Polar bonds and molecular Polar bonds and molecular polarity polarity

Degrees between ionic and Degrees between ionic and covalentcovalent

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

In the molecule FIn the molecule F22:: Each atom wants 8Each atom wants 8 Alone each has sevenAlone each has seven Together they have eightTogether they have eight Single covalent bondSingle covalent bond

Covalent bonds between unlike Covalent bonds between unlike elementselements

Oxygen requires octet – shares two Oxygen requires octet – shares two electrons with H atoms (one with each)electrons with H atoms (one with each)

Hydrogen requires two – each atom shares Hydrogen requires two – each atom shares one electron with Oone electron with O

OH H HH O

Lewis dot structuresLewis dot structures

In going from G4 – G7, a H In going from G4 – G7, a H atom is replaced by a lone pair atom is replaced by a lone pair of electronsof electrons

The total number of electrons is The total number of electrons is equal to the sum of all the equal to the sum of all the valence electronsvalence electrons

The The totaltotal number of electrons number of electrons remains the same – 8remains the same – 8

Each atom has a complete Each atom has a complete octetoctet

C

H

HH HN

HH H

OHHFH

Multiple bonds are a featureMultiple bonds are a feature

OO22 and N and N22 do not achieve octets by sharing two electrons do not achieve octets by sharing two electrons Must share more electronsMust share more electrons OO22 has double bond (four electrons shared) has double bond (four electrons shared) NN22 has triple bond (six electrons shared) – one of the has triple bond (six electrons shared) – one of the

strongest in chemistrystrongest in chemistry NN22 is very stable and unreactive – also the major product from is very stable and unreactive – also the major product from

explosivesexplosives

N NO O

Properties of covalent compoundsProperties of covalent compounds

Gases, liquids and solids at room Gases, liquids and solids at room temperaturetemperature

May be hard or soft (diamond is a covalent May be hard or soft (diamond is a covalent solid)solid)

May be soluble in polar or non-polar May be soluble in polar or non-polar solventssolvents

Solutions and melts do not conduct Solutions and melts do not conduct electricityelectricity

Most covalent compounds are molecular Most covalent compounds are molecular

Polar bondsPolar bonds

The ionic bond and the equally shared The ionic bond and the equally shared covalent bond are two extremescovalent bond are two extremes

Complete transfer of charge to equal Complete transfer of charge to equal sharing of chargesharing of charge

Many bonds fall in between: atoms of Many bonds fall in between: atoms of different elements have different attraction different elements have different attraction for electronsfor electrons

ElectronegativityElectronegativity

The degree to which an atom attracts The degree to which an atom attracts electrons towards itself in a bond with electrons towards itself in a bond with another atomanother atom highly electronegative atom attracts electronshighly electronegative atom attracts electrons weakly electronegative atom does notweakly electronegative atom does not

Table of electronegativityTable of electronegativity

Most electronegative

Least electronegative

Increasing electronegativity Increasing electronegativity difference increases polaritydifference increases polarity

Non-Non-polarpolar PolarPolar

C:CC:C C:NC:N C:OC:O C:FC:F

00 0.490.49 0.890.89 1.431.43

Electronegativity differenceElectronegativity difference

The gamut of bonding typesThe gamut of bonding typesIonicIonic Polar Polar

covalentcovalentNonpolar Nonpolar covalentcovalent

Na:FNa:F H:FH:F F:FF:F

Sodium Sodium fluoridefluoride

Hydrogen Hydrogen fluoridefluoride

FluorineFluorine

Polar bonds and polar moleculesPolar bonds and polar molecules

Any bond containing different elements will Any bond containing different elements will be polar to some degreebe polar to some degree

For a molecule to be polar will depend upon For a molecule to be polar will depend upon how the bonds are arrangedhow the bonds are arranged

A molecule may contain polar bonds and be A molecule may contain polar bonds and be itself itself nonnon-polar-polar

We need to understand the molecular We need to understand the molecular structurestructure

Lewis dot structures: doing the dotsLewis dot structures: doing the dots

Molecular structure reduced to simplest terms Molecular structure reduced to simplest terms showing only the arrangements of the valence showing only the arrangements of the valence electrons as dots in a 2-dimensional figureelectrons as dots in a 2-dimensional figure Show only valence electronsShow only valence electrons Electrons are either in:Electrons are either in:

• bondsbonds• lone pairs (stable molecules do not contain unpaired electrons – lone pairs (stable molecules do not contain unpaired electrons –

with very few exceptions)with very 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

Lewis dot structures made easyLewis dot structures made easy

Start with the skeleton of the moleculeStart with the skeleton of the molecule Least Least electronegative element is the central electronegative element is the central

atomatom S = N - AS = N - A

N = number of electrons required to fill octet for N = number of electrons required to fill octet for each atom (8 for each element, except 2 for H each atom (8 for each element, except 2 for H and 6 for B)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

Calculate Calculate NN for the molecule for the molecule Calculate Calculate AA (all the dots), (all the dots),

including charges where including charges where appropriate appropriate (add one for each –(add one for each –ve charge and subtract one for ve charge and subtract one for each +ve charge)each +ve charge)

Determine Determine SS from from SS = = NN – – AA Satisfy all octets and create Satisfy all octets and create

number of bonds dictated by number of bonds dictated by SS (may be multiples) (may be multiples)

NFNF33

NN = 8(N) + 3 x 8(F) = = 8(N) + 3 x 8(F) = 3232

AA = 5(N) + 3 x 7(F) = = 5(N) + 3 x 7(F) = 2626

SS = 32 – 26 = = 32 – 26 = 66

N FFF

N FFF

Two tests for dot structuresTwo tests for dot structures

Are the number of dots in the molecule Are the number of dots in the molecule equal to the number of valence electrons?equal to the number of valence electrons?

Are all the octets satisfied?Are all the octets satisfied? If both yes structure is validIf both yes structure is valid If either no then back to the drawing boardIf either no then back to the drawing board

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)

S OO

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

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 solutionto obtain a reasonable solution

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 electrons6 bonding electrons and 4 bonds! Means excess electrons Make bonds and complete octets on peripheral atomsMake bonds and complete octets on peripheral atoms 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 atom In this case the octet expansion involves a bonded atom

rather than a lone pairrather 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 equivalent ways) can be drawn in two equivalent 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