Chemical BondingChemical Bonding

Natural rock formation Natural rock formation in Bryce Canyon, Utah.in Bryce Canyon, Utah.

Source: M.I. Sinibaldi/The Stock Market/Corbis

Structure Determines Structure Determines Properties!Properties! A cardinal principle of chemistry is that A cardinal principle of chemistry is that

the macroscopic observed properties of the macroscopic observed properties of a material are related to its microscopic a material are related to its microscopic structurestructure..– And visa versaAnd visa versa

The microscopic structure entailsThe microscopic structure entails– the kinds of atomsthe kinds of atoms– the manner in which they are attached the manner in which they are attached – their relationship to other molecules, like their relationship to other molecules, like

and dislikeand dislike– the shape of the moleculethe shape of the molecule

Diamond, composed of carbon Diamond, composed of carbon atoms bonded together to atoms bonded together to produce one of the hardest produce one of the hardest materials known, makes a materials known, makes a beautiful gemstone.beautiful gemstone.

Chemical BondsChemical Bonds Forces that hold atoms togetherForces that hold atoms together Ionic bondsIonic bonds are the forces of are the forces of

attraction between ionsattraction between ions– ions formed by electron transferions formed by electron transfer– electrostatic forceselectrostatic forces

Covalent bondsCovalent bonds are the forces of are the forces of attraction between two atoms attraction between two atoms which are sharing electronswhich are sharing electrons

Figure 11.1: The Figure 11.1: The formation of a bond formation of a bond between two hydrogen between two hydrogen atoms.atoms.

Source: Andrey K. Geim/High Field Magnet Laboratory/University of Nijmegen

Figure 11.2: Probability representations of the Figure 11.2: Probability representations of the electron sharing in HF. (a) What the probability electron sharing in HF. (a) What the probability map would look like if the two electrons in the map would look like if the two electrons in the HH––F bond were shared equally. (b) The actual F bond were shared equally. (b) The actual situation, where the shared pair spends more situation, where the shared pair spends more time close to the fluorine atom than to the time close to the fluorine atom than to the hydrogen atom.hydrogen atom.

Ionic BondsIonic Bonds Results from reaction between Metal and NonmetalResults from reaction between Metal and Nonmetal Metal loses electrons to form cation, Nonmetal gains electrons to Metal loses electrons to form cation, Nonmetal gains electrons to

form anionform anion Ionic bond is the attraction between a positive ion and negative Ionic bond is the attraction between a positive ion and negative

ionion Larger Charge = Stronger AttractionLarger Charge = Stronger Attraction Smaller Ion = Stronger AttractionSmaller Ion = Stronger Attraction No bond is 100% ionic!!No bond is 100% ionic!! Electrostatic attraction nondirectionalElectrostatic attraction nondirectional

– no direct anion-cation pair, no direct anion-cation pair, No ionic moleculeNo ionic molecule chemical formula is empirical formula, simply giving the ratio of ions based on chemical formula is empirical formula, simply giving the ratio of ions based on

charge balancecharge balance Ions arranged in a pattern called a Ions arranged in a pattern called a crystal latticecrystal lattice

maximizes attractions between + and - ionsmaximizes attractions between + and - ions

Figure 11.8: The Figure 11.8: The structure of lithium structure of lithium fluoride.fluoride.

Covalent BondsCovalent Bonds Typical of molecular substancesTypical of molecular substances Atoms bond together to form Atoms bond together to form

moleculesmolecules– strong attractionstrong attraction

Sharing pairs of electronsSharing pairs of electrons Molecules attracted to each other Molecules attracted to each other

weaklyweakly Often found between nonmetal atomsOften found between nonmetal atoms

Figure 11.4: The three possible types of bonds: Figure 11.4: The three possible types of bonds: (a) a covalent bond formed between identical (a) a covalent bond formed between identical atoms; (b) a polar covalent bond, with both atoms; (b) a polar covalent bond, with both ionic and covalent components; and ionic and covalent components; and (c) an ionic bond, with no electron sharing.(c) an ionic bond, with no electron sharing.

