Covalent BondingCovalent Bonding

Chemical socialismChemical socialism

Bonding between nonmetals: ionic Bonding between nonmetals: ionic bonding is not an optionbonding is not an option

Ionic bonds meet requirements of elements Ionic bonds meet requirements of elements in compounds of metals and nonmetals to in compounds of metals and nonmetals to obtain noble gas configurationsobtain noble gas configurations

In the vast ocean of compounds involving In the vast ocean of compounds involving nonmetals exclusively (all organic nonmetals exclusively (all organic compounds) the avenue of electron transfer compounds) the avenue of electron transfer is not open, since all members tend to form is not open, since all members tend to form negative ionsnegative ions

Solution: electron sharingSolution: electron sharing

Still electrostaticsStill electrostatics

Balancing forces:Balancing forces: Attractive forces between nucleus and electrons of Attractive forces between nucleus and electrons of

different atomsdifferent atoms Repulsive forces between nuclei and between electronsRepulsive forces between nuclei and between electrons

As the atoms approach, electrons shift from As the atoms approach, electrons shift from approximate spherical distribution to being approximate spherical distribution to being localized between the atomslocalized between the atoms

Bond formation is result of net Bond formation is result of net attractionattraction

Coulombic force falls off with

distance (1/r2)

Distance at which balance of forces is

optimized

Internuclear repulsion

dominates

Sharing two electrons effectively Sharing two electrons effectively doubles the countdoubles the count

Each atom wants 8 (octet rule)Each atom wants 8 (octet rule) Each F atom alone has sevenEach F atom alone has seven Together they have eight eachTogether they have eight each Two shared electrons = single covalent Two shared electrons = single covalent

bondbond

Multiple bonds accommodate more Multiple bonds accommodate more extreme electron deficiencyextreme electron deficiency

OO22 and N and N22 do not achieve octets by sharing two do not achieve octets by sharing two Must share more electronsMust share more electrons OO22 has double bond has double bond NN22 has triple bond – one of the strongest in has triple bond – one of the strongest in

chemistrychemistry NN22 is very stable and unreactive – also the major is very stable and unreactive – also the major

product from explosivesproduct from explosives

Bond dissociation energyBond dissociation energy

Energy needed to break a bond into its Energy needed to break a bond into its component atomscomponent atoms

Same as energy released in forming bond Same as energy released in forming bond between atomsbetween atoms

Strength of covalent bondsStrength of covalent bonds

Covalent bonds themselves are not weakCovalent bonds themselves are not weak Bonding between molecules of covalent Bonding between molecules of covalent

compounds is weak – gases and liquidscompounds is weak – gases and liquids Where covalent bonding is found in lattices Where covalent bonding is found in lattices

(diamond, silicon etc.) melting points can be (diamond, silicon etc.) melting points can be very high (m.p. carbon 3500°C)very high (m.p. carbon 3500°C)

Ionic and covalent: two extremes of Ionic and covalent: two extremes of possibilitiespossibilities

PolarityPolarity

Unequal sharing of electronsUnequal sharing of electrons Only in homonuclear bonds are the Only in homonuclear bonds are the

electrons perfectly evenly sharedelectrons perfectly evenly shared In all other bonds the electrons are drawn In all other bonds the electrons are drawn

more towards one atom than the othermore towards one atom than the other

Electronegativity: predictor of bond Electronegativity: predictor of bond polaritypolarity

Electronegativity measures the ability of an atom in a bond Electronegativity measures the ability of an atom in a bond to attract electronsto attract electrons

Correlates with electron affinity and ionization energy:Correlates with electron affinity and ionization energy:High electron affinity = high electronegativity (nonmetals)High electron affinity = high electronegativity (nonmetals)

Low ionization energy = low electronegativity (metals)Low ionization energy = low electronegativity (metals)

Polar or non-polar?Polar or non-polar?

