BondingBonding

ObjectivesObjectives Define chemical bondDefine chemical bond Distinguish among the following bond Distinguish among the following bond

types based upon how electrons are types based upon how electrons are involved: pure covalent, polar covalent, involved: pure covalent, polar covalent, coordinate covalent, ionic, and metalliccoordinate covalent, ionic, and metallic

Draw Lewis Structures for simple Draw Lewis Structures for simple molecules.molecules.

The Chemical BondThe Chemical Bond Simply put, a chemical bond is the Simply put, a chemical bond is the

attractive force that holds two atoms attractive force that holds two atoms together.together.

Chemical bonds always describe the Chemical bonds always describe the interactions of valence electrons interactions of valence electrons between two adjacent atoms.between two adjacent atoms.

Different types of interactions result Different types of interactions result in different types of chemical bonds.in different types of chemical bonds.

Types of Chemical Bonds:Types of Chemical Bonds:

IonicIonic MetallicMetallic CovalentCovalent

The Ionic BondThe Ionic Bond Ionic Bonds occur between a Ionic Bonds occur between a

metal and a non-metalmetal and a non-metal When two atoms combine by When two atoms combine by

transfer of electrons, ions are transfer of electrons, ions are produced. The opposite charges produced. The opposite charges of the ions hold them together. of the ions hold them together.

This electrostatic force, created This electrostatic force, created by electron transfer, is called an by electron transfer, is called an ionic bondionic bond..

ExampleExample

2Na + Cl2Na + Cl22 ---> 2Na ---> 2Na++ + 2Cl + 2Cl-- If a chloride ion and a sodium ion are If a chloride ion and a sodium ion are

brought together, there will be an brought together, there will be an attractive force between them. If the ions attractive force between them. If the ions are brought almost into contact, the force are brought almost into contact, the force will be great enough to hold the two ions will be great enough to hold the two ions together. together.

Ionic BondingIonic Bonding

Ionic compounds are formed Ionic compounds are formed during the transfer of electrons during the transfer of electrons between many atoms in a solid between many atoms in a solid state.state.

A crystal lattice of regularly A crystal lattice of regularly organized, repeating pattern of organized, repeating pattern of cations and anions formscations and anions forms

The effective charge of the The effective charge of the compound is zerocompound is zero

Ionic BondingIonic Bonding Elements can be assigned Elements can be assigned

oxidation numbers for ionic oxidation numbers for ionic bonding. bonding. E.g. Sulfur, with 6 valence E.g. Sulfur, with 6 valence

electrons, will tend to gain 2 electrons, will tend to gain 2 electrons to attain the stable octet electrons to attain the stable octet configuration. The oxidation number configuration. The oxidation number of sulfur for ionic bonding is 2-. The of sulfur for ionic bonding is 2-. The negative two is its electric charge negative two is its electric charge after gaining two electrons.after gaining two electrons.

Ionic BondingIonic Bonding

Ionic compounds are characterized Ionic compounds are characterized by:by:

high melting points high melting points ability to conduct electricity in ability to conduct electricity in

the molten statethe molten state high solubility in waterhigh solubility in water ability to crystallize as sharply ability to crystallize as sharply

defined particlesdefined particles

Bond FunniesBond Funnies

Bond FunniesBond Funnies

Metallic BondingMetallic Bonding

A A metallic bondmetallic bond is defined as the is defined as the attraction of a metallic cation for attraction of a metallic cation for delocalized electrons.delocalized electrons.

Electron Sea ModelElectron Sea Model All the metal atoms contribute All the metal atoms contribute

their valence (outer energy level) their valence (outer energy level) electrons to form a “sea” of electrons to form a “sea” of electronselectrons

The valence electrons are not held The valence electrons are not held by any specific atom and can move by any specific atom and can move freely from one atom to the nextfreely from one atom to the next

Because they are free to move Because they are free to move they are called they are called delocalizeddelocalized electronselectrons

Electron Sea ModelElectron Sea Model

Alkali metalAlkali metal Alkaline earth metalAlkaline earth metal

Metallic BondingMetallic Bonding

Because the electrons are no Because the electrons are no longer held to one atom, metallic longer held to one atom, metallic cations are formed. cations are formed.

Each metal cation is bonded to the Each metal cation is bonded to the other metal cations in the lattice, other metal cations in the lattice, via the surrounding sea of via the surrounding sea of electrons.electrons.

