Types of chemical bondsTypes of chemical bonds

BondBond: Force that holds groups of two or more atoms together and : Force that holds groups of two or more atoms together and makes the atoms function as a unit.makes the atoms function as a unit.

Example: H-O-HExample: H-O-H

Bond EnergyBond Energy: Energy required to break a bond.: Energy required to break a bond.

Ionic BondIonic Bond: Attractions between oppositely charged ions.: Attractions between oppositely charged ions.

Example: NaExample: Na+ + ClCl--

Types of chemical bondsTypes of chemical bonds

Ionic CompoundIonic Compound: A compound resulting from a positive ion : A compound resulting from a positive ion (usually a metal) combining with a negative ion (usually a (usually a metal) combining with a negative ion (usually a non-metal).non-metal).

Example: MExample: M++ + X + X-- MX MX

Covalent BondCovalent Bond: Electrons are shared by nuclei.: Electrons are shared by nuclei.

Example: H-HExample: H-H

Polar Covalent BondPolar Covalent Bond: Unequal sharing of electrons by nuclei.: Unequal sharing of electrons by nuclei.

Example: H-FExample: H-F

Hydrogen fluoride is an example of a molecule that has bond Hydrogen fluoride is an example of a molecule that has bond polarity.polarity.

Lewis structuresLewis structures

Lewis StructureLewis Structure: Representation of a molecule that shows : Representation of a molecule that shows how the valence electrons are arranged among the atoms how the valence electrons are arranged among the atoms in the molecule.in the molecule.

Bonding involves the valence electrons of atoms.Bonding involves the valence electrons of atoms.

Example: NaExample: Na●● H-H H-H

Lewis structures of Lewis structures of elementselements Dots around elemental symbolDots around elemental symbol

– Symbolize valence electronsSymbolize valence electrons Thus, one must know valence electron Thus, one must know valence electron

configurationconfiguration

Lewis Structures of Lewis Structures of moleculesmoleculesSingle BondSingle Bond: Two atoms sharing one electron pair.: Two atoms sharing one electron pair.

Example: HExample: H22

Double BondDouble Bond: Two atoms sharing two pairs of electrons.: Two atoms sharing two pairs of electrons.

Example: OExample: O22

Triple BondTriple Bond: Two atoms sharing three pairs of electrons.: Two atoms sharing three pairs of electrons.

Example: NExample: N22

Resonance StructuresResonance Structures: More than one Lewis Structure can be drawn : More than one Lewis Structure can be drawn for a molecule.for a molecule.

Example: OExample: O33

Rules for Lewis structures of Rules for Lewis structures of moleculesmolecules1.1. Write out valence electrons for each Write out valence electrons for each

atomatom2.2. Connect lone electrons because lone Connect lone electrons because lone

electrons are destabilizingelectrons are destabilizing1.1. Become two shared electronsBecome two shared electrons

1.1. Called a “bond”Called a “bond”

3.3. Check to see if octet rule is satisfiedCheck to see if octet rule is satisfied1.1. Recall electron configuration resembling Recall electron configuration resembling

noble gasnoble gas1.1. In other words, there must be 8 electrons In other words, there must be 8 electrons

(bonded or non-bonded) around atom(bonded or non-bonded) around atom1.1. Non-bonded electron-pair Non-bonded electron-pair

1.1. Called “lone pair”Called “lone pair”

Let’s do some examples Let’s do some examples on the boardon the board HH22

– Duet ruleDuet rule

FF22

– Octet ruleOctet rule

OO22

NN22

Lewis structuresLewis structures

ExampleExample

Write the Lewis Structure for the following molecules:Write the Lewis Structure for the following molecules:

1)1) HH22OO

2)2) CClCCl44

1)1) Where does the carbon go & why?Where does the carbon go & why?

3)3) PHPH33

4)4) HH22SeSe

5)5) CC22HH66

Lewis structures Lewis structures continuedcontinued6)6) COCO22

7)7) CC22HH44

8)8) CC22HH22

9)9) SiOSiO22

Polyatomic ionsPolyatomic ions

If positive charge on ion If positive charge on ion Take away electron from central speciesTake away electron from central species

If negative charge on ionIf negative charge on ion Add electron to central speciesAdd electron to central species

Example:Example: HH33OO++

Your turnYour turn

NHNH44++

ClOClO--

OHOH--

Resonance structuresResonance structures

When structures can be written in When structures can be written in more than one waymore than one way– OO33

Actual molecule is “in-between”Actual molecule is “in-between”– Resonance hybridResonance hybrid

Another exampleAnother example– HCOHCO33

--

What would its resonance hybrid look What would its resonance hybrid look like?like?

Practice Practice

NONO22--

NONO33--

Formal ChargeFormal Charge Charge calculated on atom based on Charge calculated on atom based on

Lewis structureLewis structure– Yields best Lewis structure of Yields best Lewis structure of

competitors competitors FC = VE - [LE + ½(BE)]FC = VE - [LE + ½(BE)] Rules:Rules:1.1. Sum of all FC’s must equal to charge on Sum of all FC’s must equal to charge on

species, if anyspecies, if any2.2. Smaller or zero FC’s on atoms better than Smaller or zero FC’s on atoms better than

large FC’slarge FC’s3.3. Negative FC should be on most Negative FC should be on most

electronegative specieselectronegative species

Examples Examples

HBrHBr– FC on H = 1-[0 + ½ (2)] = 0FC on H = 1-[0 + ½ (2)] = 0– FC on Br = 7 – [6+ ½ (2)] = 0FC on Br = 7 – [6+ ½ (2)] = 0– Net sum of FC’s = charge on ion = 0Net sum of FC’s = charge on ion = 0