The following are loose definitions for The following are loose definitions for polar/non-polar bonds:polar/non-polar bonds:

If difference in electronegativity If difference in electronegativity < 0.4,< 0.4, Non-polarNon-polar

If difference in electronegativity If difference in electronegativity ≥ 0.4, ≥ 0.4, polarpolar

If difference in electronegativity ≥ 2, If difference in electronegativity ≥ 2, IonicIonic

Polarity in moleculesPolarity in molecules

Polarity is a vitally important property of Polarity is a vitally important property of matter. The special properties of water are matter. The special properties of water are a consequence of polaritya consequence of polarity

Prediction of polarity in molecules requires Prediction of polarity in molecules requires knowledge of structure in addition to knowledge of structure in addition to knowledge about polarity of individual bonds knowledge about polarity of individual bonds – stay tuned– stay tuned

Pathways to structure:Pathways to structure:Lewis dot diagrams - doing the dotsLewis dot diagrams - doing the dots

Convenient visual representation of covalent Convenient visual representation of covalent bonding in molecules: a beginning towards bonding in molecules: a beginning towards understanding molecular structure, without understanding molecular structure, without indicating anything about shapeindicating anything about shape Show only valence electronsShow only valence electrons Electrons are either in:Electrons are either in:

• bondsbonds• or lone pairs (stable molecules do not contain unpaired or lone pairs (stable molecules do not contain unpaired

electrons, with a few exceptions)electrons, with a few exceptions) Octet rule is guiding principle for distribution of electrons Octet rule is guiding principle for distribution of electrons

in the moleculein the molecule

Rules for Lewis dot structuresRules for Lewis dot structures

Guidelines for a skeleton of a moleculeGuidelines for a skeleton of a molecule1.1. Least Least electronegative element is the central atom (HOCl electronegative element is the central atom (HOCl

not HClO)not HClO)2.2. Oxygen atoms do not bond with each other except in Oxygen atoms do not bond with each other except in

peroxides or superoxidesperoxides or superoxides3.3. In ternary oxoacids (e.g. HIn ternary oxoacids (e.g. H22SOSO44), H is not bonded to the ), H is not bonded to the

central atom but to O. central atom but to O.

S = N - AS = N - A N = number of electrons required to fill octet for each atom (8 for N = number of electrons required to fill octet for each atom (8 for

each element, except 2 for H and 6 for B)each element, except 2 for H and 6 for B) A = number of valence electronsA = number of valence electrons S = number of electrons in bondsS = number of electrons in bonds

Applying the rulesApplying the rules

1.1. Calculate N for the moleculeCalculate N for the molecule

2.2. Calculate A, including charges where Calculate A, including charges where appropriate – add electrons for anion, appropriate – add electrons for anion, subtract electrons for cationssubtract electrons for cations

3.3. Determine S from S = N – ADetermine S from S = N – A

4.4. Satisfy all octets and create number of Satisfy all octets and create number of bonds as dictated by S (may be multiples)bonds as dictated by S (may be multiples)

Example of sulphur dioxideExample of sulphur dioxide

N = 24 (3 atoms @ 8)N = 24 (3 atoms @ 8) A = 18 (S = 6, O = 2 x 6 = 12 valence electrons)A = 18 (S = 6, O = 2 x 6 = 12 valence electrons) S = 6 (3 two-electron bonds)S = 6 (3 two-electron bonds) 12 non-bonded electrons (6 pairs)12 non-bonded electrons (6 pairs)

Expansion of the octetExpansion of the octet

Elements in second row invariably obey the octet Elements in second row invariably obey the octet rulerule

The heavy congeners regularly disobey itThe heavy congeners regularly disobey it Consider:Consider:

OFOF22 but SF but SF66

NClNCl33 but PCl but PCl55 Octet expansion is a consequence of the Octet expansion is a consequence of the

availability of vacant 3availability of vacant 3dd orbitals to the third row, orbitals to the third row, where there are no 2where there are no 2dd orbitals in the second row orbitals in the second row and the and the 3d3d orbitals are too high in energy orbitals are too high in energy

Investigate with dot structuresInvestigate with dot structures

Proceed with same S = N – A strategyProceed with same S = N – A strategy Octet expansion is indicated by the inability Octet expansion is indicated by the inability

to obtain a reasonable solution using the to obtain a reasonable solution using the formulaformula

Consider SFConsider SF44

N = 40, A = 28 + 6 = 34N = 40, A = 28 + 6 = 34 S = 6S = 6

6 bonding electrons and 4 bonds! Means excess 6 bonding electrons and 4 bonds! Means excess electronselectrons

Make bonds and complete octets on peripheral Make bonds and complete octets on peripheral atomsatoms

Add the excess to the central atomAdd the excess to the central atom

PClPCl55 N = 48, A = 5 x 7 + 5 = 40N = 48, A = 5 x 7 + 5 = 40 S = 8S = 8

8 bonding electrons and 5 bonds8 bonding electrons and 5 bonds Proceed as beforeProceed as before In this case the octet expansion involves a bonded In this case the octet expansion involves a bonded

atom rather than a lone pairatom rather than a lone pair

Resonance: short-comings of the dot Resonance: short-comings of the dot modelmodel

