BF3? • What is the molecular geometry of CH4 B F F

  9. Examples Lewis Structure (exception to Law of Octaves) F • What is the molecular geometry of BF3? • What is the molecular geometry of CH4 Bond Angles = 120° Trigonal Planar B F F

  10. Examples Lewis Structure (exception to Law of Octaves) F • What is the molecular geometry of BF3? • What is the molecular geometry of CH4 Bond Angles = 120° Trigonal Planar B F F H C H H H

  11. Examples Lewis Structure (exception to Law of Octaves) F • What is the molecular geometry of BF3? • What is the molecular geometry of CH4 Bond Angles = 120° Trigonal Planar B F F H Bond Angles = 120° Tetrahedral C H H H

  12. Central Atoms with Lone Pairs • Electron-pair geometry describes the arrangement of atoms and lone pairs of electrons about a central atom. • The electron-pair geometry will always be one of the five geometries presented previously. • The molecular geometry in these molecules describes the shape of the atoms present (it excludes the lone pairs).

  13. Lone Pairs • Lone pairs of electrons occupy more space around a central atom than do bonding electrons. • Lone pair-lone pair repulsion is the largest. • Lone pair-bonding pair repulsion is the next largest. • Bonding pair-bonding pair repulsion is the smallest. • In structures with lone pairs on the central atom, the bond angles are a little smaller than predicted based on the electron-pair geometry.

  14. Non-bonding Electrons & Shape Angles = 120 Trigonal Planar Angles < 120 Bent

  15. Non-bonding Electrons & Shape

  16. Non-bonding Electrons & Shape Tetrahedral Trigonal Pyramid v-shape

  17. The lone pairs of electrons are always found in the trigonal planar part of the structure to minimize repulsion.

  18. Hybrid Orbitals You may have noticed that the electron pairs in molecules have different orientations in space compared to atomic orbitals. Wave equations mathematically generated volumes of space where electrons spend most of their time, but what about molecules?

  19. Hybrid Orbitals You may have noticed that the electron pairs in molecules have different orientations in space compared to atomic orbitals. Wave equations mathematically generated volumes of space where electrons spend most of their time, but what about molecules? This brings us to the concept of hybrid orbitals, combinations of atomic orbitals, or molecular orbitals (from wave equations of electrons in molecules)

  20. Hybrid Orbitals Hybridization is a concept you might be familiar with. For example a grapefruit is a hybrid of what two fruits?

  21. Hybrid Orbitals Hybridization is a concept you might be familiar with. For example a grapefruit is a hybrid of what two fruits?

  22. Hybrid Orbitals Hybridization is a concept you might be familiar with. For example a grapefruit is a hybrid of what two fruits? Lemon and orange

  23. Hybrid Orbitals How about a nectarine?

  24. Hybrid Orbitals How about a nectarine? Plumb and a peach.

  25. Hybrid Orbitals How about a nectarine? Plumb and a peach. Broccoaflower? Broccoli and cauliflower

  26. Hybrid Orbitals How about a nectarine? Plumb and a peach. Broccoaflower? Broccoli and cauliflower And a Cocapoo?

  27. Hybrid Orbitals How about a nectarine? Plumb and a peach. Broccoaflower? Broccoli and cauliflower And a Cocapoo? Cocker spaniel and poodle

  28. Hybrid Orbitals On to Chemistry! How about an s-orbital and a p-orbital? Yes, sp orbital.

  29. Hybrid Orbitals On to Chemistry! How about an s-orbital and a p-orbital? Yes, sp orbital. How about one s-orbital and two p-orbitals?

  30. Hybrid Orbitals On to Chemistry! How about an s-orbital and a p-orbital? Yes, sp orbital. How about one s-orbital and two p-orbitals? Yes an sp2 orbital.

  31. Hybrid Orbital Notation In order to construct hybrid orbital notation, we need to separate the central atom from the surrounding electrons, usually the central atom is the largest, the most electronegative, or the one that there is one of.

  32. Hybrid Orbital Notation In order to construct hybrid orbital notation, we need to separate the central atom from the surrounding electrons, usually the central atom is the largest, the most electronegative, or the one that there is one of. When constructing a hybrid orbital diagram, all of the valence electrons of the central atom are used and only the single electrons of the atoms attached to the central atom are use.

  33. Hybrid Orbital Example First we separate the central atom from the other atoms. The central atom is A and the other atoms are called X’s Suppose we want to make a diagram of SF6 A X’s SF6

  34. Hybrid Orbital Example First we separate the central atom from the other atoms. The central atom is A and the other atoms are called X’s Suppose we want to make a diagram of SF6 A X’s SF6 Then we generate a set of degenerate hybrid orbitals to house the valence electrons

  35. Hybrid Orbital Example First we separate the central atom from the other atoms. The central atom is A and the other atoms are called X’s Suppose we want to make a diagram of SF6 A X’s SF6 Then we generate a set of degenerate hybrid orbitals to house the valence electrons F F F F F F F Insert single electrons into the degenerate hybrid orbitals

  36. Practice What are the molecular geometries of the ions: SCN- and NO2- ?

  37. Polar Bonds and Polar Molecules • Two covalently bonded atoms with different electronegativities have partial electric charges of opposite sign creating a bond dipole. • A molecule is called a polar molecule when it • has polar bonds and a shape where the bond • dipoles don’t offset each other.

  38. Examples

  39. Measuring Polarity • The permanent dipole moment () is a measured value that defines the extent of separation of positive and negative charge centers in a covalently bonded molecule.

  40. Atomic Orbitals and Bonds • A tetrahedral molecule requires that four orbitals of the central atom must overlap with an orbital of an outer atom to form a bond. • The central atom would use its s orbital and its three p orbitals, but these orbitals would not yield the 109° bond angles observed in the tetrahedral molecule.

  41. Valence-Bond Theory • Valence-bond theory assumes that covalent bonds form when orbitals on different atoms overlap or occupy the same region of space. • A sigma () bond is a covalent bond in which the highest electron density lies between the two atoms along the bond axis connecting them.

  42. Examples

  43. Valence Bond Theory • Hybridization is the mixing of atomic orbitals to generate new sets of orbitals that are then available to overlap and form covalent bonds with other atoms. • A hybrid atomic orbital is one of a set of equivalent orbitals about an atom created when specific atomic orbitals are mixed.

  44. Tetrahedral Geometry: sp3 Hybrid Orbitals A tetrahedral orientation of valence electrons is achieved by forming four sp3 hybrid orbitals form one s and three p atomic orbitals.

  45. Other sp3 Hybrid Examples

  46. sp2 Hybridization • In a covalent pi () bond, electron density is greatest above and below the bonding axis.

  47. sp Hybridization • Pi bonds will not exist between two atoms unless a sigma bond forms first.

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