VESPR Theory

VSEPR Theory

VSEPR Theory (Valence Shell Electron Pair Repulsion Theory)

A model for describing the shapes of molecules whose main postulate is that the structure around a given atom is determined by minimizing the electron pair repulsion

Therefore, the electrons and elements bonded to the central atom want to be as far apart as possible

VSEPR Steps

1. Draw the Lewis structure for the molecule

2. Count the total number of things that are around the central atom to determine the electron pair geometry

3. Imagine that the lone pairs of electrons are invisible and describe the molecular shape

Summary VSEPR and Hybridization Table

Electron Domains

Electron Domain

Geometry

PredictedBond Angle(s)

Hybridization of Central

Atom

Molecular Geometry

0 Lone Pair

1 Lone Pair 2 Lone Pair

2 Linear 180º sp Linear

3 Trigonal Planar 120º sp2 Trigonal Plan

ar

Bent

4 Tetrahedral 109.5º sp3 Tetrahedral

Trigonal Pyramidal

Bent

5 Trigonal Bipyrami

dal

90º, 120º sp3d Trigonal Bipyramidal

Seesaw T-shaped

6 Octahedral 90º sp3d2 Octahedral Square Pyramidal Square Planar

2 Electron Pairs

If there are 2 things attached to the central atom, the shape is linear

Bond angle = 180° Hybridization = sp

3 Electron Pairs

If there are 3 electron pairs the shape will be trigonal planar

Bond angle = 120°Hybridization =sp2

3 electron pairs

Now imagine that you have 3 electron pairs, but one is just a lone pair (invisible) what would it look like then?

4 electron pairs

If there are 4 electron pairs, the shape will be tetrahedral

Bond angle = 109.5°Hybridization = sp3

4 electron pairs

What if 1 of the electron pairs is a lone pair (invisible)? What would it look like then?

Trigonal Pyramidal

4 electron pairs

What if there are 2 lone pairs (invisible)? What would it look like then?

bent

5 electron pairs

If there are 5 electron pairs the shape will be Trigonal Bipyramidal

Bond angles = 90º & 120º Hybridization = sp3d

5 electron pairs

What is there is 1 lone pair (invisible)

Seesaw

5 electron pairs

What is there are 2 lone pairs (invisible)

T-shaped

6 electron pairs

If there are 6 electron pairs the shape will be octahedral

Bond angle = 90°Hybridization = sp3d2

6 electron pairs

What if there is 1 lone pair (invisible)?

Square pyramidal

6 electron pairs

What if there are 2 lone pairs (invisible)

Square planar

Formal Charge

Formal charges can be used in 1 of 2 ways…

1. Suggest where the charges are

2. Help select the most plausible structure from a set of resonance structures

1 - Suggest where the charges are

Formal charge =

e bonding of #21e bondingnon of #- veof #

Example

Calculate the formal charge on each element in the carbonate ion

CO3 2-

Example

Example

Example

The sum of the formal charges of the individual charges equals the formal charge on the molecule or ion

The formal charge for carbonate =0 + 0 + -1 + -1 = -2

2 - Help select the most plausible structure from a set of resonance structures

When choosing the most likely resonance structure

Most likely – All formal charges are zero Next likely – All formal charges add up to zero Next likely – Formal charges add up to the

lowest possible charge Next likely – Negative charge is on most

electronegative atom

Example

Which of the following resonance structures is most likely for CH2O and why?

Example

Another Example

Which is the most likely structure for N2O?

Another example

Polar bonds & polar molecules(Dipole or non dipole)

In order for a substance to be polar, the bonds within the molecule must carry different charges and cannot cancel out due to symmetry

Polar or non polar

CHF3

CO2

BCl3CH4

H2O

Polar or non polar

Polar or non polar

Rule for solubility

Like dissolves likePolar will dissolve in polarNon polar will dissolve in non polar

Bonding

Intramolecular forces – bonding within molecule (ionic or covalent)

Intermolecular forces – bonding between molecules

Intermolecular Bonding

2 factors determine if a substance is a solid, liquid, or a gas:

1. Kinetic energy

2. Intermolecular forces holding the particles together

Intermolecular Bonding

Interactions between molecules

Hydrogen bonding(H attached to N, O,

or F)

Van der Waals forces(attractions between

Dipoles – permanent or Induced)

Dipole – Dipole(polar molecules)

Permanent dipoles

London Dispersion Forces(non polar molecules)

Induced dipoles

Hydrogen bonding

H is special when it bonds with another element.

The electron is on one side leaving an exposed nucleus

An approaching charged group can get very close the H nucleus creating a lrge electrostatic attraction

These attractions are especially large when H is bonded to a highly electronegative atom like F, O, or N

Hydrogen bonding

These bonds are called Hydrogen bonds

They are VERY strong leading to 1. High boiling points

2. Viscous

Van der Waals Forces

Dipole – Dipole Remember, dipoles mean that the molecule has a

partial positive & a partial negative charges at one end

This has a significant effect only when the molecules are close together

The partial positive and partial negative will attract These attractions are called dipole dipole

attractions These come from polar molecules ONLY!!!

London Dispersion forces

Small electrostatic forces caused by the movement of the electron in molecules that n=have no permanent dipole

In all molecules – polar & non polarHeavier atoms stronger LDF because

valence electrons are further apart in larger molecules & held less tightly so they can more easily form temporary dipoles

What type of intermolecular forces are present?

Ar HCl HF CaCl2 CH4

CO NaNO3

LDF DD, LDF HB, LDF Ionic LDF DD, LDF Ionic

Which will have the …

Highest boiling point… HBr, Kr, Cl2 HBr (DD, LDF) Highest freezing point…H2O, NaCl, HF NaCl (ionic) Lowest freezing point…N2, CO, CO2

N2 (smallest non polar present LDF only) Lowest boiling point…CH4, CH3CH3, CH3CH2CH3

CH4 (smallest nonpolar, LDF only) Highest boiling point…HF, HCl, HBr HF (Hydrogen bonding)

More examples

At 25C ONF is a gas where H2O is a liquid. Why?

H2O forms H bonds which are stronger than the dipole dipole forces in ONF

At 25C Br2 is a liquid when Cl2 is a liquid. Why?

Both have only LDF, but since Br2 is heavier, the LDF are greater