Bonding / Molecular ShapesBonding / Molecular ShapesVSEPRVSEPR

Dr. Ron RusayDr. Ron Rusay

Ionic & Covalent BondsIonic & Covalent Bonds

When aWhen a nonmetalnonmetal and aand a metal metal form a form a compound: Valence electrons of the compound: Valence electrons of the metalmetal are are lostlost and the and the nonmetal gainsnonmetal gains these these electrons to achieve a Noble gas electron electrons to achieve a Noble gas electron configuration and forming an configuration and forming an Ionic BondIonic Bond..

When twoWhen two nonmetals nonmetals form a compound: form a compound: They They share share electrons to achieve a Noble gas electrons to achieve a Noble gas electron configuration and forming a electron configuration and forming a Covalent Covalent BondBond..

• Ionic compounds are formed when electron(s) are transferred. • Electrons go from less electronegative element to the more electronegative forming ionic bonds.

Ionic Compounds

Bond EnergyBond Energy

It is the energy required to break a It is the energy required to break a bond, i.e. overcome the force of bond, i.e. overcome the force of attraction.attraction.

The quantitative value provides The quantitative value provides information about the strength and information about the strength and nature of the bond.nature of the bond.

Bond EnergiesBond Energies

Bond Bond breakingbreaking requires energy requires energy (endothermic).(endothermic).

Bond Bond formationformation releases energy releases energy (exothermic).(exothermic).

HHrxnrxn = = H(bonds broken) H(bonds broken) H(bonds H(bonds

formed)formed)

Lattice EnergyLattice EnergyHess’s Law for ionic crystalline solidsHess’s Law for ionic crystalline solids

The change in energy when The change in energy when gaseous gaseous ions ions pack together to form an pack together to form an ionic solidionic solid..

MM++(g) + X(g) + X(g) (g) MX(s) MX(s)

Lattice energy is a Lattice energy is a negativenegative value value (exothermic).(exothermic).

How can the Lattice energy be calculated?How can the Lattice energy be calculated?

ElectronegativityElectronegativity

The ability of an atom in a The ability of an atom in a molecule to attract shared electrons molecule to attract shared electrons to itself.to itself.

= (H = (H X) X)actualactual (H (H X) X)expectedexpected

ElectronegativityElectronegativity

QUESTIONQUESTION

ANSWERANSWER .

D) ionic bonds

If two atoms have greatly differing electronegativities the more electronegative atom will pull on the bonding electrons so strongly the electrons will transfer from one atom to the other.

Covalent Bond PolarityCovalent Bond Polarity

A molecule, such as HF, that has a A molecule, such as HF, that has a center of positive charge and a center of center of positive charge and a center of negative charge is said to be negative charge is said to be polarpolar, or to have , or to have a a dipole momentdipole moment..δ+ δ−FH

Lewis StructureLewis Structure

Shows how valence electrons are Shows how valence electrons are arranged among atoms in a molecule.arranged among atoms in a molecule.

Reflects central idea that stability of a Reflects central idea that stability of a compound relates to noble gas electron compound relates to noble gas electron configuration.configuration.

Shows bonds in molecules and relates Shows bonds in molecules and relates to 3-dimensional shapes in structuresto 3-dimensional shapes in structures

G.N. LewisPhoto Bancroft Library, University of California/LBNL Image Library

Notes from Lewis’s notebook and his “Lewis” structure.Notes from Lewis’s notebook and his “Lewis” structure.

Footnote:G.N. Lewis, despite his insight and contributions to chemistry, was never awarded the Nobel prize.

Octet Rule: General CommentsOctet Rule: General Comments 2nd row elements C, N, O, F 2nd row elements C, N, O, F observe the octet ruleobserve the octet rule..

2nd row elements B and Be often have fewer than 8 2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive.electrons around themselves - they are very reactive.

3rd row and heavier elements 3rd row and heavier elements CANCAN exceed the octet exceed the octet rule using empty valence d orbitals.rule using empty valence d orbitals.

