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SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")! Chemical Bonding Set 5
Credits Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website m m Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.
VSEPR VSEPR V alence S hell E lectron P air R epulsion theory.V alence S hell E lectron P air R epulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions. MOLECULAR GEOMETRY
Some Common Geometries Linear Trigonal Planar Tetrahedral
VSEPR charts Use the Lewis structure to determine the geometry of the molecule Electron arrangement establishes the bond angles Molecule takes the shape of that portion of the electron arrangement Charts look at the CENTRAL atom for all data! Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would only be 1 region)
Other VSEPR charts
Structure Determination by VSEPR Water, H 2 O The electron pair geometry is TETRAHEDRAL The molecular geometry is BENT. 2 bond pairs 2 lone pairs
Structure Determination by VSEPR Ammonia, NH 3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY — the positions of the atoms — is TRIGONAL PYRAMIDAL.
You will need to memorize: Shapes:Angles: – Linear 180 – Bent – Trigonal planar 120 – Tetrahedral – Trigonal pyramidal – Trigonal bipyramidal 180, 120 – Octahedral 90
Hybridization A hybrid occurs when two things are combined and the result has characteristics of both – EX: hybrid car (uses gas and electricity) During chemical bonding, different atomic orbitals undergo hybridization.
Carbon’s Hybridization Consider methane, CH 4 The carbon atom has four valence electrons with the electron configuration of [He]2s 2 2p 2. – You may expect the two unpaired p electrons to bond with other atoms and the two paired s electrons to remain as a lone pair – However, carbon undergoes hybridization, a process in which atomic orbitals mix and form new, identical, hybrid orbitals.
The hybrid orbitals in carbon Note that each hybrid orbital contains one electron that it can share with another atom, giving carbon 4 bonding sites. The hybrid orbital is called an sp 3 orbital because the four orbitals form from one s and three p orbitals.
Other Hybridizations Anything with a tetrahedral geometry is a result of sp 3 hybridization. Carbon is the most common example. Anything with a trigonal planar shape is a result of sp 2 hybridization. AlCl 3 and nitrate ion are examples. Can result from a double bond. Anything with a linear shape is a result of sp hybridization. CO and CO 2 are common examples. These often result from double and triple bonds.
Lone Pairs & Hybridization Lone pairs also occupy hybrid orbitals. Compare the hybrid orbitals of BeCl 2 (linear) and H 2 O (bent). Why does the water molecule have sp 3 orbitals and the BeCl 2 has sp? – The two O-H bonds occupy two of the sp 3 orbitals in water, and the two lone pairs occupy the other two. – Beryllium doesn’t have any lone pairs, so it’s geometry remains liner with sp hybridization.
Homework Questions 1) Although the VSEPR model is correct in predicting that CH 4 is tetrahedral, NH 3 is trigonal pyramidal, and H 2 O is bent, the model in its simplest form does not account for the fact that these molecules do not have exactly the same bond angles (
More Homework 2) Predict the molecular structure and bond angles for the following: – HCN, PH 3, CHCl 3 3) Compare the molecular shapes and hybrid orbitals of PF 3 and PF 5 molecules. Explain why their shapes differ. HINT: PF 5 has sp 3 d hybridization (why would that be?) 4) List, in a table, the Lewis structure, molecular shape, bond angle, and hybrid orbitals for molecules of CS 2, CH 2 O, H 2 Se, CCl 2 F 2, and NCl 3.