Expanded valence Expanded valence electrons aka Molecules electrons aka Molecules that don’t obey the octet that don’t obey the octet

rulerule

Chem 11AChem 11A

Molecules with more than 4 Molecules with more than 4 electron pairselectron pairs

Molecules with more Molecules with more than 8 valence than 8 valence electrons [expanded electrons [expanded valence shell]valence shell]

Form when an atom Form when an atom can ‘promote’ one of can ‘promote’ one of more electron from a more electron from a doubly filled s- or p-doubly filled s- or p-orbital into an unfilled orbital into an unfilled low energy d-orbitallow energy d-orbital

Only in period 3 or Only in period 3 or higher because that is higher because that is where unused d-where unused d-orbitals beginorbitals begin

Why does this ‘promotion’ occur?Why does this ‘promotion’ occur?

When atoms absorb energy (heat, When atoms absorb energy (heat, electricity, etc…)their electrons electricity, etc…)their electrons become excited and move from a become excited and move from a lower energy level orbital to a lower energy level orbital to a slightly higher one.slightly higher one.

How many new bonding sites formed How many new bonding sites formed depends on how many valence depends on how many valence electrons are excited.electrons are excited.

Exceptions to the octet rule. Shows Exceptions to the octet rule. Shows sulphur achieving 8, 10 and 12 sulphur achieving 8, 10 and 12 valence electrons due to energy valence electrons due to energy input and excited electrons.input and excited electrons.

http://www.saskschools.ca/curr_contehttp://www.saskschools.ca/curr_content/chem20/covmolec/exceptns.htmlnt/chem20/covmolec/exceptns.html

Trigonal Bipyramidal (5 pairs of V.E.)Trigonal Bipyramidal (5 pairs of V.E.)

Trigonal BipyramidalTrigonal Bipyramidal Normally would have 3 bp, but the lone Normally would have 3 bp, but the lone

pair has moved from the p-orbital to pair has moved from the p-orbital to include the d-orbital, allowing for 2 include the d-orbital, allowing for 2 additional bonding sites.additional bonding sites.

Ex: PClEx: PCl55

Octahedral (6 pairs of V.E.)Octahedral (6 pairs of V.E.)

XeF4 is square planar

SF6 is octahedral

BrF5 is square pyramidal

Bond anglesBond angles In general, the greater the bond In general, the greater the bond

angle, the weaker the repulsions.angle, the weaker the repulsions. Equatorial- equatorial (120 Equatorial- equatorial (120 oo) )

repulsions are weaker than axial- repulsions are weaker than axial- equatorial (90equatorial (90oo) repulsions.) repulsions.– Equatorial: lie on the trigonal plane Equatorial: lie on the trigonal plane

(straight across)(straight across)– Axial: lies above and below the trigonal Axial: lies above and below the trigonal

plane (up and down)plane (up and down)

Remember that Remember that lone lone pairspairs cause more cause more repulsion than repulsion than bonding sites, so bonding sites, so expect the bond expect the bond angle to be changed angle to be changed should there be lone should there be lone pairs, or pairs, or doubledouble or or tripletriple bonds bonds involved involved (multiple (multiple bonds also cause bonds also cause more repulsion than more repulsion than expected)expected)

Practice:Practice:

1.1. ClFClF33

2.2. PFPF55

3.3. XeOXeO22FF22

4.4. SOFSOF44

5.5. SClSCl666.6. IFIF44

++

7.7. IClICl44--

1.1. T-shapedT-shaped2.2. Trigonal Trigonal

bipyramidalbipyramidal3.3. SeesawSeesaw4.4. Trigonal Trigonal

bipyramidalbipyramidal5.5. OctahedralOctahedral6.6. SeesawSeesaw7.7. Square planarSquare planar

HybridizationHybridization. .

