Announcements– Final Exam

• TIME: October 8, 7:30 - 9:30AM • VENUE: CTC 105• 65-Multiple Choice Questions

– 3 Questions Each Chapter 2-5– 7 Questions Each Chapter 6-8– 30 Questions From Chapter 9-11

– Saturday Review C-109 1-3PM– RESURRECTION: There is talk among the

instructors to offer 45 bonus points to anyone who scores 55/65 on the Exam. Any F would automatically turn to a D.

Electronegativity is an element’s inherent property to draw electrons to itself when chemically bonded to another atom in a molecule. The units are dimensionless (all relative measurements to Li).

RankFONClBr

Differences in elements electronegativity between bonding atoms result in the formation of polar-covalent bonds and net dipole moments in molecules.

Net Dipole MomentNo Net Dipole Moment

Polar BondPolar Bond

Polar Bond Polar Bond

Think of the dipole moment as a molecule with separated charges + and -.

For a poly-atomic molecule we must consider the vector sum of polar bonds in the molecule to see if there is a net dipole moment.

No NetDipoleMoment

DipoleMoment

DipoleMoment

DipoleMoment

No NetDipoleMoment

VSEPRT explains the geometry of molecules but NOT how covalent bonds are formed with that geometry.

Molecular formula

Lewis structure

VSEPRTGeometry

Hybrid orbitals

VSEPRT Valence BondTheoryVSEPRTLewis Structure

Valence Bond Theory explains covalent bonding by the spatial overlap of atomic orbitals on bonding atoms and the sharing of electron pairs.

Electrons that must have opposite spins.

1s1 + 1s1

Bonding in F2

1s1 + 2p5

Bonding in H2

Bonding in HF

2p5 + 2p5

sp3

hybridized orbitals

hybridization

Bonding in carbon presents a problem as combining pure atomics orbitals does not conform to experiment or explain bonding in C. VBT solves this by allowing the blending of pure atomic orbitals in a proces called hybridization.

Pure atomic orbitals (valence orbitals)

sp3

hybridized orbitals

hybridization

For example, if we look at the ground state of carbon we would expect 2 possible bonds in its 2p oribitals. This molecule is not observed, however, what we see is CH4 and tetrahedral geometry. We rationalize this by making an sp3 hybrid as shown by the “excited state” carbon. It can form 4-bonds.

Pure atomic orbitals (valence orbitals)we expect 2 bonds: CH2 but this

does not exist.

By combing different numbers of atomic orbitals we can make “different hybrids” that match one of the VSEPRT geometries. For example one sp pure orbital + 1 p-orbital combine to give and two “sp hybrids” that are linear when superimposed

s-orbital + p-orbital ----> Two sp hybrids = Linear

s-orbital + Two p-orbital --> Three sp2 hybrids = Trig Planar

s-orbital + Three p-orbitals -> Four sp3 hybrids = Tetrahedral

sp3 hybrid orbitals

The process of combining pure atomic orbitals to form “hybrid orbitals” on central bonding atoms in a molecule is called hybridization.

1. The number of hybrid orbitals obtained equals the number of atomic orbitals mixed.2. The type of and shape of a “hybrid orbital” varies with the types of atomic orbitals mixed.

3. Each hybrid orbital has a specific geometry that matches one of five VSEPRT shapes (show below).

sp3d2

Octahedral

sp sp2 sp3 sp3d

Linear TrigonalPlanar

Tetrahedral Trigonal Bipyramidal

Some generalized rules and comments on VBT and the formation of hybridized orbitals.

sp3d2

Octahedral

sp sp2 sp3 sp3d

Linear TrigonalPyramidal

Tetrahedral Trigonal Bipyramidal

Molecular formula

Lewis structure

VSEPRTGeometry

Hybrid orbitals

The goal is to understand geometry (via VSEPRT) and to relate it to a picture of covalent bonding in molecules.

