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