When writing Lewis structures, When writing Lewis structures, satisfy octets firstsatisfy octets first,, then place electrons around elements having then place electrons around elements having available d orbitalsavailable d orbitals..

QUESTIONQUESTION

ANSWERANSWER

A) N

Nitrogen does not have d orbitals to allow extra bonds to go beyond the octet rule.

Lewis Electron-Dot Symbols for Elements in Periods 2 & 3

Depicting ion formation with orbitaldiagrams and electron dot symbols

Problem: Use orbital diagrams and Lewis structures to show the formation of magnesium chloride from its ions starting with the respective atoms.

Step 1: Draw the orbital diagrams for Mg and Cl atoms.

To reach completely filled, stable electronic configurations each Mg atom loses 2 electrons, and each Cl atom gains 1 electron.

Therefore there are two Cl - ions for every one Mg 2+ ion.

Step 2: Draw the orbital diagrams for the Mg 2+ cation and 2 Cl-

anions.

2 Cl

Mg+2 + 2 Cl-

Mg

+

Mg + Cl

Cl

......

....

..Mg+2 + 2 Cl.. ....

Depicting Ion Formation with OrbitalDiagrams and Electron Dot Symbols

Remember chlorine is diatomic.Remember chlorine is diatomic.

..

....

Covalent Compounds•Share valence electrons. •1 pair = 1 bond; maximum # of atom-atom

bonds = 3.•Octet rule (“duet” for hydrogen)•Lewis structure examples:

Notice the charges: In one case they balance, can you name the compound?In the other they do not.

It has a “Formal” charge. Can you name the polyatomic ion?

QUESTIONQUESTION

ANSWERANSWER

e

D) 12

Fluorine is so electronegativ its atoms will only form single bonds. Therefore the sulfur atom must have six bonds, one to each F atom.

Important Bond Numbers(Neutral Atoms!)

H F ICl Brone bond

Otwo bonds

Nthree bonds

Cfour bonds

QUESTIONQUESTION

ANSWERANSWER

C) a triple bond between the nitrogens.

Only a triple bond will allow both atoms to complete their octet.

Lewis Structures of Simple Lewis Structures of Simple Covalently Bonded MoleculesCovalently Bonded Molecules

KClO3

CF4

CH3CH2OH Ethyl alcohol (Ethanol)

Potassium Chlorate

Carbon Tetrafluoride

CH4 Methane

Draw Lewis Structures for the following: Draw Lewis Structures for the following:

Lewis Structures of Simple Lewis Structures of Simple MoleculesMolecules

C

H

H H

H

Cl

O

O O

K+

KClO3

CF4

..

..H C O H

H

H

H

H

C

Ethyl Alcohol (Ethanol)

Potassium Chlorate Carbon Tetrafluoride

......

..

..

..

.. ...... ..

.... C

F

FF

F

......

..

.. ..

..

....

CH4

Methane

Lewis Structures of Simple Lewis Structures of Simple MoleculesMolecules

N

H

H H. .

N

H

H H

H

+

Ammonia

Ammonium Ion

C

NN

O

H H

HH

. .

. .

. .

. .

Urea

QUESTIONQUESTIONThe formaldehyde compound, also known as methanal, has been linked to indoor air pollution and related health effects. It can be used in some cases as a disinfectant and is found in some resins and glues. The correctly drawn Lewis structure of H2CO would have how many unshared electrons?

A. Zero unshared e–

B. One unshared e–

C. Two unshared e–

D. Four unshared e–

ANSWERANSWERD) is correct if the proper Lewis structure is drawn. The carbon atom shares a double bond with oxygen in this compound (in addition to two C–H bonds) This leaves four unshared e– around oxygen.

QUESTIONQUESTIONDinitrogen monoxide has several uses ranging from a dentistry anesthetic to automobile racing enhancement. Starting with two possible basic structures given here, diagram two different Lewis structures. If your first structure contains four unbonded electrons around oxygen and your second structure contains six unbonded electrons around oxygen, how many bonds would be between the N atoms in the first and second compounds?