A A metallic bondmetallic bond is the attraction of is the attraction of a metal cation for delocalized a metal cation for delocalized electronselectrons

Properties of MetalsProperties of Metals

High electrical conductivityHigh electrical conductivity Metals are malleable -they can be Metals are malleable -they can be

hammered into sheetshammered into sheets Ductile- they can be drawn into a Ductile- they can be drawn into a

wirewire Moderately high melting pointsModerately high melting points Extremely high boiling pointsExtremely high boiling points

Properties of Metal Properties of Metal ExplainedExplained

Electrical current occurs when Electrical current occurs when electrons enter a wire and push other electrons enter a wire and push other electrons down the wire.electrons down the wire.

Metals have high electrical Metals have high electrical conductivity because the delocalized conductivity because the delocalized electrons are free to move.electrons are free to move.

Properties of Metal Properties of Metal ExplainedExplained

Ductility and malleability both Ductility and malleability both occur because an applied force occur because an applied force allows metal cations to move allows metal cations to move through the delocalized electron through the delocalized electron field to a new location, without field to a new location, without suffering a complete break in the suffering a complete break in the structure of the crystal lattice structure of the crystal lattice itself.itself.

Properties of Metal Properties of Metal ExplainedExplained

Moderately high melting points occur Moderately high melting points occur because the electron sea in the solid because the electron sea in the solid state already allows some movement state already allows some movement in the lattice without a great deal of in the lattice without a great deal of additional energy. additional energy.

But to completely separate atoms But to completely separate atoms from another, because they are from another, because they are associated with many other atoms, associated with many other atoms, not just one, requires a great deal of not just one, requires a great deal of energy, hence their high boiling energy, hence their high boiling points.points.

Covalent BondingCovalent Bonding When two atoms which both need to When two atoms which both need to

gain electrons come together, sharing gain electrons come together, sharing valence electrons to complete the valence electrons to complete the octet rule, a covalent bond is formedoctet rule, a covalent bond is formed

Because electrons are not given up, Because electrons are not given up, no ions are produced.no ions are produced.

Covalent bonds generally occur Covalent bonds generally occur between non-metalsbetween non-metals

Bond EnergyBond Energy The amount of energy needed to The amount of energy needed to

break the attractive force holding 2 break the attractive force holding 2 atoms together.atoms together.

The higher the energy, the shorter The higher the energy, the shorter the bond, and therefore, the shorter the bond, and therefore, the shorter the distance between atoms.the distance between atoms.

The higher the energy, the stronger The higher the energy, the stronger the bond and the more difficult it is the bond and the more difficult it is to break.to break.

Lewis StructureLewis Structure

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

Reflects central idea that stability of Reflects central idea that stability of a compound relates to noble gas a compound relates to noble gas electron configuration. (octet-duet electron configuration. (octet-duet rule)rule)

Lewis StructuresLewis Structures

Share valence electrons to make Share valence electrons to make sure that every atom has a noble gas sure that every atom has a noble gas electron configurationelectron configuration

Two electrons shared forms a single Two electrons shared forms a single covalent bondcovalent bond

ExamplesExamplesHH22

ClCl22

Try theseTry these

OO22

NN22

HClHCl COCO22

Drawing Lewis StructuresDrawing Lewis Structures

S = N-AS = N-A

S = number of S = number of sharedshared electrons electrons

N = number of electrons N = number of electrons neededneeded (8 x (8 x number of nonhydrogen atoms + 2 x number of nonhydrogen atoms + 2 x number of hydrogen atoms)number of hydrogen atoms)

A = number of electrons A = number of electrons availableavailable (sum of (sum of the valence electrons of all atoms in the valence electrons of all atoms in molecule + 1 for each negative and – 1 molecule + 1 for each negative and – 1 for each positive charge in a polyatomic for each positive charge in a polyatomic ion)ion)

Drawing Lewis StructuresDrawing Lewis Structures

1.1. The number of bonds = number of The number of bonds = number of shared electrons/2shared electrons/2

2.2. Put the least electronegative atom Put the least electronegative atom in the centerin the center

3.3. Arrange the other atoms Arrange the other atoms symmetrically around the central symmetrically around the central atomatom

4.4. Draw in the bondsDraw in the bonds

5.5. Fill incomplete octetsFill incomplete octets

ExampleExample

Draw the Lewis structure for the Draw the Lewis structure for the moleculemolecule

CHCH22OO

ExampleExample

Draw the Lewis structure for CClDraw the Lewis structure for CCl44

ExampleExample

Draw the Lewis structure for NHDraw the Lewis structure for NH33

ExampleExample

Draw the Lewis structure for NHDraw the Lewis structure for NH44++

ExampleExample

Draw the Lewis structure for SODraw the Lewis structure for SO332-2-

Resonance, Electronegativies, Resonance, Electronegativies, PolarityPolarity

ObjectivesObjectives Distinguish among the following bond Distinguish among the following bond

types based upon how electrons are types based upon how electrons are involved: pure covalent, polar covalent, involved: pure covalent, polar covalent, coordinate covalent, ionic, and metallic. coordinate covalent, ionic, and metallic.