Bond PolarityBond Polarity Covalent bonding between unlike Covalent bonding between unlike

atoms results in unequal sharing of atoms results in unequal sharing of the electronsthe electrons– One end of the bond has larger electron One end of the bond has larger electron

density than the otherdensity than the other The result is The result is bond polaritybond polarity

– The end with the larger electron density The end with the larger electron density gets a partial negative chargegets a partial negative charge

– The end that is electron deficient gets a The end that is electron deficient gets a partial positive chargepartial positive charge

H F••

Figure 11.5: (a) The Figure 11.5: (a) The charge distribution in charge distribution in the water molecule. the water molecule. (b) The water (b) The water molecule behaves as molecule behaves as if it had a positive if it had a positive end and a negative end and a negative end, as indicated by end, as indicated by the arrow.the arrow.

Figure 11.6: (a) Polar water molecules are Figure 11.6: (a) Polar water molecules are strongly attracted to positive ions by their strongly attracted to positive ions by their negative ends. (b) They are also strongly negative ends. (b) They are also strongly attracted to negative ions by their positive attracted to negative ions by their positive ends.ends.

Figure 11.7: Polar water Figure 11.7: Polar water molecules are strongly molecules are strongly attracted to each other.attracted to each other.

Figure 11.10: When liquid Figure 11.10: When liquid oxygen is poured between the oxygen is poured between the poles of a poles of a magnet, it “sticks” until it boils magnet, it “sticks” until it boils away.away.

Source: Donald Clegg

ElectronegativityElectronegativity Measure of the ability of an atom to Measure of the ability of an atom to

attract shared electronsattract shared electrons– Larger electronegativity means atom attracts Larger electronegativity means atom attracts

more stronglymore strongly– Values 0.7 to 4.0Values 0.7 to 4.0

Increases across period (left to right) on Increases across period (left to right) on Periodic TablePeriodic Table

Decreases down group (top to bottom) on Decreases down group (top to bottom) on Periodic TablePeriodic Table

Larger difference in electronegativities Larger difference in electronegativities means more polar bondmeans more polar bond– negative end toward more electronegative atomnegative end toward more electronegative atom

Electronegativity Electronegativity DifferencesDifferences Less than 0.5 represents a Less than 0.5 represents a

nonpolar bondnonpolar bond 0.5 to 1.7 represents a polar 0.5 to 1.7 represents a polar

covalent bondcovalent bond Greater than 1.7 represents an Greater than 1.7 represents an

ionic bondionic bond

Figure 11.3: Figure 11.3: Electronegativity values Electronegativity values for for selected elements.selected elements.

Dipole MomentDipole Moment Bond polarity results in an unequal electron Bond polarity results in an unequal electron

distribution, resulting in areas of partial positive and distribution, resulting in areas of partial positive and partial negative chargepartial negative charge

Any molecule that has a center of positive charge Any molecule that has a center of positive charge and a center of negative charge in different points is and a center of negative charge in different points is said to have a said to have a dipole momentdipole moment

If a molecule has more than one polar covalent bond, If a molecule has more than one polar covalent bond, the areas of partial negative and positive charge for the areas of partial negative and positive charge for each bond will partially add to or cancel out each each bond will partially add to or cancel out each otherother

The end result will be a molecule with one center of The end result will be a molecule with one center of positive charge and one center of negative chargepositive charge and one center of negative charge

The dipole moment effects the attractive forces The dipole moment effects the attractive forces between molecules and therefore the physical between molecules and therefore the physical properties of the substanceproperties of the substance

Electron Arrangements And Electron Arrangements And Ion ChargeIon Charge

We knowWe know– Group 1A metals form ions with +1 Group 1A metals form ions with +1

chargecharge– Group 2A metals form ions with +2 Group 2A metals form ions with +2

chargecharge– Group 7A nonmetals form ions with -1 Group 7A nonmetals form ions with -1

chargecharge– Group 6A nonmetals form ions with -2 Group 6A nonmetals form ions with -2

chargecharge– Group 8A nonmetals do not form ions, Group 8A nonmetals do not form ions,

in fact they are extremely unreactivein fact they are extremely unreactive

Electron Arrangements and Ion Electron Arrangements and Ion ChargeCharge

Representative Metals Representative Metals form cations by losing form cations by losing enough electrons to enough electrons to get the same electron get the same electron configuration as the configuration as the previous noble gasprevious noble gas

