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Shapes of molecules & ions. VSEPR theory VSEPR - the Valence Shell Electron Pair Repulsion theory is used to obtain the shape of simple molecules and.

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Presentation on theme: "Shapes of molecules & ions. VSEPR theory VSEPR - the Valence Shell Electron Pair Repulsion theory is used to obtain the shape of simple molecules and."— Presentation transcript:

1 Shapes of molecules & ions

2 VSEPR theory VSEPR - the Valence Shell Electron Pair Repulsion theory is used to obtain the shape of simple molecules and ions

3 VSEPR theory VSEPR - the Valence Shell Electron Pair Repulsion theory is used to obtain the shape of simple molecules and ions It states that because of the repulsion between pairs of electrons around the central atom in the molecule or ion, the pairs of electrons are arranged to be as far away from each other as possible

4 Negative Charge Centres For VSEPR, multiple bonds (double, triple) count as if they are just one pair of electrons

5 Negative Charge Centres For VSEPR, multiple bonds (double, triple) count as if they are just one pair of electrons So VSEPR uses repulsion between negative charge centres rather than between pairs of electrons

6 4 bonding pairs - tetrahedral Bond Angle = 109.5º E.g methane - CH 4, CCl 4, SiCl 4 4 negative charge centres From: http://library.thinkquest.org/3659/structures/commonstructures.htmlhttp://library.thinkquest.org/3659/structures/commonstructures.html www.chemvc.com/~tim/VSEPR.htmlwww.chemvc.com/~tim/VSEPR.html, http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml

7 3 bonding pairs (& 1 non-bonding pair) trigonal pyramid Bond Angle = 107 º E.g. ammonia - NH 3, PCl 3 4 negative charge centres From: http://library.thinkquest.org/3659/structures/commonstructures.htmlhttp://library.thinkquest.org/3659/structures/commonstructures.html www.chemvc.com/~tim/VSEPR.htmlwww.chemvc.com/~tim/VSEPR.html, http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml

8 The bond angle decreases because the non-bonding pair of electrons exerts a greater repulsion than the bonding pair(s) From: http://www.examstutor.com/chemistry/resources/studyroom/bonding/shapes_of_molecules/

9 2 bonding pairs ( 2 non-bonding pairs) bent or v-shaped Bond angle = 105 º E.g. H 2 O, H 2 S 4 negative charge centres From: http://library.thinkquest.org/3659/structures/commonstructures.htmlhttp://library.thinkquest.org/3659/structures/commonstructures.html www.chemvc.com/~tim/VSEPR.htmlwww.chemvc.com/~tim/VSEPR.html, http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section8.rhtml

10 2 negative charge centres Shape is always linear Bond angle is 180º E.g. From: www.physchem.co.za/Bonding/Shape.htmwww.physchem.co.za/Bonding/Shape.htm www.tulane.edu/~bmitche/book/structrj.html

11 3 negative charge centres 3 bonding electron pairs, no non-bonding pairs Trigonal planar Bond angle is 120° e.g. BF 3, ethene, CO 3 2- 2 bonding pairs, 1 non-bonding pair Bent or v-shaped Bond angle is 120° E.g. SO 2, ozone - O 3 From: http://library.thinkquest.org/3659/structures/commonstructures.html, www.chm.bris.ac.uk/motm/so2/so2h.htmhttp://library.thinkquest.org/3659/structures/commonstructures.html www.chm.bris.ac.uk/motm/so2/so2h.htm

12 5 and 6 charge centres Elements in the third period can have more than 8 electrons in their outer shell because the 3 d orbitals are close in energy to the 3p orbitals. The VSEPR theory also applies You do not have to know specific bond angles

13 5 negative charge centres (HL) Trigonal bipyramidal e.g. PCl 5 From: http://library.thinkquest.org/3659/structures/commonstructures.html

14 5 negative charge centres (HL) 4 bonding pairs and 1 non-bonding pair Distorted tetrahedral From: www.chemvc.com/~tim/Predictions.html

15 6 negative charge centres (HL) Octahedral e.g. SF 6 From: http://library.thinkquest.org/3659/structures/commonstructures.html

16 6 negative charge centres (HL) 4 bonding pairs and 2 non-bonding pairs as far apart as possible (above & below plane) Square planar e.g. XeF 4 From: www.chemvc.com/~tim/Predictions.html

17 VESPRtheory All the previous molecular shapes are based on VSEPR theory VSEPR=Valence Shell Electron Pair Repulsion theory Pairs of electrons arrange themselves around the central atom so that they are as far apart from each other as possible

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