molecular geometry determining the structure of molecules
TRANSCRIPT
MOLECULAR GEOMETRY
Determining the Structure of Molecules
Molecular Structures
Molecular Formula H2OElectron Dot Structure H:Be:H
Structural Formula H – Be – H
Ball and Stick Molecular Model
MOLECULAR GEOMETRY
Structural formulas, such as NH3, provide information about bonding only. It does not provide direct information about the shape of the bond or the shape of the molecule.
The repulsion between charge clouds in the outer levels of atoms determines the arrangement of the orbitals. The orbital arrangement determines the shape of the molecules.
VSEPR
Valence Shell Electron Pair Repulsion theory
is based on the number of regions of high electron density around a central atom.
can be used to predict structures of molecules or ions by minimizing the electrostatic repulsion between the regions of high electron density.
can also be used to predict structures of molecules or ions that contain multiple bonds or unpaired electrons.
does fail in some cases.
VSEPR
In small molecules, electron pairs will spread as far apart as possible to minimize repulsive forces.
Two electron pairs = 180 apartThree electron pairs = 120 apartFour electron pairs = 109.5 apart
SHAPES WE WILL LEARN
LinearTrigonal PlanarTetrahedralPyramidalBent
LINEAR
Atoms are connected in a straight line.180 bond anglesOne or two bonded pairs of electronsExamples: HCl CO2
TRIGONAL PLANAR
Atoms are connected in a flat equilateral triangle
Three bonded pairs of electrons120 bond angleExample: BCl3
TETRAHEDRAL
Atoms are connected in a shape with four surfaces.
Four bonded pairs of electrons109.5 bond angles.Example: CH4
PYRAMIDAL
Atoms are in the shape of a pyramid. Similar to tetrahedral but only has three bonded pairs of
electrons, not four; has one unshared/lone pair of electrons. Less than 109.5 bond angle due to unshared pair of electrons. Example: NH3
BENT
Atoms are close to the shape of a tetrahedral, but the two unshared pairs of atoms exert a greater repulsive force than the two sets in the bonds.
Two shared pairs of electrons and two unshared pairs of electrons
105 bond angleExample: H2O
Other Shapes
VSEPR TABLE
Look on the back of your Molecular Geometry worksheet
HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
USE ELECTRON DOT DIAGRAMS
1. Look at the chemical formula. Figure out the location of the atoms
Hydrogen is always on the outside The least electronegative atom is the central atom
(most “electropositive”).
2. Draw the Electron Dot Diagram for each atom.
3. Count up the total amount of valence electrons for all the atoms involved.
HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
4. Determine the number of bonding pairs of electrons by dividing the total # of electrons by two.
5. Arrange a skeletal diagram of the molecule by placing the other atoms around the central atom. Place a bonding pair of electrons (2) between the central atom and each of the terminal atoms.
HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
6. Subtract the number of bonded pairs of electrons for the central atom from the total number of electrons. If there are any electrons left over, these electrons are lone pairs to be placed around the central atom.
HOW TO DETERMINE THE GEOMETRY OF A MOLECULE
7. If the terminal (outside) atoms do not have a full octet, place lone pairs of electrons around them. The rest go around the central atom.
8. If the central atom does not have four pairs of electrons around it (and it had a least four valence electrons to begin with), try converting some of the lone pairs to double or triple bonds. (Carbon, nitrogen, oxygen, and sulfur like to form multiple bonds)
EXCEPTIONS
There are exceptions to the OCTET rule:Atoms with less than an Octet: Hydrogen – only 2 valence electrons Group 2A – only 4 valence electrons Group 3A – only 6 valence electrons
Atoms with more than an Octet: Sulfur and phosphorus – 10+ valence electrons Krypton, xenon, iodine, and others with “d” orbitals will
accept more than 8.
PRACTICE
Fill in the following chart and predict the molecular shape for the following substances:
Molecule E- Dot diagram # of shared/ # of lone/ Electron Dot Shape of Bond Type
Formula all elements bonded e- unshared e- Structure Molecule (e-negativity)
PRACTICE
H2O
SiCl4NH3
Cl2N2
GaF3
MOLECULAR POLARITY
This is a result of bond dipoles (areas of unequal polarity) that do not cancel each other out.
This is the polarity of the MOLECULE not the BOND.
BOND POLARITY
You can determine the polarity of BONDS by determining the electronegativity differences of the two atoms involved.
C – C nonpolar cov. e-neg diff = 0Na – F ionic e-neg diff – 3.05C - H nonpolar cov. e-neg diff = 0.35
MOLECULAR POLARITY
But, take those same molecules and the polarity of the molecule will depend on the whole molecule, not just the bond.
C – C nonpolar equal sharingNa – F polar unequal sharingC - H polar unequal sharingFor a molecule, you must consider the
shape and the terminal atoms.
MOLECULAR POLARITY
LINEARIf the terminal atoms are the same, there are equal forces, so
it is NONPOLAR. If they are not the same, it is POLAR.BeF2 – nonpolar
HCl - polar
MOLECULAR POLARITY
TETRAHEDRALIf the terminal atoms are the same, there are
equal forces, so it is NONPOLAR. If they are not the same, it is POLAR.
CCl4 – nonpolar
CHCl3 - polar
MOLECULAR POLARITY
TRIGONAL PLANARIf the terminal atoms are the same, there
are equal forces, so it is NONPOLAR. If they are not the same, it is POLAR.
BCl3 – nonpolar
BHCl2 - polar
MOLECULAR POLARITY
PYRAMIDALBecause of the unshared pair, there are
unequal forces, so the molecule is POLAR.
NH3
MOLECULAR POLARITY
BENTBecause of the unshared pairs, there are
unequal forces, so the molecule is POLAR.
H2O
MOLECULAR POLARITY
What is the Molecular Polarity for these molecules?
Molecular Polarity
What about these?
REMEMBER!
To determine BOND POLARITY, calculate the electronegativity differences.
To determine MOLECULAR POLARITY, look at the shape of the molecule and the terminal atoms.
ORBITAL HYBRIDIZATION
This is the mixing of atomic orbitals in an atom to generate a new set of atomic orbitals.
S and p orbitals merge and there no longer are distinct orbitals.
They merge to form sp orbitals.
ORBITAL HYBRIDIZATION
ORBITAL HYBRIDIZATION
Determine the Hybridization:
sp3
sp
sp2
Do this now – scrap paper
Molecule Formula
E- Dot Diagram
for all atoms
# shared/bonded
electrons
# lone/unshared electrons
E- Dot Structure
Shape of molecule
Bond Type (electro-
negativity)
AlH3
PH3
CS2