ElectronGeometry

Molecular Geometry AXnEm Hybridization

Linear Linear AX2 spTrigonal planar

Trigonal planar V-shaped bent

AX3

AX2E1sp2

Tetrahedral

Tetrahedral Trigonal pyramidal

V-shaped bent

AX4

AX3E1 AX2E2

sp3

Trigonal bipyramidal

Trigonal bipyramidalSeesaw

T-shaped Linear

AX5

AX4E1

AX3E2

AX2E3

sp3d

OctahedralOctahedral

Square pyramidal Square planar

AX6

AX5E1

AX4E2

sp3d2

Atomic OrbitalsMixed

Hybrid OrbitalsFormed

HybridShape

Linear AX2

Trig Planar AX3

Tetrahedral AX4

Trig Bypyr AX5

Octahedral AX6

s + p s + 2 p s + 3 p s + 3 p + d s + 3 p + 2d

Two sp Three sp2

Four sp3 Five sp3d Six sp3d2

UnhybridOrbitals Leftover

Two p one p none Four d Three d

Linking VSEPRT To Valence Bond Theory Hybrids

2s

--The number of hybrid orbitals formed is equal to the number of “pure orbitals” combined!

--When superimposed the “sp-hybrid” give us bonding orbitals for a linear molecules.

An sp hybrid is formed from the combination of a one pure 1s orbital and a one 2p orbital from a central bonding atom producing two new orbitals called sp orbitals.

s-orbital p-orbitalTwo sp hybrid orbitals

sp hybrid orbitals superimposed

Hybridization

Hybridization

Show the bonding scheme and hybridized orbitals used in BeCl2

2 unhybridized p-orbitals

So after hybridization we have on the central atom, 2 pure p-orbitals and two sp hybrids.

hybridization

Isolated Be Atom

Hybridized Be Atom

Show the bonding scheme and hybridized orbitals in BeCl2

two sp hybrids on Be

two lone p-orbitals

sp hybrid:Ethylyne: HC≡CH:Linear

sp hybrid orbitals

Lone p orbitals that are not hybridized

Sigma bonds (σ bonds) and Pi bonds (π bonds)are two different types of covalent chemical bonds that form as a result of end to end spatial overlap of atomic orbitals or hybridized orbitals (σ bonds) or side to side overlap on bonding atoms (π bonds)

sp2 = Triginal planar geometry, 120˚ bond angle

3-atomic orbitals, s and two p’s combine to form 3-sp2 hybrid orbitals

An sp2 hybrid is formed from the combination of a one pure 1s orbital and a two 2p orbitals from a central bonding atom producing two new orbitals called sp2 orbitals.

Example 2: sp2 hybridizaton scheme BF3.

Boron Orbital Box Diagram

Boron Hybrid Box Diagram

Bonding of pure p-orbital in F with sp2 hybridized orbitals in BF3

Tetrahedral geometry = sp3 hybrid orbitals

sp3 = Tetrahedral geometry = 109.5˚ bond angle

Note the number of hybrids formed is the number of atomic orbitals combined!

combine to generatefour sp3 orbitals

which are representedcollectively as: sp3

Example: sp3 orbital hybridization: CH4.

the four sp3 hybrid orbitals form a tetrahedral shape

sp3 hybridization mixes one 2s orbital with three 2p orbitals to produce four sp3 orbitals on each carbon atom. End to end overlap with a 1s orbital from H gives four sigma bond in CH4.

CH4

This is the ground stateconfiguration of valence atomic orbitals

Example 3: sp3 hybrid orbitals in H2O.

What is the electronic geometry?What is the molecular geometry?What orbitals contribute to bonding?

Note the lone pairs occupy 2-of the sp3 orbitals

sp3 hybridization mixes one 2s orbital with three 2p orbitals to produce four sp3 orbitals. The e- are distributed throughout the hybrids ready for bonding. End to end overlap with a 1s orbital from H gives four sigma bond in CH4.

sp3 is tetrahedral shape. In water we have AX2E2

What is the electron geometry, the molecular geometry at each carbon atom? Use that information to determine the hybridization around each carbon atom in nicotinic acid? How many sigma and pi bonds are in nicotinic acid?

sp3d hybridization in PCl5.