Nonmetals form Nonmetals form anions by gaining anions by gaining enough electrons to enough electrons to get the same electron get the same electron configuration as the configuration as the next noble gasnext noble gas

Atom AtomsElectronConfig

Ion IonsElectronConfig

Na [Ne]3s1 Na+1 [Ne]

Mg [Ne]3s2 Mg+2 [Ne]

Al [Ne]3s23p1 Al+3 [Ne]

O [He]2s22p4 O-2 [Ne]

F [He]2s22p5 F-1 [Ne]

Figure Figure 11.9: 11.9: Relative Relative sizes of sizes of some ions some ions and their and their parent parent atoms.atoms.

Representative metals lose their valence Representative metals lose their valence electrons to form cationselectrons to form cations

Nonmetals gain electrons so their valence shell Nonmetals gain electrons so their valence shell has the same electron arrangement as the next has the same electron arrangement as the next noble gasnoble gas

There have to be enough electrons from the There have to be enough electrons from the metals atoms to supply the needed electrons metals atoms to supply the needed electrons for the nonmetal atomsfor the nonmetal atoms– Allows us to predict the formulas of ionic compoundsAllows us to predict the formulas of ionic compounds

In Polyatomic ions, the atoms in the ion are In Polyatomic ions, the atoms in the ion are connected with covalent bonds. The ions are connected with covalent bonds. The ions are attracted to oppositely charged ions to form an attracted to oppositely charged ions to form an ionic compoundionic compound

Electron Arrangements and Ionic Electron Arrangements and Ionic BondingBonding

Properties of Ionic Properties of Ionic CompoundsCompounds

All solids at room temperatureAll solids at room temperature– Melting points greater than 300°CMelting points greater than 300°C

Liquid state conducts electricity, solid state does notLiquid state conducts electricity, solid state does not– Liquid = moltenLiquid = molten

Brittle and HardBrittle and Hard Often soluble in water, and when dissolved the solution Often soluble in water, and when dissolved the solution

becomes an electrical conductorbecomes an electrical conductor– When ionic compounds containing polyatomic ions dissolve, the When ionic compounds containing polyatomic ions dissolve, the

covalent bonds holding the polyatomic ion do not break, the ion covalent bonds holding the polyatomic ion do not break, the ion stays together even though it separates from the other ionstays together even though it separates from the other ion

– All strong electrolytesAll strong electrolytes

Bonding and Structure of Ionic Bonding and Structure of Ionic CompoundsCompounds Crystal LatticeCrystal Lattice = geometric pattern = geometric pattern

determined by the size and charge of the ionsdetermined by the size and charge of the ions Anions larger than cationAnions larger than cation

– Almost alwaysAlmost always– Anions larger than parent atom, Cations smaller than Anions larger than parent atom, Cations smaller than

parent atomparent atom Anions generally considered “hard” spheres Anions generally considered “hard” spheres

packed as efficiently as possible, with the packed as efficiently as possible, with the cations occupying the “holes” in the packingcations occupying the “holes” in the packing

Arrangement results in each cation being Arrangement results in each cation being surrounded by as many anions as will fitsurrounded by as many anions as will fit– And visa versaAnd visa versa– Maximizes attractions between ionsMaximizes attractions between ions

Lewis Symbols of Atoms Lewis Symbols of Atoms and Ionsand Ions

Also known as electron dot symbolsAlso known as electron dot symbols Use symbol of element to represent nucleus and inner Use symbol of element to represent nucleus and inner

electronselectrons Use dots around the symbol to represent valence electronsUse dots around the symbol to represent valence electrons

