Week 10/Tu: Units ‘23,24’ Valence Bonds 10/Tu: Units ‘23,24’ Valence Bonds ... Valence Bond Theory: a) Use the VSEPR & Lewis Diagrams to determine structure b) ... Valence
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Unit 22: VESPR -- Electron Arrangements -- 3D shapes Unit 23,24: Structure & Orbitals -- Molecular polarity -- Molecular Shapes and Orbitals -- Valence bond theory -- sigma and pi bonds Unit 25: Gases -- properties of gases -- gas laws Issues: CONNECT Lewis Drawing … Homework resumes this week, due Saturday 8AM
In general the energy levels of elements are different which leads to the unequal sharing of electrons in chemical bonds. Electronegativity is the quantitative measure of the unequal sharing that we have already discussed. Thus, most chemical bonds are polarized to a greater or lesser extent. The molecular electron distribution can be polarized (or unequally distributed) depending on the atoms in the molecule and its shape …
1) Lewis Diagrams for Main Group elements (8 atomic diagrams) 2) VSEPR Structures for molecules (5 arrangements, 11 shapes) à symmetric shapes: non-polar molecules à asymmetric shapes: polar molecules The polarization of water molecules is widely known and gives water unique properties.
H O H
EN = 2.1 3.5 2.1 Δ EN = 1.4 each bond
4 pairs on Oxygen – tetrahedral 2 lone pairs on Oxygen – bent
O C O
2 pairs on Carbon – linear (ignore multiple bonds for arrangement) 0 lone pairs on Carbon – linear
1) Lewis Diagrams for Main Group elements (8 atomic diagrams) 2) VSEPR Structures for molecules (5 arrangements, 11 shapes) à symmetric shapes: non-polar molecules à asymmetric shapes: polar molecules
3 pairs on Nitrogen – trigonal planar (ignore multiple bonds for arrangement) 0 lone pairs on Nitrogen – trigonal planar Resonance structure …
2 pairs on central Nitrogen – linear (ignore multiple bonds for arrangement) 0 lone pairs on central Nitrogen – linear
1) Lewis Diagrams for Main Group elements (8 atomic diagrams) 2) VSEPR Structures for molecules (5 arrangements, 11 shapes)
3) Valence Bond Theory: a) Use the VSEPR & Lewis Diagrams to determine structure b) Combine the available valence orbitals to describe structure à Hybridize (mix) atomic orbitals as necessary to correspond to the electron distribution in the molecule. à the number of orbitals is conserved; 2 AO’s give 2 VBO’s, etc. à Each mixed (hybrid) orbital can hold one pair of electrons, that are either a bonding pair or a or a non-bonding (lone) pair c) Use (remaining) atomic orbitals to describe multiple bonds if present. Note that there are only five “arrangements” of electrons in
the VSEPR theory. Thus, only five hybrids (mixtures) of atomic orbitals needed to describe these arrangements.