OHOH--

– FC on O = 6 – [6 + ½(2)] = -1FC on O = 6 – [6 + ½(2)] = -1– FC on H = 1 – [0 + ½(2)] = 0FC on H = 1 – [0 + ½(2)] = 0– Net sum of FC’s = charge on ion = -1Net sum of FC’s = charge on ion = -1

Practice Practice

HH22OO22

HH33OO++

Aberrant compoundsAberrant compounds

Odd-electron speciesOdd-electron species– NONO

– NONO22

Aberrant compoundsAberrant compounds

Incomplete octetIncomplete octet– BHBH33

Aberrant compoundsAberrant compounds

Expanded octetExpanded octet– Some central atoms can exceed an Some central atoms can exceed an

octetoctet Third period and higher elements Third period and higher elements

can do thiscan do this– E.g., Al, Si, P, S, Cl, As, Br, Xe, etc.E.g., Al, Si, P, S, Cl, As, Br, Xe, etc.– d-orbitals can accommodate extra d-orbitals can accommodate extra

electronselectrons

ExamplesExamples

AsIAsI55

XeFXeF22

Practice Practice

SClSCl66 XeFXeF44

Aberrant compoundsAberrant compounds

Write this out:Write this out:– SOSO44

2-2-

Can we reduce the formal Can we reduce the formal charges?charges?– If so, how?If so, how?

We can also find the average FCWe can also find the average FC– Let’s take a lookLet’s take a look

Aberrant compoundsAberrant compounds

Its OK to expand the octet for Its OK to expand the octet for those atoms that can take it in those atoms that can take it in order to lower FC’sorder to lower FC’s

Practice Practice

SOSO332-2-

POPO333-3-

SOSO22

SOSO33

HH22SeOSeO44

Electroneutrality principleElectroneutrality principle

Electrons distributed so that charges Electrons distributed so that charges on atoms are closest to zeroon atoms are closest to zero

If “-” charge present, should be on If “-” charge present, should be on mostmost electronegative atom electronegative atom

(so, “+” charge should be on (so, “+” charge should be on leastleast electronegative atom)electronegative atom)

Good for deciding which resonance Good for deciding which resonance structure is beststructure is best

Example: OCNExample: OCN--

ElectronegativityElectronegativity

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

Example: Fluorine has the highest electronegativity.Example: Fluorine has the highest electronegativity. SimilarSimilar electronegativities between elements give non- electronegativities between elements give non-

polar covalent bonds (0.0-0.4)polar covalent bonds (0.0-0.4) DifferentDifferent electronegativities between elements give electronegativities between elements give

polar covalent bonds (0.5-1.9)polar covalent bonds (0.5-1.9) If the difference between the electronegativities of two If the difference between the electronegativities of two

elements is about 2.0 or greater, the bond is ionicelements is about 2.0 or greater, the bond is ionic

ElectronegativityElectronegativity

ExampleExample

For each of the following pairs of bonds, For each of the following pairs of bonds, choose the bond that will be more polar.choose the bond that will be more polar.

Al-P vs. Al-NAl-P vs. Al-N C-O vs. C-SC-O vs. C-S

Dipole momentDipole moment

Dipole Moment Dipole Moment A molecule that has a center of positive A molecule that has a center of positive

charge and a center of negative chargecharge and a center of negative charge Will line up on electric fieldWill line up on electric field

In Debye unitsIn Debye units 1 D = 3.34 x 101 D = 3.34 x 10-30-30 C C m m

Examples Examples

FF22

COCO22

HH22OO

NHNH33

BFBF33

CClCCl44

Molecular polarityMolecular polarity

Net-dipole moment leads to Net-dipole moment leads to molecular polaritymolecular polarity

Thus the following two that have Thus the following two that have net-dipole moments are polar:net-dipole moments are polar:– HH22OO

– NHNH33

Molecular structureMolecular structure

Molecular StructureMolecular Structure: or geometric structure refers to the : or geometric structure refers to the three-dimensional arrangement of the atoms in a three-dimensional arrangement of the atoms in a molecule.molecule.

Bond AngleBond Angle: The angle formed between two bonds in a : The angle formed between two bonds in a molecule.molecule.

Molecular structure:Molecular structure:VSEPRVSEPRThe VSEPR ModelThe VSEPR Model: The valence shell electron pair : The valence shell electron pair

repulsion model is useful for predicting the molecular repulsion model is useful for predicting the molecular structures of molecules formed from nonmetals.structures of molecules formed from nonmetals.

The structure around a given atom is determined by The structure around a given atom is determined by minimizing repulsions between electron pairs.minimizing repulsions between electron pairs.

The bonding and nonbonding electron pairs (lone pairs) The bonding and nonbonding electron pairs (lone pairs) around a given atom are positioned as far apart as around a given atom are positioned as far apart as possible.possible.

Molecular Structure:Molecular Structure:VSEPRVSEPRSteps for Predicting Molecular Structure Using the VSEPR Steps for Predicting Molecular Structure Using the VSEPR

ModelModel

1. Draw the Lewis structure for the molecule.1. Draw the Lewis structure for the molecule.

2. Count the electron pairs and arrange them in the way 2. Count the electron pairs and arrange them in the way that minimizes repulsion (that is, put the lone pairs as that minimizes repulsion (that is, put the lone pairs as far apart as possible).far apart as possible).

3. Determine the positions of the atoms from the way the 3. Determine the positions of the atoms from the way the electron pairs are shared.electron pairs are shared.

4. Determine the name of the molecular structure from the 4. Determine the name of the molecular structure from the positions of the atoms.positions of the atoms.

Example Example

Br2Br2 CO2CO2 CF4CF4 PF3PF3

Your turnYour turn

NHNH44++

XeFXeF44

AsIAsI55

SFSF3 3 ++

II3 3 --