The dot structure of OThe dot structure of O33 (or SO (or SO22) can be drawn in two ) can be drawn in two equivalent waysequivalent ways

Neither is correct in of itselfNeither is correct in of itself The “true” structure is an average of the two “resonance hybrids”The “true” structure is an average of the two “resonance hybrids” Lewis model considers bonds as being between two atomsLewis model considers bonds as being between two atoms In many molecules, the bonding can involve 3 or more atomsIn many molecules, the bonding can involve 3 or more atoms This phenomenon is called delocalizationThis phenomenon is called delocalization In OIn O33 the bonding electrons are delocalized over all three O atoms the bonding electrons are delocalized over all three O atoms

Benzene: a classic example of Benzene: a classic example of delocalizationdelocalization

The top figure shows the six orbitals on the The top figure shows the six orbitals on the carbon atoms fused together into a ring of carbon atoms fused together into a ring of circulating chargecirculating charge

The lower figure shows the Lewis The lower figure shows the Lewis representation of two “resonance” representation of two “resonance” structures, and the conventional ring within structures, and the conventional ring within a hexagona hexagon

Formal chargesFormal charges

Formal charge is a measure of the degree to which at atom Formal charge is a measure of the degree to which at atom gains or loses electrons in formation of covalent bondsgains or loses electrons in formation of covalent bonds

Formal charge = No. valence electrons in free atom – No. Formal charge = No. valence electrons in free atom – No. of valence electrons in bonded atomof valence electrons in bonded atom

Useful for distinguishing between reasonable and Useful for distinguishing between reasonable and unreasonable resonance structures – the most likely unreasonable resonance structures – the most likely structure will have the lowest number of formal chargesstructure will have the lowest number of formal charges

A formal charge is on the individual atom and not on the A formal charge is on the individual atom and not on the molecule/ion. Sum of the formal charges = ion chargemolecule/ion. Sum of the formal charges = ion charge

Formal charges:Formal charges:Counting the electronsCounting the electrons

Each electron in a bond counts halfEach electron in a bond counts half Each non-bonded electron counts oneEach non-bonded electron counts one

Formal charge = total valence electrons - Formal charge = total valence electrons - ½(½(∑∑bonding electrons) - bonding electrons) - ∑(nonbonding ∑(nonbonding electrons)electrons)

Worked example with COClWorked example with COCl22

AtomAtom # valence # valence electrons electrons (=group (=group number)number)

# bonding # bonding electronselectrons

# lone # lone pair pair electronselectrons

Formal Formal chargecharge

CarbonCarbon 44 88 00 4 – (8x0.5 4 – (8x0.5 + 0) = 0+ 0) = 0

OxygenOxygen 66 44 44 6 – (4x0.5 6 – (4x0.5 + 4) =0+ 4) =0

ChlorineChlorine 77 22 66 7 – (2x0.5 7 – (2x0.5 + 6) = 0+ 6) = 0

Formal charges and selection of Formal charges and selection of preferred resonance structurespreferred resonance structures

There are two possible resonance structures for an amide There are two possible resonance structures for an amide which both satisfy the octet requirements (each drawing which both satisfy the octet requirements (each drawing has 18 dots) has 18 dots)

Left one has no formal charges - favourableLeft one has no formal charges - favourable Right one has formal charges - unfavourableRight one has formal charges - unfavourable

NH2

O

R C

O

+NH2R C

-

Both resonance structures have Both resonance structures have formal chargesformal charges

In the case of nitrous oxide, both resonance structures In the case of nitrous oxide, both resonance structures have formal chargeshave formal charges

On the left, the negative charge is on the O atom and on On the left, the negative charge is on the O atom and on the right it is on the N atomthe right it is on the N atom

The lower energy structure has the negative charge on the The lower energy structure has the negative charge on the more electronegative atommore electronegative atom

-N N O

+N N O- +

Distinguishing possible bonding Distinguishing possible bonding arrangementsarrangements

If the skeleton is not known, formal charges can If the skeleton is not known, formal charges can distinguish the more likely arrangementsdistinguish the more likely arrangements

(a) HClO or (b) HOCl?(a) HClO or (b) HOCl? Draw the Lewis structures and compute the formal Draw the Lewis structures and compute the formal

charges to show thatcharges to show that In (a), the formal charge on Cl is +1 and on O is -1In (a), the formal charge on Cl is +1 and on O is -1 In (b), the formal charges on O and Cl are both 0In (b), the formal charges on O and Cl are both 0