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Topic 7: Part I - Bonding What is a Bond?. A force that holds atoms together. Why? We will look at it in terms of energy. Bond energy the energy required to break a bond. Why are compounds formed? Because it gives the system the lowest energy. Ionic Bonding.

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Topic 7 part i bonding what is a bond
Topic 7: Part I - BondingWhat is a Bond?

  • A force that holds atoms together.

  • Why?

  • We will look at it in terms of energy.

  • Bond energy the energy required to break a bond.

  • Why are compounds formed?

  • Because it gives the system the lowest energy.


Ionic bonding
Ionic Bonding

  • An atom with a low ionization energy reacts with an atom with high electron affinity.

  • The electron moves.

  • Opposite charges hold the atoms together.


What about covalent compounds
What about covalent compounds?

  • The electrons in each atom are attracted to the nucleus of the other.

  • The electrons repel each other,

  • The nuclei repel each other.

  • They reach a distance with the lowest possible energy.

  • The distance between is the bond length.


Topic 7 part i bonding what is a bond

Energy

0

Internuclear Distance


Topic 7 part i bonding what is a bond

Energy

0

Internuclear Distance


Topic 7 part i bonding what is a bond

Energy

0

Internuclear Distance


Topic 7 part i bonding what is a bond

Energy

0

Internuclear Distance


Topic 7 part i bonding what is a bond

Energy

0

Bond Length

Internuclear Distance


Topic 7 part i bonding what is a bond

Energy

Bond Energy

0

Internuclear Distance


Covalent bonding
Covalent Bonding

  • Electrons are shared by atoms.

  • These are two extremes.

  • In between are polar covalent bonds.

  • The electrons are not shared evenly.

  • One end is slightly positive, the other negative.

  • Indicated using small delta d.


Topic 7 part i bonding what is a bond

d+

d-

H - F


Topic 7 part i bonding what is a bond

d+

H - F

d+

H - F

d-

d-

d-

H - F

d+

d+

d-

H - F

H - F

d+

d-

d-

H - F

d+

d+

d+

H - F

H - F

d-

d-


Topic 7 part i bonding what is a bond

d+

H - F

d+

H - F

d-

d-

d-

H - F

d+

d+

d-

H - F

H - F

d+

d-

d-

H - F

d+

d+

d+

H - F

H - F

d-

d-

-

+


Topic 7 part i bonding what is a bond

d+

d-

d+

d-

H - F

H - F

d+

d-

H - F

d+

d-

d+

d-

H - F

H - F

d+

d-

d+

d-

d+

d-

H - F

H - F

H - F

-

+


Electronegativity
Electronegativity

  • The ability of an electron to attract shared electrons to itself.

  • Pauling method

  • Imaginary molecule HX

  • Expected H-X energy = H-H energy + X-X energy 2

  • D = (H-X) actual - (H-X)expected


Electronegativity1
Electronegativity

  • D is known for almost every element

  • Gives us relative electronegativities of all elements.

  • Tends to increase left to right.

  • decreases as you go down a group.

  • Noble gases aren’t discussed.

  • Difference in electronegativity between atoms tells us how polar.


Topic 7 part i bonding what is a bond

If difference is < 0.5, bond is non polar

If difference is >0.5 and <1.7, bond is polar covalent

If difference is >1.7, bond is ionic


Topic 7 part i bonding what is a bond

Polar

Covalent

Ionic

Electronegativity

difference

Bond

Type

Zero

Covalent

Covalent Character

decreases

Ionic Character increases

Intermediate

Large


Dipole moments
Dipole Moments

  • A molecule with a center of negative charge and a center of positive charge is dipolar (two poles),

  • or has a dipole moment.

  • Center of charge doesn’t have to be on an atom.

  • Will line up in the presence of an electric field.


How it is drawn

d+

d-

H - F

How It is drawn


Which molecules have them
Which Molecules Have Them?

  • Any two atom molecule with a polar bond.

  • With three or more atoms there are two considerations.

  • There must be a polar bond.

  • Geometry can’t cancel it out.


Geometry and polarity
Geometry and polarity

  • Three shapes will cancel them out.

  • Linear


Geometry and polarity1
Geometry and polarity

  • Three shapes will cancel them out.

  • Planar triangles

120º


Geometry and polarity2
Geometry and polarity

  • Three shapes will cancel them out.