Use the electronegativity differences Use the electronegativity differences between bonding atoms to predict the between bonding atoms to predict the type of bond. type of bond.

ExampleExample

Draw the Lewis Structure for SODraw the Lewis Structure for SO33. .

ResonanceResonance Formal Definition: Description of the Formal Definition: Description of the

ground state of a molecule with ground state of a molecule with delocalized electrons as an average delocalized electrons as an average of several Lewis structures. of several Lewis structures.

The actual ground state doesn't The actual ground state doesn't switch rapidly between the separate switch rapidly between the separate structures: it is an average.structures: it is an average.

We call the true structure a We call the true structure a resonance hybridresonance hybrid

ExampleExample

Draw the Lewis Structure for SODraw the Lewis Structure for SO22. .

ExampleExample

Draw the Lewis Structure for Draw the Lewis Structure for benzene, Cbenzene, C66HH66. This molecule is a . This molecule is a cyclic hydrocarbon. cyclic hydrocarbon.

Electronegativity and Electronegativity and BondingBonding

Electrons are Electrons are transferredtransferred between atoms when the between atoms when the difference in electronegativity difference in electronegativity between them is high. This between them is high. This results in an results in an ionicionic bond. bond.

If the electronegativity difference If the electronegativity difference between two reacting atoms is between two reacting atoms is small, electron small, electron sharingsharing occurs.This results in a occurs.This results in a covalentcovalent bond. bond.

The Pauling Electronegativity The Pauling Electronegativity ValuesValues

Electronegativity and Electronegativity and BondingBonding

At what point in electronegativity At what point in electronegativity difference does the changeover occur? difference does the changeover occur? The answer is not simple. The answer is not simple.

The electronegativity of an atom The electronegativity of an atom varies slightly depending upon the varies slightly depending upon the atom with which it is combining. atom with which it is combining.

Another factor is the number of other Another factor is the number of other atoms with which the atom is atoms with which the atom is combining. combining.

Three Possible Three Possible Types of BondsTypes of Bondsa) nonpolar a) nonpolar covalentcovalentb) polar covalentb) polar covalentc) ionicc) ionic

Electronegativity and Electronegativity and BondingBonding

A scale showing the percent of A scale showing the percent of transfer of electrons (percent transfer of electrons (percent ionic character) has been ionic character) has been constructed. The amount of constructed. The amount of transfer depends on the transfer depends on the electronegativity differenceelectronegativity difference between two atoms.between two atoms.

The Relationship Between the Ionic Character of The Relationship Between the Ionic Character of a Covalent Bond and the Electronegativity a Covalent Bond and the Electronegativity

Difference of the Bonded AtomsDifference of the Bonded Atoms

How to decide?How to decide?

1.71.7 3.33.300

CovalentCovalent IonicIonicENEN

NonpolarNonpolar

0.40.4

PolarPolar

50% ionic50% ionic

Halogen BondingHalogen Bonding

Covalent bonds tend to have Covalent bonds tend to have overlapping electron cloudsoverlapping electron clouds

Ionic bonds tend to have distinct Ionic bonds tend to have distinct electron cloudselectron clouds

Are these bonds ionic or covalent?Are these bonds ionic or covalent?

The Polar Covalent BondThe Polar Covalent Bond Since the electronegativity of each element Since the electronegativity of each element

differs, we should consider that in a covalent differs, we should consider that in a covalent bond one of the atoms attracts the shared bond one of the atoms attracts the shared pair more strongly than the other. The pair more strongly than the other. The resulting bond is said to be resulting bond is said to be polar covalentpolar covalent. .

Since one atom in the bond will attract the Since one atom in the bond will attract the electrons more strongly, there will be a electrons more strongly, there will be a partial negative charge partial negative charge ((--)) near that end of near that end of the bond.the bond.

The atom holding the electrons less strongly The atom holding the electrons less strongly will have a partial positive charge will have a partial positive charge (()) near near its end.its end.

Polar MoleculesPolar Molecules Polar bonds, unless symmetrically arranged, Polar bonds, unless symmetrically arranged,

produce polar molecules. produce polar molecules. In CClIn CCl44, the four C—Cl bonds are each , the four C—Cl bonds are each polarpolar but but

their symmetrical arrangement around the C their symmetrical arrangement around the C produces a produces a nonpolarnonpolar molecule. molecule.

