Chemical Chemical BondingBonding

Lewis Structures, Polarity and Lewis Structures, Polarity and Bond ClassificationBond Classification

Lewis TheoryLewis Theory

Developed by Lewis, Kossel, and Developed by Lewis, Kossel, and LangmuirLangmuir

Between 1916-1919 they made Between 1916-1919 they made several important proposals several important proposals about bonding, which was made about bonding, which was made into the Lewis Theoryinto the Lewis Theory

Elements of the Lewis Elements of the Lewis TheoryTheory

1.1. Valence electrons play a fundamental Valence electrons play a fundamental role in chemical bondingrole in chemical bonding

2.2. Sometimes bonding involves the Sometimes bonding involves the TRANSFERTRANSFER of one or more electrons of one or more electrons from one atom to another. This leads from one atom to another. This leads to ion formation and to ion formation and IONIC BONDSIONIC BONDS..

3.3. Sometimes bonding involves Sometimes bonding involves SHARING SHARING electrons between atoms, this leads to electrons between atoms, this leads to COVALENT BONDSCOVALENT BONDS..

More Lewis Theory…More Lewis Theory…

4.4. Electrons are transferred or shared Electrons are transferred or shared such that each atom gains a more such that each atom gains a more stable electron configurationstable electron configuration

usually changes to Noble gas usually changes to Noble gas configurationconfiguration

Eg. Having 8 outer Eg. Having 8 outer electronselectrons

this arrangement (having 8 this arrangement (having 8 valence electrons) is called an valence electrons) is called an OCTETOCTET

Electron Dot DiagramsElectron Dot Diagrams

Show the valence electrons of an Show the valence electrons of an atom / ionatom / ion

Chemical symbol represents the Chemical symbol represents the nucleus and the inner electronsnucleus and the inner electrons

Dots represent the valence electronsDots represent the valence electrons

Things to Note:Things to Note:1.1. Since elements in the same family Since elements in the same family

have the same number of valence have the same number of valence electrons, their dot diagrams will look electrons, their dot diagrams will look VERY similar (just different symbols)VERY similar (just different symbols)

2.2. Lewis dot diagrams only work Lewis dot diagrams only work well for representative well for representative elementselements- Transition metals, Lanthanides, and Transition metals, Lanthanides, and

Actinides have incompletely filled Actinides have incompletely filled inner shells (“ d ” or “ f ” orbitals), so inner shells (“ d ” or “ f ” orbitals), so we can’t make simple Lewis we can’t make simple Lewis diagrams for themdiagrams for them

Back to Bonding…Back to Bonding…

Remember, we have already talked Remember, we have already talked about 2 types of bondingabout 2 types of bonding

–IonicIonic–CovalentCovalent

Now there is another type of Now there is another type of bonding to know!bonding to know!

INTERMOLECULAR BONDINGINTERMOLECULAR BONDING

1.1. Ionic BondingIonic Bonding Forces that hold ionic compounds Forces that hold ionic compounds

together based on the electrostatic together based on the electrostatic attraction of cations and anionsattraction of cations and anions

4 Steps to forming the bond…4 Steps to forming the bond…1. Form a positive ion – by loss of 1. Form a positive ion – by loss of

1 or 1 or more electrons to become more electrons to become isoelectronic isoelectronic with noble gaseswith noble gases

2. Form a negative ion – by 2. Form a negative ion – by gaining 1 gaining 1 or more electrons to or more electrons to become become isoelectronic (Lewis isoelectronic (Lewis Diagram will show Diagram will show an an OCTET OCTET of 8 of 8 electrons for the anionelectrons for the anion

3.3. Oppositely charged ions attract each other Oppositely charged ions attract each other (electrostatic attraction) and form an ionic (electrostatic attraction) and form an ionic bondbond

4.4. An ionic crystal grows – cations are An ionic crystal grows – cations are surrounded by anions and vice versasurrounded by anions and vice versa

- - formula unitformula unit is the smallest is the smallest collection of collection of ions that is ions that is electrically electrically neutralneutral

- Lewis structures for ionic bonds - Lewis structures for ionic bonds represent one formula unit represent one formula unit

- ionic crystal is not a single molecule, - ionic crystal is not a single molecule, but but a collection of ions (“lattice” a collection of ions (“lattice” structure)structure)

Example with NaCl:Example with NaCl:

Properties of Ionic Properties of Ionic CompoundsCompounds

Neutral overall (positives cancel out the Neutral overall (positives cancel out the negatives)negatives)

No unique molecules (all bonded together)No unique molecules (all bonded together) Decreased reactivity compared to atomsDecreased reactivity compared to atoms Conduct electricity when melted or Conduct electricity when melted or

dissolveddissolved

IONIC BONDSIONIC BONDS are are STRONGSTRONG, so ionic solids , so ionic solids have have HIGH MELTING TEMPERATURES!HIGH MELTING TEMPERATURES!