  • Tetrahedral


Geometry and polarity3
Geometry and polarity

  • Others don’t cancel

  • Bent


Geometry and polarity4
Geometry and polarity

  • Others don’t cancel

  • Trigonal Pyramidal


Partial ionic character
Partial Ionic Character

  • There are probably no totally ionic bonds between individual atoms.

  • Calculate % ionic character.

  • Compare measured dipole of X-Y bonds to the calculated dipole of X+Y- the completely ionic case.

  • % dipole = Measured X-Y x 100 Calculated X+Y-

  • In the gas phase.


Topic 7 part i bonding what is a bond

75%

% Ionic Character

50%

25%

Electronegativity difference


How do we deal with it
How do we deal with it?

  • If bonds can’t be ionic, what are ionic compounds?

  • And what about polyatomic ions?

  • An ionic compound will be defined as any substance that conducts electricity when melted.

  • Also use the generic term salt.


The covalent bond
The Covalent Bond

  • The forces that cause a group of atoms to behave as a unit.

  • Why?

  • Due to the tendency of atoms to achieve the lowest energy state.

  • It takes 1652 kJ to dissociate a mole of CH4 into its ions

  • Since each hydrogen is hooked to the carbon, we get the average energy = 413 kJ/mol


Topic 7 part i bonding what is a bond

  • CH3Cl has 3 C-H, and 1 C - Cl

  • the C-Cl bond is 339 kJ/mol

  • The bond is a human invention.

  • It is a method of explaining the energy change associated with forming molecules.

  • Bonds don’t exist in nature, but are useful.

  • We have a model of a bond.


Bond energy and enthalpy
Bond Energy and Enthalpy

  • Bond energy can be used to calculate approximate energies for reactions.

  • H2(g) + F2(g)  2HF(g)

  • This reactions breaks one H-H and one F-F bond and forms two H-F bonds.

    • Energy terms associated with bond breaking have positive signs. Energy is added (endothermic).

    • Energy terms associated with bond forming have negative signs. Energy is released (exothermic).


Topic 7 part i bonding what is a bond

  • ∆H = sum of energies required to break bonds (+ signs) plus the sum of energies released (- signs) in forming new bonds.

  • ∆H = ∑D (bonds broken) - ∑D (bonds formed)

  • D is the bond energy per mole of bonds and is always positive.

  • ∆H = DH-H + DF-F – 2DH-F

    • (432 kJ) + (154 kJ) – 2(565 kJ)

    • = -544 kJ


Topic 7 part i bonding what is a bond

  • Ex. 8.5 plus the sum of energies released (- signs) in forming new bonds.

  • Using the bond energies listed in Table 8.4, calculate ∆H for the reaction of methane with chlorine and fluorine to give Freon-12 (CF2Cl2)

  • CH4(g) + 2Cl2(g) + 2F2(g)  CF2Cl2(g) + 2HF(g) + 2HCl(g)


Localized electron model
Localized Electron Model plus the sum of energies released (- signs) in forming new bonds.

  • Simple model, easily applied.

  • A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms.

  • Three Parts to the Model

  • Valence electrons using Lewis structures

  • Prediction of geometry using VSEPR

  • Description of the types of orbitals (Chapt 9)


Lewis structure
Lewis Structure plus the sum of energies released (- signs) in forming new bonds.

  • Shows how the valence electrons are arranged.

  • One dot for each valence electron.

  • A stable compound has all its atoms with a noble gas configuration. (octet rule)

  • Hydrogen follows the duet rule.

  • The rest follow the octet rule.

  • Bonding pair is the one between the symbols.


Rules
Rules plus the sum of energies released (- signs) in forming new bonds.

  • Sum the valence electrons.

  • Use a pair to form a bond between each pair of atoms.

  • Arrange the rest to fulfill the octet rule (except for H and the duet).

  • H2O

  • A line can be used instead of a pair.


Exceptions to the octet
Exceptions to the octet plus the sum of energies released (- signs) in forming new bonds.

  • BH3

  • Be and B often do not achieve octet

  • Have less than and octet, for electron deficient molecules.

  • SF6

  • Third row and larger elements can exceed the octet

  • Use 3d orbitals?