Describe σ (sigma) and π (pi) bondsDescribe σ (sigma) and π (pi) bondsState and explain the meaning of the

term hybridizationDiscuss the relationships between Lewis structures, molecular shapes and types of hybridization (sp, sp2,

sp3).

hybridizationhybridization the concept of mixing atomic orbitals to the concept of mixing atomic orbitals to

form new form new hybrid orbitalshybrid orbitals Used to Used to help explainhelp explain some atomic some atomic

bonding properties and the shape of bonding properties and the shape of molecular orbitals for molecules. molecular orbitals for molecules.

The valence orbitals (outermost s and p The valence orbitals (outermost s and p orbitals) are hybridised (mathematically orbitals) are hybridised (mathematically mixed) before bonding, converting some mixed) before bonding, converting some of the dissimilar s and p orbitals into of the dissimilar s and p orbitals into identical hybrid spidentical hybrid spnn orbitals orbitals

We must know sp, spWe must know sp, sp22, and sp, and sp33 hydrid hydrid orbitalsorbitals

http://www.mhhe.com/http://www.mhhe.com/physsci/chemistry/physsci/chemistry/essentialchemistry/essentialchemistry/flash/hybrv18.swfflash/hybrv18.swf

Video to visualize the concept Video to visualize the concept of hybridization, only of hybridization, only

concerned with sp, spconcerned with sp, sp22 and and spsp33

Hybrid orbitalsHybrid orbitals Carbon has 4 Carbon has 4

valence electrons.valence electrons. 2 electrons paired 2 electrons paired

up in the s-orbital, up in the s-orbital, and 2 electrons and 2 electrons unpaired in the p-unpaired in the p-orbital.orbital.

So why does it So why does it commonly make 4 commonly make 4 bonding sites?bonding sites?

One of carbon’s paired s-orbital One of carbon’s paired s-orbital electrons is ‘promoted’ to the empty electrons is ‘promoted’ to the empty p-orbitalp-orbital

This produces a carbon in an excited This produces a carbon in an excited state which has 4 unpaired electrons state which has 4 unpaired electrons (4 equivalent bonding sites)(4 equivalent bonding sites)

This is our explanation as to why This is our explanation as to why carbon has 4 bonding sites and not 2.carbon has 4 bonding sites and not 2.

spsp33 hybrid orbital hybrid orbital formed by mixing the formed by mixing the

outermost s- and all outermost s- and all three outermost p- three outermost p- orbitals to form four orbitals to form four spsp33 hybrids. hybrids.

The furthest these The furthest these four [negatively four [negatively charged, and charged, and therefore repulsive] therefore repulsive] orbitals can get from orbitals can get from each other is the each other is the corners of a corners of a tetrahedron (109.5°).tetrahedron (109.5°).

Overlap four s-orbitals from four hydrogens (blue) Overlap four s-orbitals from four hydrogens (blue) with four spwith four sp33 hybrids on carbon leads to formation hybrids on carbon leads to formation of of bondsbonds, each containing one electron from the , each containing one electron from the

carbon and one from the hydrogencarbon and one from the hydrogen

Examples of spExamples of sp33 hybrids hybrids

Methane, ammonia, water and hydrogen fluoride.Methane, ammonia, water and hydrogen fluoride. Note that the orbitals not involved in bonding to Note that the orbitals not involved in bonding to

hydrogen are still hybridised, but end up as lone hydrogen are still hybridised, but end up as lone pairs of electrons (symbolised by the two dots in pairs of electrons (symbolised by the two dots in the diagram above). the diagram above).

spsp22 hybrid orbital hybrid orbital formed when only one formed when only one

s- and two p-orbitals s- and two p-orbitals are involved. are involved.

This leaves one This leaves one remaining p orbital, remaining p orbital, which may be which may be involved in forming a involved in forming a double bond.double bond.

