Vsepr

VSEPR

Valence Shell Electron Pair Repulsions

Covalent Bond: A Model

• Chemical bonds can be viewed as forces that cause a group of atoms to act as a unit

• They result from the tendency of a system to seek its lowest possible energy

• Bonds occur when collections of atoms are more stable (lower in energy) than the separate atoms

Note: The next three slides will repeat at the end. This is preliminary intro info that may make more sense at the end.

Example: Methane

• 1652 kJ of energy are required to break a mole of methane into separate C and H atoms

• OR 1652 kJ of energy are released when one mole of methane is formed from one mole of C atoms and four moles of H atoms

• Methane is therefore a stable molecule relative to its stable atoms

• Since there are four H atoms arranged around the central C, it is natural to envision four individual attractions between C and H (bonds)

• Each bond has an associated bond energy, found by dividing the total energy by four (1652/4 = 413 kJ)

• The positive Bond Energy value indicates the energy required to break the bond between C and H atoms

Bonding Model• Models originate from our observations of

the properties of nature• Atoms can form stable groups by sharing

electrons, shared electrons give a lower energy state because simultaneously attracted to two nuclei

• Remember: Models are human inventions that allow us to explain and predict. A model is a useful way of thinking; they include simplifications and assumptions. A model does not equal reality.

Covalent Bonds

• Electron pair(s) – shared between two atoms– attracted to both nuclei

• Location of a single shared pair– Directly between two nuclei– Maximizes attractions with shortest distance between

two positive nuclei– Minimizes repulsions with negative electrons between

positive nuclei that would repel one another

Multiple covalent bonds around the same atom determine the shape

• Negative e- pairs with same charge repel each other

• Repulsions push the pairs as far apart as possible

Single bonds

• Sigma bond ө• Overlap of orbitals allow electron pair to be

shared between the two atoms

• Electron pair shared directly between two nuclei

• Only one pair may be shared in this space - just as only one pair of electrons may occupy a single atomic orbital

Double and Triple Bonds

• Pi bonds π• Since the space between the nuclei is

occupied, e- pair is shared above and below the plane or front and back

• Overlap of p-orbital lobes allow for this sharing above and below OR front and back

Lewis Structures

• Drawn to show the bonds between the atoms in the structure

• Only shows whether single, double or triple bonds

• Does not show the shape

Lewis Structure

• Represents the arrangement of valence electrons among atoms in the molecule

• Rules based upon observations of thousands of molecules, which show that in most stable compounds the atoms achieve noble gas configurations

• Duet Rule – hydrogen stable with only a pair of e-

• Octet Rule – other atoms stable with 4 pairs of e-

Rules for Drawing Lewis Structures

1) Count the number of valence electrons2) Draw the skeleton structure- the central is

generally listed first in formula3) Distribute electrons to give each atom a stable

octet4) Reconcile # e-

a) Do you have enough electrons? You may need to use double or triple bonds.

b) Do have too many electrons? You may need to explain the octet, but only if empty d-orbital available

Determine # Valence e- from column #

Electron Clouds repel each other, thus structure around an atom is determined

principally by minimizing repulsions

2 electron pairs (2 EP) around central atom

• Two clouds pushed as far apart as possible– Greatest angle possible 180º– LINEAR shape


• Three clouds pushed as far apart as possible– Greatest angle possible 120º– TRIGONAL PLANAR shape

(3) (flat)


• Four clouds pushed as far apart as possible– Greatest angle no longer possible

in two dimensions– Requires three-dimensional– TETRAHEDRAL shape

Orbital Hybridization #1

• Atomic orbitals such as s and p are not well suited for overlapping and allowing two atoms to share a pair of electrons

• Remember: best location of shared pair is directly between two atoms

• e- pair spends little time in best location– With overlap of two s-orbital– With overlap of two p-orbitals


• Hybrid orbitals (cross of atomic orbitals)• Remember: The pink flower hybrid cross

of the red and white flower• Hybrid orbitals

– Shape more suitable for bonding• One large lobe and one very small lobe• Large lobe oriented towards other nucleus

– Angles more suitable for bonding• Angles predicted from VSEPR

Overlap of two s-orbitals

NOT A GOOD LOCATION-

far from one nucleus

Note: shared in this overlap the e- pair would spend most of the time in an unfavorable location