N–N–O N–N–O

A. 2, 3B. 1, 3C. 2, 2D. 1, 2

ANSWERANSWERA) provides the correct number of bonds for both NNO

compounds. This is an example of resonance. By following the Lewis electron dot structure rules for both compounds, the first compound should have double bonds between N=N and N=O. The second compound would have a triple bond between the N atoms and a single bond between N and O.

Formal ChargeFormal Charge

Equals the number of Equals the number of valence valence electrons electrons of the free atom minus [the of the free atom minus [the number of unshared valence electrons number of unshared valence electrons in the molecule + 1/2 the number of in the molecule + 1/2 the number of shared valence electrons in the shared valence electrons in the molecule].molecule].

QUESTIONQUESTIONThe N2O molecule has sometimes found use as an aerosol propellant. One structure that satisfies the Lewis electron dot rules is N=N=O. In this case, what would be the formal charge of the middle N atom?

A. ZeroB. +1C. –1D. I am not sure how to determine the formal charge on

an atom in a compound.

ANSWERANSWERB) for this structure is correct. Add together the total unbonded e– and ½ the shared e–. Subtract this from the assigned valence number of electrons based on the group number for the element from the periodic table. For the middle N, these values would be: 5 – (0 + 1/2 ( 8)) = + 1.

0.143 nm

Visual Interpretation

Color coded electron density distribution: blue-lowest, red highest, green balanced

Determining Bond Polarity from Electronegativity Values

Draw arrows to represent the dipole moment of each using ENs:

a) the EN of O = 3.5 and of H = 2.1: O - H the EN of O = 3.5 and of Cl = 3.0: O - Cl the EN of C = 2.5 and of P = 2.1: C - P the EN of P = 2.1 and of N = 3.0: P - N the EN of N = 3.0 and of S = 2.1: N - S the EN of C = 2.5 and of Br = 2.8: C - Br the EN of As = 2.0 and of O = 3.5: As - O

b) Rank the bonds in increasing order of polarity.

Determining Bond Polarity from Electronegativity Values

Solutions:

a) the EN of O = 3.5 and of H = 2.1: O - H the EN of O = 3.5 and of Cl = 3.0: O - Cl the EN of C = 2.5 and of P = 2.1: C - P the EN of P = 2.1 and of N = 3.0: P - N the EN of N = 3.0 and of S = 2.1: N - S the EN of C = 2.5 and of Br = 2.8: C - Br the EN of As = 2.0 and of O = 3.5: As - O

b) C - Br < C - P < O - Cl < P - N < N - S < O - H < As - O 0.3 < 0.4 < 0.5 < 0.9 < 0.9 < 1.4 < 1.5

ResonanceResonance Occurs when more than one valid Occurs when more than one valid

Lewis structure can be written for a Lewis structure can be written for a particular molecule.particular molecule.

These are These are resonance structuresresonance structures. . The actual structure is an average of The actual structure is an average of the resonance structures.the resonance structures.

Resonance: Delocalized Electron-Pairs

Ozone : O3 ......

..

O O

O ........ ..OOO..

I II

O

O

O

..

........

Resonance Hybrid Structure

One pair of electron’s resonates between the two locations!!

....

Resonance and Formal ChargeResonance and Formal Charge

Not as goodNot as good Better Better

COO(-1)(0)(+1)COO(0)(0)(0)

Acetic acid Acetic acid

Complete the Lewis Structure.Complete the Lewis Structure.

Acetic acidAcetic acid

The nitrogen atom in theammonium ion has a formalcharge of +1, but the actualpositive charge of the ion isdistributed around the hydrogen atoms and the nitrogen atom has a partial negative charge.

Localized Electron ModelLocalized Electron Model

1.1. Description of valence electron arrangement Description of valence electron arrangement (Lewis structure).(Lewis structure).

