Notes: 9.4 OBJECTIVES: Students will be able to (SWBAT) Predict the shapes of molecules QUESTIONS TO PONDER: What is VSEPR? Why is VSEPR important in determining molecular shape? What is the difference between molecular and electron geometry? Why is a water molecule shaped differently than expected from its Lewis structure? What are some of the exceptions to the VSEPR model?

Molecular Shape (Chapter 9, Section 4)

9.4 – Molecular Shape VSEPR (Valence Shell Electron Pair Repulsion) Model or Theory Model used to determine 3-D molecular shape Based on arrangement that minimizes repulsion of shared and unshared electron pairs around central atom Unshared pairs repel more than shared pairs Unshared/lone pairs are not visible The shapes of molecules determine whether or not molecules can get close enough to each other to react.

Electron Geometry vs. Molecular Geometry First, draw the molecular Lewis Structures Electron Geometry (what determines shape): Lone and shared pairs determine the geometry Use VSEPR chart to determine electron geometry Molecular Geometry (what you see): Only shared pairs determines the geometry Use VSEPR chart to determine molecular geometry

← shared pair → ← unshared pair ↓ DASH – going back WEDGE – sticking out

Steps to Determine Molecular Shape Draw Lewis Structure of molecule Count # of bonded atoms around central atom Count unshared (lone) pairs (2 dots) Use VSEPR chart (like one in the book, pg 260) to determine shape *Remember, electron pairs will position themselves around central atoms to avoid the most repulsion (move farthest away from each other)

3D Model using VSEPR for NH3

Example NH3

Example NH3 Total # of bonded atoms = 3

Example NH3 Total # of lone pairs = 1

Example ***remember, lone pairs are not visible! NH3 ↓ ↓ ↓ ↓ According to VSEPR chart  NH3 will have trigonal pyramidal shape

Exceptions to the Octet Rule Central atoms in some molecules will not have an octet Boron  can have less than an octet; often has only 3 pairs of e- (trigonal planar) Phosphorus  can have an expanded (more than) octet, up to 5 pairs Sulfur  can have expanded octet up to 6 pairs

More than 1 central atom Many molecules form longer chains Anything with multiple C or N’s often form longer chains of molecules with C and N as the “backbone”

Example Example: C6H14 (hexane)  each C is a “central atom” Structural Formula (model) 3-D (projection) formula