GOOD SPOT between both nuclei

Overlap of two p-orbitals

One atom & its p-orbital The other atom & its p-orbital

represents the nucleus

BAD location far from other nucleus GOOD SPOT

between both nuclei

Hybrid Orbitals yield more favorable shape for overlap

• Atomic orbitals are not shaped to maximize attractions nor minimize repulsions

• Hybrid orbital shape – One large lobe oriented towards other atom– Notice the difference in this shape compared

to p-orbital shape

Angles and Shape

• Atomic orbitals are not shaped to maximize attractions nor minimize repulsions

• BUT the angles are also not favorablep-orbitals are oriented at 90º to

each otherOther angles are required 180º,

120º or 109.5º


• Each e-pair requires a hybrid orbital• If two hybrid orbitals required than two atomic

orbitals must be hybridized, an s and a p orbital forming two sp orbitals at 180º

sp hybrids

2 EP 4 EP3 EP

sp2 hybrids sp3 hybrids

Electron-Pair Geometryvs

Molecular Geometry

• Electron-pair geometry– Where are the electron pairs– Includes

• bonding pairs (BP) – shared between 2 atoms• nonbonding pairs (NBP) – lone pair

• Molecular geometry– Where are the atoms– Includes only the bonding pairs

Examples of 3 EP• 3 BP + 0 NBP = 3 EP

– 3 EP = EP geom is trigonal planar– All locations occupy by an atom, – so molecular geometry is also trigonal planar

• 2 BP + 1 NBP = 3 EP– 3 EP = EP geom is trigonal planar– Only two bonding pairs– One of the locations is only lone pair of e-– so molecular geometry is bent

O

O O

N

O O

O

Carbonate Ion (CO32-)

Nitrate Ion (NO3-)

Nitrite Ion (NO2-)

Examples of 4 EP• 4 BP + 0 NBP = 4 EP

– Both EP geom and molecular geom– tetrahedral

• 3 BP + 1 NBP = 4 EP– 4 EP so EP geom is tetrahedral– Molecular geom is TRIGONAL PYRAMIDAL – No atom at top location

• 2 BP + 2 NBP = 4 EP– 4 EP so EP geom is tetrahedral– Molecular geom is BENT – no atoms at two locations

4 BP + 0 NBP = 4 EP

TETRAHEDRAL

Cl Cl

Cl

Cl

S

3 BP + 1 NBP = 4 EP

TRIGONAL PYRAMIDAL

NH H

H

●●

lone pair of e-

NBP

H H

H

N

107

2 BP + 2 NBP = 4 EP

BENT

O H

H

●●

lone pair of e-

NBP H

H

O

●●

lone pair of e-

NBP

104.5

H

Hydronium Ion (H3O+)

Ammonia Molecule (NH3)

Summary of 4 EP

Exceptions to Octet Rule

• Reduced Octet – H only forms one bond- only one pair of e-– Be tends to only form two bonds

• only two pair of e-

– B tends to only form three bonds• only three pair of e-

• Expanded Octet– Empty d-orbitals can be used to

accommodate extra e-– Elements in the third row and lower can expand– Up to 6 pairs of e- are possible

Lewis Structures in Which the Central Atom Exceeds an Octet

5 EP

Trigonal bipyramidal

Orbital hybridizationRequires 5 hybrid orbitals

So, 5 atomic orbitals required

sp3d

Trigonal planar shape

5 EP = trigonal pyramidal

molecular geometry5 BP + 0 NBP = 5 EP 4 BP + 1 NBP = 5 EP

5 EP = trigonal pyramidal

molecular geometry 3 BP + 2 NBP = 5 EP 2 BP + 3 NBP = 5 EP

6 EP

Octahedral

Orbital hybridizationRequires 5 hybrid orbitalsSo, 5 atomic orbitals requiredsp3d

6 EP = octahedral

6 BP

+0 NBP

6 EP

6 EP = octahedral

5 BP+1 NBP 6 EP

6 EP = octahedral

4 BP+2 NBP 6 EP

Summary: Molecular Geometry of Expanded Octets

Summary of EP Geometry

2 EP 3 EP 4 EP 5 EP 6 EP

Summary of EP Geometry

Predict the geometry, angles and orbital hybridization

Predict the geometry, angles and orbital hybridization

Covalent Bond: A Model

• Chemical bonds can be viewed as forces that cause a group of atoms to act as a unit

• They result from the tendency of a system to seek its lowest possible energy

• Bonds occur when collections of atoms are more stable (lower in energy) than the separate atoms

Example: Methane

• 1652 kJ of energy are required to break a mole of methane into separate C and H atoms

• OR 1652 kJ of energy are released when one mole of methane is formed from one mole of C atoms and four moles of H atoms

• Methane is therefore a stable molecule relative to its stable atoms

• Since there are four H atoms arranged around the central C, it is natural to envision four individual attractions between C and H (bonds)

• An average bond energy associated with each bond is found by dividing the total energy by four (1652/4 = 413 kJ)

• The positive Bond Energy value indicates the energy required to break the bond between C and H atoms

Bonding Model• Models originate from our observations of

the properties of nature• Atoms can form stable groups by sharing

electrons, shared electrons give a lower energy state because simultaneously attracted to two nuclei

• Remember: Models are human inventions that allow us to explain and predict. A model is a useful way of thinking; they include simplifications and assumptions. A model does not equal reality.

Bond Energy and Enthalpy

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