3D Molecular Geometry Shades and Behaviours of 3D Molecules In order to understand the shapes the molecules form we must adhere to rules that are based on the repulsion of valence shell electrons! VSEPR = Valence Shell Electron Pair Repulsion
Reasons for Geometry boils down to 5 things 1. Electrons all have the same negative charge 2. Like charges repel therefore Electrons repel 3. Bonded pairs surrounding the nucleus repel other bonded pairs and other electrons 4. Lone pairs surrounding the nucleus repel other bonded pairs and other electrons 5. Valence electrons are oriented in such a way as to be as far apart from one
another as possible For each of the following models, A WILL REPRESENT THE CENTRAL ELEMENT, X THE OUTER ELEMENT and E THE ELECTRON LONG PAIRS Shapes of 3D Molecules Molecule BeCl2
(AX2) Shape Molecular Geometry Linear
Reason for Shape The 2 pairs of bonding electrons maximize their distance from each
other 180o apart Shapes of 3D Molecules Molecule BF3 (AX3) (AX3E2)
Shape Molecular Geometry Trigonal Planar
Reason for Shape The bonding pairs must be equally far from each other The 3 bonding pairs maximize distance
from each other 120o Shapes of 3D Molecules Molecule Shape
Molecular Geometry Reason for Shape The bonding pairs must be equally far
from each other CH4 (AX4) Tetrahedral Maximize distance from each other
Shapes of 3D Molecules Molecule Shape Molecular Geometry
NH3 (AX3E) Bent Reason for Shape
Because the lone pairs are closer to the nucleus they have great repulsion, push the Hs down Shapes of 3D Molecules
Molecule H2O (AX2E2) Shape Molecular
Geometry Reason for Shape Bent The two sets of lone
pairs further push the Hs down and together Shapes of 3D Molecules Molecule Shape
Molecular Geometry Reason for Shape To maximize
distance 3 equatorial Cls are 120o PCl5 (AX5) Trigonal Bipyramidal
Top and bottom Cls are 90o apart Shapes of 3D Molecules Molecule Shape
Molecular Geometry Reason for Shape As more bonding pairs fill up locations
around the nucleus the space between the pairs becomes smaller and smaller SF6 (AX6) Octahedradron
The molecule becomes less stable due to increased repulsion Shapes of 3D Molecules Molecule
Shape Molecular Geometry Reason for Shape
Similar to Octahedradron except two lone pairs occupy the axial positions. XeF4
(AX4E2) Square Planar Whiteboarding! Grab a whiteboard and pens Form a group of 3 to 4 with people you have never worked with!
Steps to Make VSEPR Diagrams Combining two Concepts Electronegativity + Shape of the Molecule We will now combine two concepts to introduce a new one Polarity Remember Electronegativity Values
Polarity Molecules are said to be polar is the electron density is NOT evenly spread around the molecule This causes part of the molecule to be partially positive and part of the molecule to be partially negative Two Factors
1) Electronegativity 2) Symmetry of the Molecule (based on shape) Determining the Dipole + Dipole In the bonding unit we determined if a bond was polar based on the electronegativity difference of the two elements
To determine if the entire molecule is polar we must also check the symmetry (or lack there of) of the molecule Symmetry vs. Asymmetry Case 1: Non-Polar
Case 2: Polar and Asymmetric Case 3: Polar and Symmetric Polar or not polar?
Case 3: Polar and Symmetric Not Polar Case 4: Polar and Asymmetric
Polar or not polar? Case 4: Polar and Asymmetric Polar
Same Shape, Different Polarity Question to ask yourself Polar or not Polar? Does the molecule contain polar bonds, and if so, do the bond dipoles cancel each other or not? Whiteboarding!
Please collect a whiteboard and pens! Work in groups of 3 Polarity The extent of polarity depends on the size of the dipole The size of the dipole depends on the difference in electronegativities and the symmetry (or lack there of) of the molecule
Polarity When two substances have opposite polarities, they do not mix. Like mixes with Like In-Class Work Collect the Worksheet Shapes of Molecules from
the front of the class Work your way through the questions, the answers will be posted online
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