– put one electron on each side first, then pairput one electron on each side first, then pair Elements in the same group have the same Lewis symbolElements in the same group have the same Lewis symbol

– Because they have the same number of valence Because they have the same number of valence electronselectrons

Li• Be• •B• •C• •N• •O: :F: :Ne:• •

•

• • • •

•• •• •• ••

••

Li• Li+1 :F: [:F:]-1

•

•• ••

••

G. N. G. N. Lewis Lewis in his in his lab.lab.

Source: The Bancroft Library

Writing Lewis Structures of Writing Lewis Structures of Molecules Molecules

Count the Count the totaltotal number of valence electrons from number of valence electrons from all the atomsall the atoms

Attach the atoms together with one pair of Attach the atoms together with one pair of electronselectrons– A line is often used as shorthand for a pair of electrons A line is often used as shorthand for a pair of electrons

that attach atoms togetherthat attach atoms together Arrange the remaining electrons in pairs so that all Arrange the remaining electrons in pairs so that all

hydrogen atoms have 2 electrons (1 bond) and hydrogen atoms have 2 electrons (1 bond) and other atoms have 8 electrons (combination of other atoms have 8 electrons (combination of bonding and nonbonding)bonding and nonbonding)

Occasionally atoms may violate this ruleOccasionally atoms may violate this rule– Nonbonding pairs of electrons are also know as Lone PairsNonbonding pairs of electrons are also know as Lone Pairs

Covalent BondsCovalent Bonds Single Covalent Bond the atoms share 2 electronsSingle Covalent Bond the atoms share 2 electrons, ,

– ((1 pair)1 pair) Double Covalent Bond the atoms share 4 electronsDouble Covalent Bond the atoms share 4 electrons, ,

– ((2 pairs)2 pairs) Triple Covalent Bond the atoms share 6 electrons,Triple Covalent Bond the atoms share 6 electrons,

– ((3 pairs)3 pairs) Bond Strength = Triple > Double > SingleBond Strength = Triple > Double > Single

– For bonds between same atoms, CFor bonds between same atoms, CN > C=N > C—NN > C=N > C—N– Though Double not 2x the strength of Single and Triple Though Double not 2x the strength of Single and Triple

not 3x the strength of Singlenot 3x the strength of Single Bond Length = Single > Double > TripleBond Length = Single > Double > Triple

– For bonds between same atoms, C—N > C=N > CFor bonds between same atoms, C—N > C=N > CN N

ResonanceResonance When there is more than one Lewis structure for When there is more than one Lewis structure for

a molecule that differ a molecule that differ onlyonly in the position of the in the position of the electrons they are called electrons they are called resonance structuresresonance structures– Lone Pairs and Multiple Bonds in different Lone Pairs and Multiple Bonds in different

positionspositions The actual molecule is a combination of all the The actual molecule is a combination of all the

resonance formsresonance forms– It does It does notnot resonate between the two forms, though resonate between the two forms, though

we often draw it that waywe often draw it that way

•••• •• ••••••••

•• ••O S O O S O•••••• ••••

••••

••••

Problems with Lewis Problems with Lewis StructuresStructures

Some atoms do not tend to follow the octet ruleSome atoms do not tend to follow the octet rule– B and Be often found octet deficientB and Be often found octet deficient– Elements in the 3Elements in the 3rdrd Period or below often have Period or below often have

expanded octetsexpanded octets Some molecules have an odd number of Some molecules have an odd number of

electronselectrons Impossible to accurately draw Lewis structure of Impossible to accurately draw Lewis structure of

molecules that exhibit resonancemolecules that exhibit resonance Sometimes the Lewis Structure does not Sometimes the Lewis Structure does not

accurately describe a structure that explains all accurately describe a structure that explains all the observed properties of the moleculethe observed properties of the molecule

