# Molecular Geometry Notes. February 3, 2014 You will be able to describe the molecular geometry of a molecule. Correct HW 6B A note on nomenclature Notes.

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Molecular Geometry Notes

February 3, 2014 You will be able to describe the molecular geometry of a molecule. Correct HW 6B A note on nomenclature Notes Cool video of the day: Check out THIS ionic crystal lattice!!THIS

Binary Nomenclature First element by name (with prefix, ONLY if there is more than one atom) Second element as an “-ide”, with prefix Does not work for molecules with more than 2 atoms

CO NO SF 2 SO 2 NO 2 NCl 3 CCl 4 H 2 O N 2 F 6 N 2 O 5 Carbon monoxide Nitrogen monoxide Sulfur difluoride Sulfur dioxide Nitrogen dioxide Nitrogen tricloride Carbon tetrachloride Dihydrogen monoxide Dinitrogen hexafluoride Dinitrogen pentoxide

Molecular Geometry Applies to any Lewis diagram. (Involved in covalent bonding)

VSEPR Theory Valence Shell Electron Pair Repulsion Each bonding group or lone pair of electrons spreads out as far as possible from every other bonding group or lone pair of electrons. Think 3D!

Electron Pair vs. Molecular Geometry Electron pair geometry describes both bonds AND lone pairs located around the central atom. Molecular geometry describes only the bonds located around the central atom. Molecular geometry is a subset of electron pair geometry.

Practice  How many bonding groups are around the central atom? 44  How many lone pairs around the central atom? 00

Practice  How many bonding groups are around the central atom? 33  How many lone pairs are around the central atom? 11

Practice  How many bonding groups are around the central atom? 33  How many lone pairs are around the central atom? 00

Practice  How many bonding groups are around the central atom? 22  How many lone pairs are around the central atom? 00

Electron Pair Geometry: Overview (Not in notes) The shape of the molecule including both bonds and lone pairs Linear – 2 e - groups from central atom Trigonal Planar – 3 e - groups from center Tetrahedral – 4 e - groups from center Any of these lobes (electron groups) can be either lone pairs or covalent bonds

REMEMBER ELECTRON PAIR GEOMETRY and MOLECULAR GEOMETRY are not the same thing. – EPG describes all electron groups (including lone pairs) – MG describes only bonding groups

2 Electron Groups Electron pair geometry: Linear – 2 bonding groups – 0 lone pairs – Molecular geometry: Linear – Example: CO 2 2 bonding groups 0 lone pairs Double and triple bonds count as just ONE bonding group!!!

3 Electron Groups Electron pair geometry: Trigonal Planar – 3 bonding groups – 0 lone pairs – Molecular geometry: Trigonal planar – Example: NO 3 - Double and triple bonds count as just ONE bonding group This Lewis structure also shows the molecular shape (trigonal planar).

3 Electron Groups Electron pair geometry: Trigonal Planar – 2 bonding groups – 1 lone pair – Molecular geometry: Bent – Example: NO 2 - Double and triple bonds count as just ONE bonding group Lone pair is one of the three electron groups around the central atom

4 Electron Groups Electron pair geometry: Tetrahedral – 4 bonding – 0 lone pairs – Molecular geometry: Tetrahedral – Example: CH 4 This Lewis structure does NOT show the tetrahedral molecular shape. It DOES show the four electron groups.

4 Electron Groups Electron pair geometry: Tetrahedral – 3 bonding – 1 lone pair – Molecular geometry: Trigonal Pyramidal – Example: NH 3 This Lewis structure does NOT show the molecular shape. It DOES show the four electron groups. Lone pair is one of the four electron groups around the central atom

4 Electron Groups Electron pair geometry: Tetrahedral – 2 bonding groups – 2 lone pairs – Molecular geometry: Bent/Angular – Example: H 2 O This Lewis structure shows both the molecular shape and the four electron groups Lone pairs are two of the four electron groups around the central atom

Molecular Geometry: Overview The shape of the molecule including just bonds between atoms. 2 e - groups from central atom: – If 2 bonds/0 lone pairs: Linear 3 e - groups from central atom: – 3 bonds/0 lone pairs: Trigonal planar – 2 bonds/1 lone pair: Bent 4 e - groups from central atom: – 4 bonds/0 lone pairs: Tetrahedral – 3 bonds/1 lone pair: Trigonal pyramidal – 2 bonds/2 lone pairs: Bent

Sketches of Molecular Geometry Sketches (on homework, quizzes, and tests) should show the shape of the molecule in three dimensions:

Exceptions to the Octet Rule (SF 6, BrF 5, XeF 4, IF 7 ) (Beyond the scope of this class)

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