2. Covalent Bonding2. Covalent Bonding Bonds formed by sharing of Bonds formed by sharing of

electrons between atomselectrons between atoms Electrons are attracted by nuclei of Electrons are attracted by nuclei of

both atoms involvedboth atoms involvedElectrons spend most of the time between Electrons spend most of the time between

the the two atoms - forming the bondtwo atoms - forming the bond

Covalent bonds are Covalent bonds are VERYVERY strong strong

Bonds between MOLECULES vary in Bonds between MOLECULES vary in strength though, so melting points varystrength though, so melting points vary

Covalent compounds can form Covalent compounds can form MULTIPLE MULTIPLE BONDSBONDS

- use #’s (bond order) to describe - use #’s (bond order) to describe how many how many bonds are being made bonds are being made

1 = single ; 2 = double ; 3 = triple1 = single ; 2 = double ; 3 = triple

(bond order shows how many pairs of (bond order shows how many pairs of electrons are shared)electrons are shared)

Octet Rule:Octet Rule: Atoms tend to form bonds until Atoms tend to form bonds until

they are surrounded by 8 valence they are surrounded by 8 valence electronselectrons

Exception #1 – Hydrogen will form Exception #1 – Hydrogen will form bonds to have 2 valence electronsbonds to have 2 valence electrons

There are other exceptions to the There are other exceptions to the octet rule (more to come later)octet rule (more to come later)

Bonding Terminology:Bonding Terminology:

Bonding ElectronsBonding Electrons – – the electrons that are the electrons that are shared in the bondshared in the bond

Lone Pairs / NonLone Pairs / Non--bonding Electronsbonding Electrons – – electrons not involved electrons not involved in bondingin bonding

Bond Classification using Bond Classification using ElectronegativityElectronegativity

Type of bond is based Type of bond is based on the on the electronegativity electronegativity difference of the atoms difference of the atoms involved in the bondinvolved in the bond

Electronegativity Electronegativity values can be found in values can be found in textbook (p. 241)textbook (p. 241)

Between 1.6 and 2.0 Between 1.6 and 2.0 need to look at atoms need to look at atoms in bond to classify (if a in bond to classify (if a metal is present, then metal is present, then bond is ionic)bond is ionic)

Bond Bond TypeType

ElectronegativElectronegativity Differenceity Difference

Non-Non-polarpolar

CovaleCovalentnt

0.40.4

Polar- Polar- CovaleCovalentnt

between 0.4 between 0.4 and 1.6and 1.6

IonicIonic 2.02.0

Electronegativities of AtomsElectronegativities of Atoms

What is a Polar-Covalent What is a Polar-Covalent Bond?Bond?

Electrons in the bond are Electrons in the bond are shared unequallyshared unequally because of electronegativity differences because of electronegativity differences between bonding atomsbetween bonding atoms

the electrons spend more time on the the electrons spend more time on the atom that is more electronegativeatom that is more electronegative

More on Polar Bonds…More on Polar Bonds…

Polar molecules have partial charges Polar molecules have partial charges on themon them– The atom where the electron spends The atom where the electron spends

more time is partially negativemore time is partially negative– The atom where the electron spends The atom where the electron spends

less time is partially positiveless time is partially positive The The greatergreater the electronegativity the electronegativity

difference, the difference, the more polarmore polar the bond the bond (extreme case is an ionic bond!)(extreme case is an ionic bond!)

3. Intermolecular 3. Intermolecular BondingBonding Bonding between molecules!Bonding between molecules!

Two TypesTwo Types

-- Dipole-Dipole Forces Dipole-Dipole Forces

-- London London ForcesForces

3a. Dipole-Dipole Forces3a. Dipole-Dipole Forces

Formed with polar covalent moleculesFormed with polar covalent molecules The partial positive (The partial positive (δδ++) and partial ) and partial

negative charges (negative charges (δδ--) attract each other ) attract each other forming electrostatic bonds (like forming electrostatic bonds (like WEAKWEAK ionic bonds… but are NOT true bonds)ionic bonds… but are NOT true bonds)

Ex. COEx. CO22

δδ++ δδ++δδ-- δδ--

Dipole-Dipole Force

Special Case: Hydrogen Special Case: Hydrogen bondingbonding

Occur when H bonds with F, O, or N Occur when H bonds with F, O, or N (large electronegativity difference, so (large electronegativity difference, so stronger than regular dipole-dipole stronger than regular dipole-dipole forces)forces)

Common example: attraction between Common example: attraction between water molecules water molecules

Oxygen is more Oxygen is more electronegative so is electronegative so is partially negative (partially negative (δδ-)-)

Hydrogen is less Hydrogen is less electronegative, so is electronegative, so is partially positive (partially positive (δδ+)+)

The attraction between the The attraction between the two partial charges is two partial charges is shown with dotted lines shown with dotted lines betweenbetween the water the water moleculesmolecules