The paramagnetic behavior of OThe paramagnetic behavior of O22

Some Geometric Some Geometric FiguresFigures

LinearLinear– 2 atoms on opposite sides of central 2 atoms on opposite sides of central

atomatom– 180° bond angles180° bond angles

Trigonal PlanarTrigonal Planar– 3 atoms form a triangle around the 3 atoms form a triangle around the

central atomcentral atom– PlanarPlanar– 120° bond angles 120° bond angles

TetrahedralTetrahedral– 4 surrounding atoms form a 4 surrounding atoms form a

tetrahedron around the central atomtetrahedron around the central atom– 109.5° bond angles109.5° bond angles

180°

120°

109.5°

Predicting Molecular Predicting Molecular GeometryGeometry

VSEPR TheoryVSEPR Theory– Valence Shell Electron Pair RepulsionValence Shell Electron Pair Repulsion

The shape around the central atom(s) can be predicted The shape around the central atom(s) can be predicted by assuming that the areas of electrons on the central by assuming that the areas of electrons on the central atom will repel each otheratom will repel each other

Each Bond counts as 1 area of electronsEach Bond counts as 1 area of electrons– single, double or triple all count as 1 areasingle, double or triple all count as 1 area

Each Lone Pair counts a 1 area of electronsEach Lone Pair counts a 1 area of electrons– Even though lone pairs are not attached to other atoms, they Even though lone pairs are not attached to other atoms, they

do “occupy space” around the central atomdo “occupy space” around the central atom– Lone pairs generally “push harder” than bonding electrons, Lone pairs generally “push harder” than bonding electrons,

affecting the bond angleaffecting the bond angle

ShapesShapes LinearLinear

– 2 areas of electrons around the central atom, both 2 areas of electrons around the central atom, both bondingbonding

Or two atom molecule as trivial caseOr two atom molecule as trivial case

TrigonalTrigonal– 3 areas of electrons around the central atom3 areas of electrons around the central atom– All Bonding = All Bonding = trigonal planartrigonal planar– 2 Bonding + 1 Lone Pair = 2 Bonding + 1 Lone Pair = trigonal benttrigonal bent

TetrahedralTetrahedral– 4 areas of electrons around the central atom4 areas of electrons around the central atom– All Bonding = All Bonding = tetrahedraltetrahedral– 3 Bonding + 1 Lone Pair = 3 Bonding + 1 Lone Pair = trigonaltrigonal pyramidpyramid– 2 Bonding + 2 Lone Pair = 2 Bonding + 2 Lone Pair = tetrahedral bent or V-tetrahedral bent or V-

shapedshaped

Computer graphics of (a) Computer graphics of (a) a linear a linear molecule containing molecule containing three atoms.three atoms.

Source: Frank Cox

Computer graphics of (b) Computer graphics of (b) a trigonal a trigonal planar molecule.planar molecule.

Source: Frank Cox

Computer graphics of Computer graphics of (c) a tetrahedral (c) a tetrahedral molecule.molecule.

Source: Frank Cox

Figure Figure 11.11: The 11.11: The tetrahedral tetrahedral molecular molecular structure of structure of methane.methane.

Figure Figure 11.12: The 11.12: The molecular molecular structure of structure of methane.methane.

Figure 11.13: (a) the tetrahedral arrangement Figure 11.13: (a) the tetrahedral arrangement of electron pairs around the nitrogen atom in of electron pairs around the nitrogen atom in the ammonia molecule. (b) Three of the the ammonia molecule. (b) Three of the electron pairs around nitrogen are shared with electron pairs around nitrogen are shared with the hydrogen atoms as shown, and one is a the hydrogen atoms as shown, and one is a lone pair. (c) The NH3 molecule has the trigonal lone pair. (c) The NH3 molecule has the trigonal pyramid structure.pyramid structure.

Figure 11.14: (a) The tetrahedral arrangement Figure 11.14: (a) The tetrahedral arrangement of the four electron pairs around oxygen in the of the four electron pairs around oxygen in the water molecule. (b) Two of the electron pairs are water molecule. (b) Two of the electron pairs are shared between oxygen and the hydrogen shared between oxygen and the hydrogen atoms. (c) The V-shaped molecular structure of atoms. (c) The V-shaped molecular structure of the water molecule.the water molecule.