2.2. Prediction of geometry (VSEPR model).Prediction of geometry (VSEPR model). 3.3. Description of atomic orbital types used to Description of atomic orbital types used to

share electrons or for lone pairs.share electrons or for lone pairs.

Coupled with molecular orbital theory, highly reliable Coupled with molecular orbital theory, highly reliable conceptual images of molecular shape can be conceptual images of molecular shape can be obtained.obtained.

QUESTIONQUESTIONBiologically the NO molecule plays several important roles in human physiology. Of particular importance and interest is its role in maintaining blood flow and pressure. A properly diagramed Lewis structure of NO would have how many e– not involved in bonding?

A. ZeroB. ThreeC. Five D. Seven

ANSWERANSWERC) or D) provides an answer when considering that NO is an odd electron species and that N and O both only have s and p orbitals available for bonding; 11 electrons total! NO is an exception to the Lewis octet rule. The answer is D) since C) would have 9 e-s total for N, a triple bond, and involve formal charges.Does NO (N=O) have to have a formal charge, either N or O, or can it be neutral?

MOLECULE of the Year:

http://chemconnections.org/Reading/Nitric%20Oxide/Science-1992-Koshland-1861.pdf

http://chemconnections.org/Reading/Nitric%20Oxide/Science-1992-Culotta-1862-5.pdf

Formal ChargeFormal Charge

Equals the number of Equals the number of valence electrons valence electrons of the of the free atom minus [the number of unshared valence free atom minus [the number of unshared valence electrons in the molecule + 1/2 the number of electrons in the molecule + 1/2 the number of shared valence electrons in the molecule].shared valence electrons in the molecule].

Adding/subtracting atoms and electrons.Adding/subtracting atoms and electrons.

Formal charge = number of valence electrons –(number of lone pair electrons +1/2 number of bonding electrons)

Formal charge = 5 – (3+1/2 4) = 0

VSEPR ModelVSEPR ModelValence Shell Electron Pair RepulsionValence Shell Electron Pair Repulsion

VSEPR ModelVSEPR Model

The molecular structure which The molecular structure which surrounds a given atom is determined surrounds a given atom is determined principallyprincipally by minimizing electron pair by minimizing electron pair repulsions through maximizing repulsions through maximizing separations.separations.

VSEPR ModelVSEPR Model

The molecular structure of a given atom is determined The molecular structure of a given atom is determined principallyprincipally by minimizing electron pair (bonded & free) by minimizing electron pair (bonded & free) repulsions through maximizing separations.repulsions through maximizing separations.

Some examples of minimizing interactions.

Some examples of minimizing interactions.

Predicting a VSEPR StructurePredicting a VSEPR Structure

1.1. Draw Lewis structure.Draw Lewis structure. Using your imagination in 3 dimensions:Using your imagination in 3 dimensions: 2.2. Put pairs as far apart as possible.Put pairs as far apart as possible. 3.3. Determine positions of atoms from the Determine positions of atoms from the

way electron pairs are shared.way electron pairs are shared. 4.4. Select the geometrical name of the Select the geometrical name of the

molecular structure from positions of the molecular structure from positions of the atoms, or……. skip 2 &3 and match to the atoms, or……. skip 2 &3 and match to the names: names:

http://chemconnections.org/VSEPR-jmol/

Linear Linear LinearLinear

Trigonal PlanarTrigonal Planar Trigonal PlanarTrigonal Planar

Trigonal Planar Trigonal Planar BentBent

TetrahedralTetrahedral TetrahedralTetrahedral

TetrahedralTetrahedral Trigonal PyramidalTrigonal Pyramidal

TetrahedralTetrahedral BentBent

Trigonal BipyramidalTrigonal Bipyramidal Trigonal BipyramidalTrigonal Bipyramidal