In HIn H22O and HF, however, the polar bonds are not O and HF, however, the polar bonds are not symmetrically arranged and produce a polar symmetrically arranged and produce a polar molecule.molecule.

Properties of Polar Properties of Polar SubstancesSubstances

Polar molecules are generally Polar molecules are generally soluble in water (hydrophilic).soluble in water (hydrophilic).

Non-polar molecules are Non-polar molecules are generally insoluble in water generally insoluble in water (hydrophobic).(hydrophobic).

Properties of Polar Properties of Polar SubstancesSubstances

Substances composed of Substances composed of nonpolarnonpolar molecules molecules are generally gases or low-boiling liquids. are generally gases or low-boiling liquids.

Substances composed of Substances composed of polarpolar molecules molecules generally have higher boiling pointsgenerally have higher boiling points Hence, many polar molecules are solids under Hence, many polar molecules are solids under

normal conditions. normal conditions. A polar molecule is sometimes called a dipole. A polar molecule is sometimes called a dipole. A dipole has asymmetrical charge distribution. A dipole has asymmetrical charge distribution.

In the teacher demonstration, which covalent In the teacher demonstration, which covalent molecules were polar? Non-polar?molecules were polar? Non-polar?

So, just how many So, just how many Covalent Bonds are there?Covalent Bonds are there?

Pure CovalentPure Covalent Polar CovalentPolar Covalent Coordinate CovalentCoordinate Covalent

Pure CovalentPure Covalent

Each atom contributes one Each atom contributes one electronelectron

Electrons are shared equally Electrons are shared equally between atomsbetween atoms

A nonpolar bond resultsA nonpolar bond results If all bonds in the compound are If all bonds in the compound are

nonpolar, the molecule is nonpolar, the molecule is nonpolarnonpolar

Polar CovalentPolar Covalent

Electrons are not shared equally Electrons are not shared equally between atomsbetween atoms

A dipole moment exists, so a polar A dipole moment exists, so a polar bond resultsbond results

If the polar bonds are arranged If the polar bonds are arranged symmetrically, the molecule is symmetrically, the molecule is nonpolar, but if not, the molecule is nonpolar, but if not, the molecule is polarpolar

Coordinate CovalentCoordinate Covalent

Only one atom contributes Only one atom contributes electrons to the bondelectrons to the bond

NHNH33 + H + H++ NH NH4 4 ++

HH

HH

HH

HH

NN++

HH

HH

N:N:

HH

Chemistry at WorkChemistry at Work

More More Chemistry at Chemistry at

WorkWork

VSEPR TheoryVSEPR Theory

Valence shell electron pair Valence shell electron pair repulsion theoryrepulsion theory ( (VSEPRVSEPR) (1957) is ) (1957) is a model in chemistry that aims to a model in chemistry that aims to generally represent the shapes of generally represent the shapes of individual molecules. individual molecules.

Molecular GeometryMolecular Geometry VSEPR theory is based on the idea VSEPR theory is based on the idea

that the geometry of a molecule or that the geometry of a molecule or polyatomic ion is determined polyatomic ion is determined primarily by:primarily by: Repulsion among the pairs of electrons Repulsion among the pairs of electrons

associated with a central atom. associated with a central atom. Electron pairs may be bonding or Electron pairs may be bonding or

nonbonding (also called lone pairs). nonbonding (also called lone pairs). Only valence electrons of the central Only valence electrons of the central

atom influence the molecular shape in a atom influence the molecular shape in a meaningful way.meaningful way.

Electron RepulsionElectron Repulsion Lone pair electrons take up more Lone pair electrons take up more

space than bonded pair electronsspace than bonded pair electrons A molecule must avoid or minimize A molecule must avoid or minimize

these repulsions to remain/stay these repulsions to remain/stay stable (lower in energy).stable (lower in energy).

Steric Electron PairsSteric Electron Pairs

The steric number is the sum of the The steric number is the sum of the total number of neighbors of a total number of neighbors of a central atom in a chemical compound central atom in a chemical compound and the number of lone pairs on it. and the number of lone pairs on it. E.g. In methane, the steric number for E.g. In methane, the steric number for

the carbon atom is 4. the carbon atom is 4. N.B. For our purposes, single bonds, N.B. For our purposes, single bonds,

double bonds, and triple bonds all double bonds, and triple bonds all count as one steric pair. count as one steric pair.

Predicting Molecular Predicting Molecular GeometryGeometry

In order to achieve this:In order to achieve this: Draw a Lewis structureDraw a Lewis structure

Include all bondsInclude all bonds Include all lone pairs of electrons Include all lone pairs of electrons

Find the steric number on the central Find the steric number on the central atom.atom.