Shorter and fatter Shorter and fatter molecular orbitals molecular orbitals than spthan sp33

spsp22 hybrid orbital hybrid orbital The furthest these orbitals can get from one The furthest these orbitals can get from one

another is a trigonal planar, with the spanother is a trigonal planar, with the sp22 hybrids hybrids arranged at 120° to each other in a plane. arranged at 120° to each other in a plane.

The remaining p-orbital is at right angles the The remaining p-orbital is at right angles the plane of the molecule.plane of the molecule.

sp hybrid orbitalsp hybrid orbital formed using just one s- formed using just one s-

and one p-orbital. and one p-orbital. Two equivalent sp hybrid Two equivalent sp hybrid

orbitals are formed from orbitals are formed from them, and the 2 p-orbitals them, and the 2 p-orbitals remaining may contribute remaining may contribute to a triple bond.to a triple bond.

The set of 2 sp hybrid The set of 2 sp hybrid orbitals has a linear orbitals has a linear arrangement. (180arrangement. (180oo))

sp hybridisation is sp hybridisation is characteristic of the triple characteristic of the triple bond. (1 bond. (1 σσ-bond and 2 π -bond and 2 π (pi) bonds)(pi) bonds)

http://www.chemguide.co.uk/basicorg/bonding/ethyne.html#top

Sigma bond (Sigma bond (σσ-bond)-bond) When s and/or hybrid orbitals overlap When s and/or hybrid orbitals overlap

'end-on', 'end-on', sigmasigma bonds (σ) are formed bonds (σ) are formed They have a single area of electron density They have a single area of electron density

between the nuclei of the two atoms between the nuclei of the two atoms whose orbitals are overlapping. whose orbitals are overlapping.

In the diagrams below, σ bond is shownIn the diagrams below, σ bond is shown

Sigma bond (Sigma bond (σσ-bond)-bond)

results from head-on overlap of orbitalsresults from head-on overlap of orbitals electron density is symmetric about the electron density is symmetric about the

internuclear axis: between nuclei.internuclear axis: between nuclei.

π (pi) bondsπ (pi) bonds p orbitals can overlap p orbitals can overlap sidewayssideways too: when this too: when this

happens two lobes of electron density are formed happens two lobes of electron density are formed between the atoms. between the atoms.

From the diagram, you can see that the double From the diagram, you can see that the double bond in ethene is composed of one σ plus one π bond in ethene is composed of one σ plus one π bondbond

π (pi) bonds π (pi) bonds

electron density is electron density is above and below the above and below the internuclear axis.internuclear axis.

The electron pair The electron pair found in a pi bond can found in a pi bond can move easily from top move easily from top to bottom, don’t to bottom, don’t assume one electron assume one electron above and one below!above and one below!

π bonds are π bonds are vulnerable to attack, vulnerable to attack, as they are far from as they are far from the nucleus, it is the nucleus, it is weaker than a weaker than a σσ-bond-bond

Predicting shapePredicting shape The shape is dictated by the The shape is dictated by the σσ-bonds and -bonds and

the non-bonding electron pairs (lone pairs)the non-bonding electron pairs (lone pairs)– They allow more rotation (movement)They allow more rotation (movement)

π-bonds do not affect the shape of the π-bonds do not affect the shape of the molecule (double bonds or triple bonds)molecule (double bonds or triple bonds)– They are more rigidThey are more rigid– That’s why we refer to bonding sites when That’s why we refer to bonding sites when

using VSEPR, not paying attention to whether it using VSEPR, not paying attention to whether it was single, double or triple bondedwas single, double or triple bonded..

Delocalization of Delocalization of electronselectrons

14.3.1 Describe the 14.3.1 Describe the delocalization of (pi) π- electrons delocalization of (pi) π- electrons and explain how this can account and explain how this can account for the structure of some speciesfor the structure of some species

Delocalised electronsDelocalised electrons The term 'delocalised' refers to an electron The term 'delocalised' refers to an electron

which is not 'attached' to a particular atom which is not 'attached' to a particular atom or to a specific bond. or to a specific bond.