Trigonal BipyramidalTrigonal Bipyramidal SeesawSeesaw

Trigonal BipyramidalTrigonal Bipyramidal T-shapeT-shape

Trigonal BipyramidalTrigonal Bipyramidal LinearLinear

OctahedralOctahedral OctahedralOctahedral

OctahedralOctahedral Square PyramidalSquare Pyramidal

OctahedralOctahedral Square PlanarSquare Planar

Orbital Orbital GeometryGeometry

Molecular Molecular GeometryGeometry Bond AngleBond Angle

00

00

11

00

11

22

00

11

22

33

00

11

22

# of lone pairs# of lone pairs

Important

in

Organic

Compounds

QUESTIONQUESTIONThe compound BF3 can react with silicon to help etch computer chips. After diagraming the complete Lewis structure for this compound, determine its shape and provide one of the following names for the shape you drew.

A. Trigonal planarB. Trigonal bipyramidalC. T–shapedD. Linear

ANSWERANSWERA) correctly names the shape of BF3. This compound has three electron densities around the B atom. These equal densities will repel each other equally thus producing a compound with 120° angles for the F to B to F bonds.

QUESTIONQUESTION

ANSWERANSWER .

D) 109

The electron pair arrangement around selenium is tetrahedral. Two of the electron pairs are bonding pairs, so the angle of H-Se-H is 109.

QUESTIONQUESTION

ANSWERANSWER 4

D) XeF

XeF4 has four bonding pairs of electrons, with two nonbonding pairs keeping the bonds set in a plane.

Summary:Summary:

http://chemconnections.org/VSEPR-jmol/

http://chemconnections.org/general/chem120/VSEPR/index.html

Representing MoleculesRepresenting Molecules Formulas, Structures & ModelsFormulas, Structures & Models

Lewis Structures / VSEPR Lewis Structures / VSEPR Molecular Models Molecular Models

Using all of the tools. Using all of the tools. Computer Generated ModelsComputer Generated Models

Covalent Compounds•Equal sharing of electrons: nonpolar covalent bond, same electronegativity (e.g., H2)• Unequal sharing of electrons between atoms of different electronegativities: polar covalent bond (e.g., HF)

• Dipole moments are experimentally measured.• Polar bonds have dipole moments.

dipole moment (D) = = e x d(e) : magnitude of the charge on the atom(d) : distance between the two charges

Bond Dipole & Dipole Moment

Bond PolarityBond Polarity

A molecule, such as HF, that has a A molecule, such as HF, that has a center of positive charge and a center of center of positive charge and a center of negative charge is negative charge is polarpolar, and has a , and has a dipole momentdipole moment. The partial charge is . The partial charge is represented by represented by δδ and the polarity with a and the polarity with a vector arrow.vector arrow.δ+ δ−FH

When identical polar bonds point in opposite directions,the effects of their polaritiescancel, giving no net dipolemoment. When they do notpoint in opposite directions,there is a net effect and a netmolecular dipole moment,designated δ

Molecular Molecular Polarity & Dipole Polarity & Dipole MomentMoment

Resultant Molecular Dipole = 0

QUESTIONQUESTIONThe bonds between carbon and oxygen in CO2 and the bonds between carbon and oxygen in acetic acid have exactly the same difference in electronegativity. Yet CO2 is a non-polar gas, whereas acetic acid is polar. Which statement provides a factual reason for this difference?

A. CO2 has no dipole moment because the symmetry of the C–O bonds places them 180°C apart; thus, the polarity predicted in the bonds is cancelled by molecular shape. Since acetic acid is polar, the shape must not allow for canceling the bond polarity.

B. Polarity in a bond does not always predict polarity in a molecule because the dipole moment could be increased causing the molecule to be less polar.

C. The partial negative charge of each oxygen, compared to carbon, is decreased in CO2 because it gets shared among two atoms.

D. The C–O bond in acetic acid remains polar because the other parts of the molecule form a tetrahedron.

ANSWERANSWERA) provides the connection between molecular shape and

possible cancellation of bond polarity. Since CO2 has one oxygen on each side of the central C atom (180°) apart, the increased pull on electron charge of oxygen over carbon gets canceled by the same strength of pull from the other oxygen.