Determining the Geometry of Determining the Geometry of MethaneMethane

Draw the Lewis Structure for Draw the Lewis Structure for methanemethane

What is the steric number?What is the steric number? How should the bonds be spaced to How should the bonds be spaced to

minimize repulsion?minimize repulsion? What shape is this?What shape is this? What is the bond angle?What is the bond angle?

Solution: MethaneSolution: Methane

The methane molecule (CHThe methane molecule (CH44) is ) is tetrahedral because there are four tetrahedral because there are four pairs of electrons. The four hydrogen pairs of electrons. The four hydrogen atoms are positioned at the vertices atoms are positioned at the vertices of a tetrahedron, and the bond angle of a tetrahedron, and the bond angle is 109.5°. is 109.5°.

Determining the Geometry of Determining the Geometry of AmmoniaAmmonia

Draw the Lewis Structure for Draw the Lewis Structure for ammoniaammonia

What is its steric number?What is its steric number? How should the bonds be spaced to How should the bonds be spaced to

minimize repulsion?minimize repulsion? What shape is this?What shape is this? What is the bond angle?What is the bond angle?

Solution: AmmoniaSolution: Ammonia The ammonia molecule (NHThe ammonia molecule (NH33) has three ) has three

pairs of electrons involved in bonding, but pairs of electrons involved in bonding, but there is a lone pair of electrons on the there is a lone pair of electrons on the nitrogen atom. nitrogen atom.

It is not bonded with another atom; It is not bonded with another atom; however, it influences the overall shape however, it influences the overall shape through repulsions. through repulsions.

As in methane, there are 4 regions of As in methane, there are 4 regions of electron density. electron density.

Therefore, the overall orientation of the Therefore, the overall orientation of the regions of electron density is tetrahedral. regions of electron density is tetrahedral.

Solution: AmmoniaSolution: Ammonia

But, there are only three outer atoms. But, there are only three outer atoms. The overall shape of the molecule is The overall shape of the molecule is

trigonal pyramidal because the lone pair is trigonal pyramidal because the lone pair is not "visible." not "visible."

This makes the angle <109°.This makes the angle <109°. The shape of a molecule is found from the The shape of a molecule is found from the

relationship of the atoms even though it relationship of the atoms even though it can be influenced by lone pairs of can be influenced by lone pairs of electrons.electrons.

HybridizationHybridization

Method 2: How to Draw a Method 2: How to Draw a Lewis Structure for a Lewis Structure for a

CompoundCompound

I. Draw a skeleton I. Draw a skeleton structurestructure..

A skeleton structure is a rough map showing the A skeleton structure is a rough map showing the arrangement of atoms within the molecule. In arrangement of atoms within the molecule. In general, you need to determine the skeleton general, you need to determine the skeleton experimentally, but here are a few guidelines for experimentally, but here are a few guidelines for predicting skeleton structures from molecular predicting skeleton structures from molecular formulas. formulas.

Central atoms are usually Central atoms are usually the atoms with highest valence, or the atoms with highest valence, or the largest atoms, or the largest atoms, or the least electronegative atom. the least electronegative atom.

H and the halogens are usually outside atoms. H and the halogens are usually outside atoms. Don't put more than four atoms around a central Don't put more than four atoms around a central

atom unless the central atom is third period or lower. atom unless the central atom is third period or lower.

II. Count total valence electrons.

Add the number of electrons in the valence shells of all atoms in the molecule.

If the molecule is charged, add an electron for each negative charge and subtract an electron for each positive charge.

Noble gas compounds are very uncommon (except on general chemistry tests!) Should you encounter one, each noble gas atom has 8 valence electrons.

III. Connect the structure.III. Connect the structure. Draw a bond between the central atom Draw a bond between the central atom

and each outside atom. and each outside atom. Each bond uses 2 valence electrons. Each bond uses 2 valence electrons.

IV. Place electrons on outside atoms.

Use remaining electrons to satisfy the octets for each of the outside atoms.

If you run out of electrons at this point, the skeleton structure was wrong. Go back to step I.

V. Place all remaining electrons on the central

atom. If there are more than 8 electrons on the central atom, and

the central atom is not third period or lower, you counted the number of valence electrons incorrectly. Go back to step II.

If the octet on the central atom is not complete, try sharing lone pairs of outside atoms to form double or triple bonds. Write one multiply bound structure for each outside atom with a lone pair to share; these are resonance structures.

If you can't get an octet on the central atom, at this point, check to see whether the total number of valence electrons for this molecule is odd. It's impossible to give octets to all atoms in an odd electron molecules. Get as close to an octet as possible by forming multiple bonds.