Isolated P atom

Trigonal Bipyramidal Electron Geometry AX5E0Trigonal BiPyramidal Molecular Geometry

The sp3d2 hybrid orbitals in SF6Octahedral Electron Geometry AX6E0Octahedral Molecular Geometry

ElectronGeometry

Molecular Geometry AXnEm Hybridizaton

Linear Linear AX2 spTrigonal planar

Trigonal planar V-shaped bent

AX3

AX2E1sp2

Tetrahedral

Tetrahedral Trigonal pyramidal

V-shaped bent

AX4

AX3E1 AX2E2

sp3

Trigonal bipyramidal

Trigonal bipyramidalSeesaw

T-shaped Linear

AX5

AX4E1

AX3E2

AX2E3

sp3d

OctahedralOctahedral

Square pyramidal Square planar

AX6

AX5E1

AX4E2

sp3d2

Determine the VSEPRT geometry, the bond angles and the hybridization of each indicated atom in the following molecule? How many sigma and pi bonds are in the molecule?

Determine the electron domain, molecular geometry, the bond angles and the hybridization of each indicated atom in the following molecule? How many sigma and pi bonds are in the molecule?

tetrahedral, 180, sp3

sp3

sp2

sp

sp2

bent, <109.5, sp3

trig planar 120˚, sp2

linear 180˚, sp

Describe the types of bonds and orbitals in acetone, (CH3)2CO and in CO2 and in HCN?

Molecular formula

Lewis structure

VSEPRTGeometry

Hybrid orbitals

Step 1 Step 2 Step 3

Describe the types of bonds and orbitals in acetone, (CH3)2CO.PLAN:

Draw the Lewis structures to ascertain the arrangement of groups and shape at each central atom. Postulate the hybrid orbitals taking note of geometries predicted from VSEPRT. Draw the orbitals and show overlap.

SOLUTION:

sp3 hybridized

sp3 hybridized

sp2 hybridized

σ bondsπ bond

Postulating Hybrid Orbitals in a Molecule

PROBLEM: Use partial orbital diagrams to describe mixing of the atomic orbitals of the central atom leads to hybrid orbitals in each of the following:

PLAN: Use the Lewis structures to ascertain the arrangement of groups and shape of each molecule. Postulate the hybrid orbitals. Use partial orbital box diagrams to indicate the hybrid for the central atoms.

(a) Methanol, CH3OH (b) Sulfur tetrafluoride, SF4

SOLUTION: (a) CH3OH The groups around C are arranged as a tetrahedron.O also has a tetrahedral arrangement with 2 nonbonding e- pairs.

Postulating Hybrid Orbitals in a Molecule

(a) Methanol, CH3OH

SOLUTION: (a) CH3OH The groups around C are arranged as a tetrahedron.O also has a tetrahedral arrangement with 2 nonbonding e- pairs.

single C atom hybridized C atom single O atom hybridized

O atom

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Postulating Hybrid Orbitals in a Molecule

(b) SF4 has a seesaw shape with 4 bonding and 1 nonbonding e- pairs.

S atomhybridized

S atom

Bond order is the number of bonds between two bonded atoms.

S C NS

F

F

F

FF

F

N N

Bond order = 3C-N: Bond order = 2

S-C: Bond order = 2

S-FBond order = 1

– Single bond between 2 atoms = order = 1– Double bond between 2 atoms = order = 2– Triple bond between 2 atoms = order = 3

Higher bond orders give shorter bond lengths and require more energy to break a bond.

Bond LengthsTriple bond < Double Bond < Single Bond

Note how bond energies (energy required to break a bond) goes up as bond order increases.