• Resultant Molecular Dipoles > 0• Solubility: Polar molecules that

dissolve or are dissolved in like molecules

Molecular Size, Shape & Molecular Size, Shape & PropertiesProperties

Ozone and WaterOzone and Water

• The Lotus flower• Water & dirt repellancy

0.1278 nm

The “Lotus Effect” The “Lotus Effect” BiomimicryBiomimicry

http://www.bfi.org/Trimtab/spring01/biomimicry.htmhttp://www.bfi.org/Trimtab/spring01/biomimicry.htm

Lotus petals have micrometer-scale roughness, Lotus petals have micrometer-scale roughness, resulting in water contact angles up to 170° resulting in water contact angles up to 170°

See the Left image in the illustration on the right.See the Left image in the illustration on the right.

Wax

The “Lotus Effect” The “Lotus Effect” BiomimicryBiomimicry

http://www.sciencemag.org/cgi/content/full/299/5611/1377/DC1http://www.sciencemag.org/cgi/content/full/299/5611/1377/DC1

Isotactic polypropylene (i-PP) melted between two Isotactic polypropylene (i-PP) melted between two glass slides and subsequent crystallization provided a glass slides and subsequent crystallization provided a smooth surface. Atomic force microscopy tests smooth surface. Atomic force microscopy tests indicated that the surface had root mean square (rms) indicated that the surface had root mean square (rms) roughness of 10 nm. roughness of 10 nm.

A) The water drop on the resulting surface had a A) The water drop on the resulting surface had a contact angle of 104° ± 2 contact angle of 104° ± 2

B) the water drop on a superhydrophobic i-PP coating B) the water drop on a superhydrophobic i-PP coating surface has a contact angle of 160°.surface has a contact angle of 160°.

Science, 299, (2003), pp. 1377-1380, H. Yldrm Erbil, A. Levent Demirel, Yonca Avc, Olcay Mert

http://chemconnections.org/COT/VSEPR1/BONUS: Questions #2, 3, & 4

http://chemconnections.org/COT/VSEPR1/VSEPR/VSEPR-WKS-I-12.pdfDVC Student Project Group

An electrically charged rod attracts a streamof chloroform buthas no effect on a stream of carbontetrachloride.

QUESTIONQUESTION

ANSWERANSWER 3

3

C) PCl

Only PCl has a lone pair that forces the bonds into a trigonal pyramidal configuration.

Acetic acid Acetic acid

•Is acetic acid polar or non-polar? Is acetic acid polar or non-polar? •Will acetic acid be miscible in water? Will acetic acid be miscible in water?

QUESTIONQUESTION

ANSWERANSWER

B) 2

Among the central atom molecules, only water has lone pairs distorting its shape into a bent configuration. HF is a linear molecule with two atoms with different electronegativities.

Proteins & Small MoleculesProteins & Small MoleculesThe interaction of a large protein bio-polymer, The interaction of a large protein bio-polymer, acetylcholinesterase, with a relatively small molecule of acetylcholinesterase, with a relatively small molecule of acetylcholine. A general process similar to the way that acetylcholine. A general process similar to the way that scientists think we smell.scientists think we smell.

Molecular Shape & Smell Theory Molecular Shape & Smell Theory

Historical view of a few smell receptors.Historical view of a few smell receptors.

4 October 2004

The Nobel Assembly at Karolinska Institutet has today decided to award

The Nobel Prize in Physiology or Medicine for 2004

jointly to

Richard Axel and Linda B. Buck

for their discoveries of

"odorant receptors and the organization of the olfactory system"

S-(+)-d-carvoneS-(+)-d-carvone R-(-)-l-carvoneR-(-)-l-carvone carawaycaraway spearmint spearmint

They discovered a large gene family, comprised of some 1,000 different genes (three per cent of our genes) that give rise to an equivalent number of olfactory receptor types. These receptors are located on the olfactory receptor cells, which occupy a small area in the upper part of the nasal epithelium and detect the inhaled odorant molecules.