  • I3-


Exceptions to the octet1
Exceptions to the octet plus the sum of energies released (- signs) in forming new bonds.

  • When we must exceed the octet, extra electrons go on central atom.

  • ClF3

  • XeO3

  • ICl4-

  • BeCl2


Resonance
Resonance plus the sum of energies released (- signs) in forming new bonds.

  • Sometimes there is more than one valid structure for an molecule or ion.

  • NO3-

  • Use double arrows to indicate it is the “average” of the structures.

  • It doesn’t switch between them.

  • NO2-

  • Localized electron model is based on pairs of electrons, doesn’t deal with odd numbers.


Vsepr
VSEPR plus the sum of energies released (- signs) in forming new bonds.

  • Lewis structures tell us how the atoms are connected to each other.

  • They don’t tell us anything about shape.

  • The shape of a molecule can greatly affect its properties.

  • Valence Shell Electron Pair Repulsion Theory allows us to predict geometry


Vsepr1
VSEPR plus the sum of energies released (- signs) in forming new bonds.

  • Molecules take a shape that puts electron pairs as far away from each other as possible.

  • Have to draw the Lewis structure to determine electron pairs.

  • bonding

  • nonbonding lone pair

  • Lone pair take more space.

  • Multiple bonds count as one pair.


Vsepr2
VSEPR plus the sum of energies released (- signs) in forming new bonds.

  • The number of pairs determines

    • bond angles

    • underlying structure

  • The number of atoms determines

    • actual shape


Vsepr3

Electron plus the sum of energies released (- signs) in forming new bonds.

pairs

Bond

Angles

Underlying

Shape

2

180°

Linear

3

120°

Trigonal Planar

4

109.5°

Tetrahedral

90° &

120°

Trigonal Bipyramidal

5

6

90°

Octagonal

VSEPR


Actual shape
Actual shape plus the sum of energies released (- signs) in forming new bonds.

Non-BondingPairs

ElectronPairs

BondingPairs

Shape

2

2

0

linear

3

3

0

trigonal planar

4

4

0

tetrahedral

4

3

1

trigonal pyramidal

4

2

2

bent


Actual shape1
Actual Shape plus the sum of energies released (- signs) in forming new bonds.

Non-BondingPairs

ElectronPairs

BondingPairs

Shape

5

5

0

trigonal bipyrimidal

5

4

1

See-saw

5

3

2

T-shaped

5

2

3

linear


Topic 7 part i bonding what is a bond

Actual Shape plus the sum of energies released (- signs) in forming new bonds.

Non-BondingPairs

ElectronPairs

BondingPairs

Shape

6

6

0

Octahedral

6

5

1

Square Pyramidal

6

4

2

Square Planar

6

3

3

T-shaped

6

2

1

linear


Topic 7 part i bonding what is a bond

The five electronic arrangements plus the sum of energies released (- signs) in forming new bonds.

All electron pairs are bonded !


Topic 7 part i bonding what is a bond

All the plus the sum of energies released (- signs) in forming new bonds.molecular shapes…..

V-shaped

These shapes contain bonded and lone electron pairs !


Vsepr theory
VSEPR Theory plus the sum of energies released (- signs) in forming new bonds.

Two electron pairs around the central atom - both bonded.

LINEAR


Vsepr theory1
VSEPR Theory plus the sum of energies released (- signs) in forming new bonds.

Three electron pairs around the central atom – all bonded.

TRIGONAL PLANAR


Vsepr theory2
VSEPR Theory plus the sum of energies released (- signs) in forming new bonds.

Four electron pairs around the central atom – all bonded.

TETRAHEDRAL


Vsepr theory3
VSEPR Theory plus the sum of energies released (- signs) in forming new bonds.

Five electron pairs around the central atom – all bonded.

Trigonal bipyramidal


Vsepr theory4
VSEPR Theory plus the sum of energies released (- signs) in forming new bonds.

Six electron pairs around the central atom – all bonded.

OCTAHEDRAL


No central atom
No central atom plus the sum of energies released (- signs) in forming new bonds.

  • Can predict the geometry of each angle.

  • build it piece by piece.


How well does it work
How well does it work? plus the sum of energies released (- signs) in forming new bonds.

  • Does an outstanding job for such a simple model.

  • Predictions are almost always accurate.

  • Like all simple models, it has exceptions.