Because water is polar it Because water is polar it can dissolve ionic can dissolve ionic compoundscompounds– The partial charges of water The partial charges of water

attract the ions of the ionic attract the ions of the ionic compundcompund

Hydrogen Bonds Between Water

Molecules

3b. London Forces3b. London Forces

Formed by temporary (instantaneous) Formed by temporary (instantaneous) charges on an atom when electrons move to charges on an atom when electrons move to unsymmetric positions around the nucleusunsymmetric positions around the nucleus

London forces are present between London forces are present between ANYANY molecules (including polar molecules) molecules (including polar molecules) when they are close togetherwhen they are close together

Are the Are the WEAKESTWEAKEST type of bond type of bond (unsymmetric electron location isn’t (unsymmetric electron location isn’t always occurring to cause the attraction) always occurring to cause the attraction)

Writing Lewis StructuresWriting Lewis Structuresfor Ionic Compoundsfor Ionic Compounds

Draw the Lewis Dot Diagram for each of Draw the Lewis Dot Diagram for each of the ions involved - including the ion the ions involved - including the ion chargecharge

Place the ions beside each otherPlace the ions beside each other Remember: the metal will have no Remember: the metal will have no

valence electrons and the non-metal will valence electrons and the non-metal will have a full valence shellhave a full valence shell

Example: NaClExample: NaCl

Your turn to try…Your turn to try…

Draw Lewis Structures for the following Draw Lewis Structures for the following ionic saltsionic salts– KBrKBr– MgClMgCl22– LiLi22SS– KK33PP

NOTE: If you count the valence electrons NOTE: If you count the valence electrons of the atoms involved and then count how of the atoms involved and then count how many electrons are in your diagram, they many electrons are in your diagram, they should be the same #!should be the same #!

Drawing Lewis Structures Drawing Lewis Structures for for

Covalent CompoundsCovalent Compounds Steps to follow:Steps to follow:

1. Count the total number of 1. Count the total number of valence electrons for the molecule valence electrons for the molecule and adjust for positive or negative and adjust for positive or negative charges on the moleculecharges on the molecule

2. Determine which atoms are 2. Determine which atoms are bonded together and put 2 electrons bonded together and put 2 electrons between them to represent the bondbetween them to represent the bond

3. – place remaining valence electrons to 3. – place remaining valence electrons to complete the octets of the atoms around complete the octets of the atoms around the central atom(s). If any remain, place the central atom(s). If any remain, place them in pairs on the central atom(s).them in pairs on the central atom(s).

4. If the central atom has less than 8 4. If the central atom has less than 8 electrons, have a neighboring atom share electrons, have a neighboring atom share electrons with the “defficient” atom by electrons with the “defficient” atom by putting an extra pair of electrons into a putting an extra pair of electrons into a shared bond (repeat if needed)shared bond (repeat if needed)

5. If desired, you can replace the bonding 5. If desired, you can replace the bonding pair(s) of electrons with dashes to represent pair(s) of electrons with dashes to represent the bondsthe bonds

Lewis Structures for Covalent Lewis Structures for Covalent Compounds that Obey the Octet Compounds that Obey the Octet

RuleRule

Examples to do together:Examples to do together:–OO22

–HOPOHOPO–CC22HH44

–NHNH44++

–COCO332-2-

Lewis Structures for Covalent Lewis Structures for Covalent Compounds that DON’T Obey the Compounds that DON’T Obey the

Octet RuleOctet Rule

Electron Deficient Molecules:Electron Deficient Molecules:– Be, B, and Al are common Be, B, and Al are common

exceptions to the octet ruleexceptions to the octet ruleBe can only share 4 electronsBe can only share 4 electronsB and Al can share up to 6 B and Al can share up to 6 electrons onlyelectrons only

–Ex. BFEx. BF33

Expanded Octet Examples:Expanded Octet Examples:– Elements in the 3Elements in the 3rdrd + 4 + 4thth periods periods

frequently have more than 8 valence frequently have more than 8 valence electrons when covalently bonding electrons when covalently bonding (extra electrons are in d-orbitals)(extra electrons are in d-orbitals)

– Write the Lewis structure the same Write the Lewis structure the same way, except extra electrons will be way, except extra electrons will be placed on the central atomplaced on the central atomP and S are common examplesP and S are common examplesEx. PClEx. PCl55, SF, SF44

What is on the TEST?What is on the TEST? Periodic TrendsPeriodic Trends

– Atomic and Ionic Atomic and Ionic RadiiRadii

– Ionization EnergyIonization Energy– Electron Affinity Electron Affinity

and and ElectronegativityElectronegativity

Lewis TheoryLewis Theory– Elements of the TheoryElements of the Theory– Ionic BondsIonic Bonds

propertiesproperties

– Covalent BondsCovalent Bonds Multiple BondsMultiple Bonds PolarityPolarity

– Hydrogen BondingHydrogen Bonding– London ForcesLondon Forces– Writing Lewis Writing Lewis

StructuresStructures Simple Ionic CompoundsSimple Ionic Compounds Structures that Obey the Structures that Obey the

Octet RuleOctet Rule Structures that DON’T Structures that DON’T

obey the Octet Ruleobey the Octet Rule