1 Recap – Last Lecture Lewis structures give us a stable arrangement of bonds and lone pairs. Based on 8 e - for C, N, O & F. 8, 10 or 12 e - for P, S, Cl, Br, I (when these are the central atoms). Resonance occurs where two or more valid Lewis structures may be drawn which differ only in position of electrons. The actual structure is a weighted average of resonance structures and is more stable than expected. The resonance structure(s) with the greatest contribution to the actual structure can be identified using the valency of oxygen as a guide.

2 VSEPR Theory Lewis 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.

3 VSEPR Theory 1.Draw Lewis Structure. 2.Count number of electron areas. Count both bonding pairs and non-bonding pairs. Count a multiple bonds as one area of electrons. 3.Determine the arrangement of electron areas. Electron pairs want to be as far away from each other as possible. 4.Use atom positions to name molecular geometry.

4 Electron Pair Arrangements Two electron areas: –Atoms at the opposite ends of a line. –180º between areas of electrons. –Called linear. –eg CO 2

5 Electron Pair Arrangements Three electron areas: –Atoms at the corners of a triangle. –120º between electron pairs. –Called trigonal planar. –Eg BF 3

6 Electron Pair Arrangements Four electron areas: –Atoms at the corners of a tetrahedron. –109.5º between electron pairs. –Called tetrahedral. –Eg CH 4

7 Electron Pair Arrangements Five electron areas: –Atoms at the corners of a trigonal bipyramid. –Some electron pairs separated by 120º degrees, other by 90º. –Called trigonal bipyramidal.

8 Electron Pair Arrangements Six electron areas: –Atoms at the corners of an octahedron. –90º between electron pairs. –Called octahedral.

9 Remove one arm from the electron area arrangement for each lone pair present. Trigonal Planar (3 electron areas) Molecular Geometry Figure 10.4 Silberberg

10 Molecular Geometry - Example Molecules with multiple bonds eg COCl 2 total 24 e - 3 areas of electrons about C, so trigonal planar arrangement of electrons No lone pairs on C so molecular geometry is also trigonal planar ~120 

11 Tetrahedral (4 electron areas) Molecular Geometry

12 Molecular Geometry Repulsion: lone pair-lone pair > lone pair- bond pair > bond pair-bond pair  107  

Structures derived from a trigonal bipyramid Molecular Geometry

14 Structures derived from an octahedron Molecular Geometry All positions are identical

Number of lone pairs of electrons trigonal planarbent 4 tetrahedraltrigonal pyramidalbent 5 trigonal bipyramidalsee-sawT-shapedlinear 6 octahedralsquare pyramidalsquare planarT-shaped Total number of electron areas Summary of Molecular Geometry

16 Applications – shape and function Morphine is an alkaloid derived from the opium poppy. It is effective at blocking the perception of pain by the brain while allowing the normal function of the nervous system. A derivative is heroin, which is devastatingly addictive. Heroin users become physically dependant on opioids. Methadone is used to treat heroin addiction. It is an agonist i.e. it binds to the opioid receptors and causes an opioid response. It is prescribed as a way of regulating and ultimately reducing heroin addiction. Naloxone is an antagonist (i.e. it binds to the opioid receptors but does not give a response) used to treat heroin overdose. It binds to the opioid receptors in the brain, displacing heroin and reversing the effects of the narcotic (eg respiratory depression).

By the end of this lecture, you should be able to: −work out the number of bonding and non-bonding pairs from the Lewis structure of a molecule. −predict the distribution of these pairs around an atom. −place any lone pairs in appropriate positions to minimize the overall electron pair repulsion. −predict and describe the molecular shape. −be able to complete the worksheet (if you haven’t already done so…). 17 Learning Outcomes:

18 Questions to complete for next lecture: 1.Draw the shapes of the following molecules and ions. Name the molecular geometry and give approximate bond angles. (a)SO 3 (b)NH 4 + (c)PBr 4 - (d)CH 2 O (e)ICl 2 - (f)H 3 O +