Chapter 5_ Bonding Theories.pdf

Chapter 5 Bonding Theories: Explaining Molecular Geometry Here isAP where Chemistry your presentation 2024-2025begins Valence-Shell Electron Pair Repulsion Theory (VSEPR) A model predictin...

Chapter 5 Bonding Theories: Explaining Molecular Geometry Here isAP where Chemistry your presentation 2024-2025begins Valence-Shell Electron Pair Repulsion Theory (VSEPR) A model predicting the arrangement of valence electron pairs around a central atom that minimizes their mutual repulsion to produce the lowest energy orientations. Electron Pair Geometry: the three-dimensional arrangement of bonding pairs and lone pairs of electrons about a central atom. Molecular Geometry: the three-dimensional arrangement of the atoms in a molecule. When there are no lone pairs of electrons, Electron-Pair Geometry = Molecular Geometry  Key Points About VSEPR From a Lewis Structure, we can determine the Steric Number (SN): ○ The sum of the number of atoms bonded to a central atom plus the number of lone pairs of electrons on the central atom. This steric number will provide us with the electron-pair geometry: ○ 2 = linear ○ 3 = trigonal planar ○ 4 = tetrahedral ○ 5 = trigonal bipyramidal ○ 6 = octahedral Difference Between Electron-Pair and Molecular Geometry Remember, the molecular geometry is only the same as the electron-pair geometry if there are no lone pairs on the central atom. If lone pairs are present, the molecular geometry will be different and based on the positions of the bonded atoms. Bond Angles The angle (in degrees) defined by lines joining the centers of two atoms to a third atom to which they are chemically bonded. The electron-pair geometry provides a theoretical bond angle. However, we see that bond angles differ in molecules when lone pairs exist due to VSEPR. ○ Repulsion between lone pairs and bonding pairs is greater than repulsion between bonding pairs. ○ Repulsion caused by a lone pair is greater than repulsion caused by a double bond. ○ Repulsion caused by a double bond is greater than repulsion caused by a single bond. ○ Two lone pairs of electrons on a central atom exert a greater repulsive force on the atom’s bonding pairs than does one lone pair. For molecules with lone pairs, you just need to know its bond angles are less than the theoretical angles for the electron-pair geometry. SN = 2 (Linear) Electron- # Bonded # Lone Molecular Bond Example Hybridization Pair Atoms Pairs Geometry Angles Geometry (Bonds) Linear 2 0 Linear 180o CO2 sp SN = 3 (Trigonal Planar) Electron- # Bonded # Lone Molecular Bond Example Hybridization Pair Atoms Pairs Geometry Angles Geometry (Bonds) Trigonal 3 0 Trigonal Planar 120o CH2O sp2 Planar Trigonal 2 1 Bent