Moleules

2.2
2.3 Shapes of molecules and ions
2.3a Electron pair repulsion theory a demonstrate an understanding of the use of electron-pair repulsion theory to interpret and predict the shapes of simple molecules and ions
In a chemical species like a molecule or ion, pairs of electrons create negative centres that surround the central atom. These electron pairs repel each other and arrange themselves to be as far apart as possible, thus minimising repulsion.
[pic](source: http://www.mikeblaber.org/oldwine/chm1045/notes/Geometry/VSEPR/Geom02.htm)

Lone pairs repel more strongly than bonding pairs of electrons therefore a lone pair distorts the shape of a molecule. Nitrogen has 5 outer electrons so ammonia will have four electron pairs around the central nitrogen atom and be tetrahedral with respect to electron pairs. Ammonia, NH3 has a lone pair which generates more repulsion than the 3 bonding pairs of electrons. The shape of NH3 is therefore distorted tetrahedral.

2.3b Known molecular shapes b recall and explain the shapes of BeCl2, BCl3, CH4, NH3, NH4+, H2O, CO2, gaseous PCl5 and SF6 and the simple organic molecules listed in Units 1 and 2
|molecule |shape wrt negative centres |shape wrt atoms |bonds and lone pairs around central atom |
|BeCl2 |linear |linear |2 single bonds |
|CO2 |linear |linear |2 double bonds |
|HCl |tetrahedral |linear |1 single bond, 3 lone pairs |
|BCl3 |trigonal planar |trigonal planar |3 single bonds |
|CH4 |tetrahedral |tetrahedral |4 single bonds |
|PCl5 |trigonal bipyramidal |trigonal bipyramidal |5 single bonds |
|SF6 |octahedral |octahedral |6 single bonds |
|SO32- |tetrahedral |pyramidal |1 double, 2 coordinate, 1 lone pair |
|CO32- |trigonal planar |trigonal planar |2 single bonds, 1 double bond |
|NO3- | trigonal planar |trigonal planar |2 double bonds, 1 single bond |
|SO2 |trigonal planar |bent |2 double bonds, 1 lone pair |
|H2O |tetrahedral |bent |2 single bonds, 2 lone pairs |
|NH4+ |tetrahedral |tetrahedral |3 single bonds, 1 coordinate bond |
|NH3 |tetrahedral |pyramidal |3 single bonds, 1 lone pair |

Task 2.3a.1 Draw diagrams to show the bonding in all of the above species.
Task 2.3a.2 Draw diagrams to show the shapes of the above species.
Story 2.3a Trainer SF6 Climate Change Danger

2.3c Predicting shapes of unknown molecules c apply the electron-pair repulsion theory to predict the shapes of molecules and ions analogous to those in 2.3b
Task 2.3c Predict and draw the shape for the following molecules or ions: BeF2, SeO2, SCl6, H2S, AlCl3, SiCl4, PF5, PO3-, SiO32-, Cl2O.

2.3d Predicting bond angles d demonstrate an understanding of the terms bond length and bond angle and predict approximate bond angles in simple molecules and ions
2 pairs linear BeCl2, angle = 180o
3 pairs trigonal planar BCl3 , angle = 120 o
4 pairs tetrahedral angle = 109.5o CH4 , NH4+ (or bent) ammonia/water less than 109.5 o
5 pairs trigonal bipyramidal angles 120 o and 90 o, PCl5(g)
6 pairs octahedral (or square planar) , angles = 90 o SF6
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2.3e Carbon structures e discuss the different structures formed by carbon atoms, including graphite, diamond, fullerenes and carbon nanotubes, and the applications of these, eg the potential to use nanotubes as vehicles to carry drugs into cells.

Giant atomic structures. [pic]The structure of Diamond.

o It is hard and has a very high melting point and boiling point because each atom is held firmly in place by 4 strong, short, covalent bonds and a lot of energy is required to break these strong bonds. o Doesn’t conduct electricity even when molten as no charged particles to carry charge. o Insoluble in water as forces between solvent and carbon atoms are too weak. o Thermal conductor as rigid structure allows heat to be passed through vibrations. o High density (3.51gcm-3) because atoms are packed tightly together.
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Task 1.3e.2
[pic]The structure of graphite. [pic]
[pic] [pic]
Graphite is soft as there are weak bonds between layers, thus allowing layers to slide over each other. (Large distances between layers imply weak bonds.) ‚
Graphite has a high melting point and boiling point as 3 strong covalent bonds hold each atom in place.
Graphite conducts heat and electricity in one direction due to delocalised electrons between the layers.
Low density (2.25gcm-3) because the layers are far apart.

Buckminsterfullerenes
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60 C atoms in 12 pentagons and 20 hexagons = a football! http://www.youtube.com/watch?v=xVZRGcg-BXI Nanotubes
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http://www.youtube.com/watch?v=yuUpYUuH2zs&feature=related

Edexcel AS chemistry p151-p153