Delocalized electrons are contained within Delocalized electrons are contained within an orbital that extends over several an orbital that extends over several adjacent atoms. adjacent atoms.

Classically, delocalized electrons can be Classically, delocalized electrons can be found in double bonds and in aromatic found in double bonds and in aromatic systems systems

Double bonds = 1 sigma Double bonds = 1 sigma andand 1 pi bond 1 pi bond Delocalisation is often represented with Delocalisation is often represented with

resonance structures or resonance hybridresonance structures or resonance hybrid

Resonance structuresResonance structures

the nitrate ion can be viewed the nitrate ion can be viewed as ifas if it it resonates between the three different resonates between the three different structures above. structures above.

Nitrate doesn’t change from one to the Nitrate doesn’t change from one to the next, but behaves as a combination of all next, but behaves as a combination of all structuresstructures

Resonance is possible whenever a Lewis Resonance is possible whenever a Lewis structure has a structure has a multiplemultiple bond and an bond and an adjacent atom with at least adjacent atom with at least oneone lone pair. lone pair.

The following is the general form for The following is the general form for resonance in a structure of this type. resonance in a structure of this type.

PracticePractice Try to show the Try to show the

individual Lewis individual Lewis structures for the structures for the HCOHCO22

-- ion ion Show its resonance Show its resonance

structure toostructure too

Practice drawing these resonance Practice drawing these resonance structures:structures:

1.1. NONO33--

2.2. NONO22--

3.3. COCO332-2-

4.4. OO33

5.5. RCOORCOO--

6.6. Benzene (CBenzene (C66HH66))

Think about thisThink about this Kekule claimed that Kekule claimed that

the inspiration for the the inspiration for the cyclic structure of cyclic structure of benzene came from a benzene came from a dream.dream.

What role do the less What role do the less rational ways of rational ways of knowing play in the knowing play in the acquistion of scientific acquistion of scientific knowledge?knowledge?

Bibliography and sites to visitBibliography and sites to visit http://www.tutorvista.com/content/chemisthttp://www.tutorvista.com/content/chemist

ry/chemistry-iii/chemical-bonding/types-cory/chemistry-iii/chemical-bonding/types-covalent-bonds.phpvalent-bonds.php– Good site on types of covalent bondsGood site on types of covalent bonds

http://www.mikeblaber.org/oldwine/chm10http://www.mikeblaber.org/oldwine/chm1045/notes/Geometry/VSEPR/Geom02.htm45/notes/Geometry/VSEPR/Geom02.htm– Used for expanded valence shell picturesUsed for expanded valence shell pictures

http://www.kentchemistry.com/links/http://www.kentchemistry.com/links/bonding/lewisdotstruct.htmbonding/lewisdotstruct.htm– Puts the lewis diagrams together and explain Puts the lewis diagrams together and explain

them. Including expanded shellthem. Including expanded shell

http://www.mpcfaculty.net/mark_bishop/http://www.mpcfaculty.net/mark_bishop/resonance.htmresonance.htm– Resonance structures pictures and notesResonance structures pictures and notes

http://en.wikipedia.org/wiki/Delocalizationhttp://en.wikipedia.org/wiki/Delocalization– Notes on delocalisation of electronsNotes on delocalisation of electrons

http://www.steve.gb.com/science/http://www.steve.gb.com/science/atomic_structure.htmlatomic_structure.html– Amazing website for hybrid orbitalsAmazing website for hybrid orbitals

http://library.thinkquest.org/C006669/http://library.thinkquest.org/C006669/data/Chem/bonding/shapes.htmldata/Chem/bonding/shapes.html– Good review of all shapesGood review of all shapes

http://www.chemguide.co.uk/http://www.chemguide.co.uk/basicorg/bonding/ethene.htmlbasicorg/bonding/ethene.html– spsp22 hybridisation and ethene as an hybridisation and ethene as an

example coveredexample covered