Recap Resonance

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Recap Resonance. Elements from the third period and beyond form compounds with 8 or more electrons around the atom. This flexibility may result in quite different resonance structure being possible. - PowerPoint PPT Presentation

Transcript of Recap Resonance

Welcome to Chemistry 1001

1Recap ResonanceElements from the third period and beyond form compounds with 8 or more electrons around the atom.

This flexibility may result in quite different resonance structure being possible.

The resonance structure(s) with the greatest contribution to the actual structure can be identified using the valency of oxygen as a guide.

Example: ClO4-

Chem 1001 Lecture 18Dr Chiara Neto12VSEPR TheoryLewis structures give bonding arrangements but do not imply any molecular shape. For this we use:

Valence Shell Electron Pair Repulsion Theory

This relies on minimising repulsion between areas of electrons (bond pairs and lone pairs) around the central atom.Chem 1001 Lecture 19Dr Chiara Neto23VSEPR TheoryDraw Lewis Structure.

Count number of electron pairs.Count both bonding pairs and non-bonding pairs.Count multiple bonds as only one area of electrons.

Determine the arrangement of electron pairs.Electron pairs want to be as far away from each other as possible.

Use atom positions to name molecular geometry.This is the atom positions.

Chem 1001 Lecture 19Dr Chiara Neto34Electron Pair ArrangementsTwo electron pairs:Atoms at the opposite ends of a line.180 degrees between areas of electrons.Called linear.eg CO2

Chem 1001 Lecture 19Dr Chiara Neto45Electron Pair ArrangementsThree electron pairs:Atoms at the corners of a triangle.120 degrees between electron pairs.Called trigonal planar.Eg BF3

Chem 1001 Lecture 19Dr Chiara Neto56Electron Pair ArrangementsFour electron pairs:Atoms at the corners of a tetrahedron.109.5 degrees between electron pairs.Called tetrahedral.Eg CH4

Chem 1001 Lecture 19Dr Chiara Neto67Remove one arm from the electron pair arrangement for each lone pair present.

Trigonal Planar (3 e- pairs)

Molecular Geometry

Figure 10.4 SilberbergChem 1001 Lecture 19Dr Chiara Neto78Tetrahedral (4 e- pairs)

Molecular Geometry

Figure 10.5 Silberberg

Chem 1001 Lecture 19Dr Chiara Neto89Molecular GeometryRepulsion: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

109.5

107

104.5Chem 1001 Lecture 19Dr Chiara Neto910Molecular Geometry

Figure 10.9 SilberbergChem 1001 Lecture 19Dr Chiara Neto1011Molecular Geometry - ExampleMolecules with multiple bonds eg COCl2 total 24 e-3 areas of electrons about C, so trigonal planar arrangement of electrons

No lone pairs so molecular geometry is also trigonal planar~120

Chem 1001 Lecture 19Dr Chiara Neto1112Molecular Geometry - ExampleCases when there is no single central atomJust apply the VSEPR rules to each central atom in turn.

3 areas of electrons about each C, so trigonal planar arrangement of electrons about each C~120Chem 1001 Lecture 19Dr Chiara Neto1213Dipole MomentsAny bond between two different atoms will be polar.

A molecule has a permanent dipole moment if it contains polar bonds and it is not a symmetrical shape.

Note: Cations and anions are not polar the overall charge overwhelms any local d+ vs d- effects. Chem 1001 Lecture 19Dr Chiara Neto1314Dipole MomentsPolar molecules

Non-polar molecules

HFH2OCHCl3

N2CO2

CCl4Chem 1001 Lecture 19Dr Chiara Neto14By the end of this lecture, you should:work out the number of bonding and non-bonding pairs from the Lewis structure of a moleculepredict the distribution of these pairs around an atompredict and describe the molecular shapedetermine if a permanent dipole exists

be able to complete the worksheet (if you havent already done so)

15Learning Outcomes:16Questions to complete for next lecture:Draw the shapes of the following molecules and ions and give approximate bond anglesBH3NH4+CS2CH2OCH3ClH3O+Chem 1001 Lecture 41617Questions to complete for next lecture:What are the approximate C-C-C bond angles in the two molecules below?Are these molecules flat?