Friday, the 13th of November 2009 AD

[A quiz over 6.4 is scheduled for Tuesday, and the Ch. 6 test is scheduled for Thursday, along with the outline. We're supposed to do the aluminum lab Tuesday or Wednesday.]

Molecular Geometry

• properties of molecules depend not only on bonding, but also molecular geometry
• molecular geometry-3D arrangement of a molecule's atoms
• polarity of each bond + molecular geometry ---> molecular polarity
• --> the uneven disrtibution of electron density

1) strongly influences the forces that act between molecules in liquids and solids

• chemical formula reveals little about the molecular geometry, so we need something... better... Like...

VSEPR THEORY!!!

• diatomic molecules can only be linear because there is only 2 atoms
• use VSEPR to predict molecular geometry of complicated molecules

1) it takes into account the location of all the electron pairs surrounding the bonding of atoms

• Valence Shell Electron-Pair Repulsion
• VSEPR Theory- repulsion between sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible (the electrons want to have the greatest distance they can have between them)

ex. - BeF2 {only 2 electron pairs shared between the atoms, so they will be 180 degrees apart, making the molecule's geometry linear}

If A--central atom in a molecule
B--atoms bonded to A
Then according to VSEPR, BeF2 is an AB2 molecule (i.e. linear)

• AB3-3 A-B bonds stay farthest apart by pointing to the corners of an equilateral triangle {120 degree bonds}
• AB4- distance between electron pairs maxed if the A-B bonds point to the corners of a tetrahedron {109.5 degree bonds}

[past AB4, there are exceptions to the octet rule---the nonmetals 3rd period and below

• VSEPR also acounts for the unshared electron pairs

-->like ammonia (NH3) and water (H2O)

• Lewis structure of ammonia shows central atom has an unshared electron pair
• VSEPR postulates that the lone pair occupies space around the N atom just as bonding pairs do
• NH3 = AB4 molecule, but with a pyramid shape instead of a tetrahedron

KEY POINT--SHAPE OF MOLECULE CONCERNS THE POSITION OF ATOMS ONLY

• H2O with 2 unshared pairs has a "bent," or angular, geometry
• bonds and electrons take up different amounts of space (electrons take up more than bonds)
• unshared electron pairs repel other electron pairs more strongly than bonding pairs, which pushes the bonded atoms together
• same basic principles of VSEPR that have been described can be used to determine the geometry of several additional types of molecules -- AB2E, AB2E2, AB5, AB6

1) treat double and triple bonds the same as single bonds

2) treat polyatomic ions similar to molecules

And that is where we stopped.

Study for the quiz with your book, because we took only 2 lines of notes over sect. 4