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!!

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• SF2

• SO2

• NO2

• NCl3

• CCl4

• H2O

• N2F6

• N2O5

Carbon monoxideNitrogen monoxideSulfur difluorideSulfur dioxideNitrogen dioxideNitrogen triclorideCarbon tetrachlorideDihydrogen monoxideDinitrogen hexafluorideDinitrogen 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

How many lone pairs around the central atom? 0

Practice

How many bonding groups are around the central atom? 3

How many lone pairs are around the central atom? 1

Practice

How many bonding groups are around the central atom? 3

How many lone pairs are around the central atom? 0

Practice

How many bonding groups are around the central atom? 2

How many lone pairs are around the central atom? 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: CO2

• 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: NO3-

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: NO2-

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: CH4

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: NH3

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: H2O

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(SF6, BrF5, XeF4, IF7 )

(Beyond the scope of this class)