Electron Pair & Molecular Geometry

Introduction:

1) The valence – shell electron – pair repulsion model (VSEPR) can predict the shape of molecules and ions by proposing that valence – shell electron pairs are arranged about each atom such that these pairs are as far away from one another as possible to minimize electron – pair repulsions.

2) Electron pair geometry is the shape that is obtained by assuming the electron pairs are groups around the central atom.

a. There are only five types of electron pair geometry:

i. Linear – two groups around central atom

ii. Trigonal planar – three groups around central atom

iii. Tetrahedral – four groups around the central atom

iv. Trigonal bipyramidal – five groups around the central atom

v. Octahedral – six groups around the central atom

3) Molecular geometry is the shape that is obtained by assuming the electron pairs are absent around the central atom.

a. There are multiple types of molecular geometries depending on the number of bonding groups (or substituents) and the number of loan pairs.

b. The table below shows all possible molecular geometries.

4) Molecular polarity is dependent upon the generation of dipole moments due to differences in electronegativity between bonding substituents and the geometry of the resultant molecule.

a. Nonpolar molecules have no net dipole.

b. Polar molecules have net nonzero dipole moments.

Geometries Predicted by the VSEPR Model

Valence Electron Pairs	Groups Bonded to Central Atom‡	Nonbonding Electron Pairs on Central Atom	VSEPR* Formula	Approx. Bond Angles	Electron Pair Geometry	Molecular Geometry	Examples
2	2	0	AX2	180o	linear	linear	BeF2
3	3	0	AX3	120o	trigonal planar	trigonal planar	BCl3
3	2	1	AX2E	<120o	trigonal planar	angular, bent	SnCl2
4	4	0	AX4	109.5o	tetrahedral	tetrahedral	CH4
4	3	1	AX3E	<109.5o	tetrahedral	trigonal pyramidal	PH3
4	2	2	AX2E​2	<109.5o	tetrahedral	angular, bent	H2O
5	5	0	AX5	90o/120o	trigonal bipyramidal	trigonal bipyramidal	PI5
5	4	1	AX4E	>90o/>120o	trigonal bipyramidal	irregular tetrahedral	SCl4
5	3	2	AX3E2	<90o	trigonal bipyramidal	T – shaped	ClF3
5	2	3	AX2E3	180o	trigonal bipyramidal	linear	XeF2
6	6	0	AX6	90o	octahedral	octahedral	SF6
6	5	1	AX5E	>90o	octahedral	square pyramidal	BrF5
6	4	2	AX4E2	90o	octahedral	square planar	ICl4–

‡ A double or triple bond counts as a single group.

*A = central atom, X = substituent, E = electron pair

Net Dipoles of Predicted Molecular Geometries

VSEPR* Formula	Molecular Geometry	Net Dipole†
AX	Linear	Can be nonzero
AX2	Linear	Zero
AX2	Bent	Can be nonzero
AX3	Trigonal planar	Zero
AX3	Trigonal pyramidal	Can be nonzero
AX3	T – shaped	Can be nonzero
AX4	Tetrahedral	Zero
AX4	Square planar	Zero
AX4	Irregular tetrahedral	Can be nonzero
AX5	Trigonal bipyramidal	Zero
AX5	Square pyramidal	Can be nonzero
AX6	Octahedral	Zero

*A = central atom, X = substituent, E = electron pair

†Assuming all atoms are identical

Substances to include in the Table of Chemical and Physical Properties:

NONE

Procedure:

1) Draw out the appropriate Lewis structure for all of the molecules or ions in the two charts below.

2) Using the online molecule viewer () construct these molecules. This is for visualization purpose only. Please attach 2-3 molecules for after saving the picture files.

3) Based on lecture video and instruction provided here determine the following information about each:

a. Number of bonding electron pairs between the central atom and the substituents

b. Number of nonbonding electron pairs on the central atom

c. Electron pair geometry

d. Molecular geometry

e. Approximate bond angles between the central atom and its substituents

f. Polarity of the resultant molecule or ion

4) After drawing Lewis structure, scan and upload it within your lab report.

Discussion:

1) Determine all structures in your data table.

2) Discuss the factor(s) that govern(s) the polarity of molecules.

3) What is meant by hybrid orbitals? Give two examples of the process of orbital hybridization.

4) What is formal charge? Explain the formula and how this can assist at determining correct Lewis structures.

Molecules/Ions that Obey Octet Rule

Molecule/Ion	Lewis Structure	Atoms Bonded to Central atom	Nonbonding Electron Pairs on Central Atom	Electron Pair Geometry	Molecular Geometry	Approx. Bond Angles	Polar or Nonpolar
CH4		4	0	Tetrahedral	Tetrahedral	109.5o	Nonpolar
OF2
NH3
SF2
H3O+
BrF2+
CO32–
BF4–
BeCl2
Molecule/Ion	Lewis Structure	Groups Bonded to Central Atom	Nonbonding Electron Pairs on Central Atom	Electron Pair Geometry	Molecular Geometry	Approx. Bond Angles	Polar or Nonpolar
SO42–		4	0	Tetrahedral	Tetrahedral	~109.5o	Polar
AsF5
BrF4–
XeF4
I3–
BrF3
SnF62–
PF5
ICl2–

Banerjee

General Chemistry-I Lab

Note: BeCl2 does not satisfy the octet.

Molecules/Ions that Do Not Obey Octet Rule