2.1 Vsepr Notes


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Notes to accompany unit 2.1 of the Edexcel AS chemistry course

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2.1 Vsepr Notes

  1. 1. Unit 2.1 - Shapes of molecules and ions Each molecule of every compound has a specific shape . The geometry of a molecule is extremely important to us as chemists as it can determine its physical properties, reactivity and biological activity… The drug molecule thalidomide has 2 different A water molecule isomers, one of which (R) is an effective treatment for morning sickness and the other (S) causes birth defects Shapes of ionic compounds • Ionic and covalent bonding both involve __________________ forces • In ionic bonding the forces are non- directional • Ions attract and ___________ in all directions • Ionic compounds form ________________ structures, which maximise the maximise the attractive forces between ________________ charged ions and minimise the repulsion between ________________ charged ions Shapes of covalent molecules • Unlike ionic bonds, covalent bonds are highly directional • Covalently bonded molecules have very specific shapes with set 3D geometries • These geometries are determined by valence shell electron pair repulsion (VSEPR) theory Remember beryllium chloride (BeCl2) and water? Dot-cross Shape Dot-cross Shape Beryllium chloride Water Valence shell electron- pair repulsion (VSEPR) theory • The shape of a molecule depends on the number of pairs of ______________ around a central atom • Pairs of electrons _____________ one another so they stay as far apart as possible
  2. 2. • Draw a dot- cross diagram, count the pairs of electrons and decide on a structure. Simple! Compound Dot- cross diagram Shape Boron chloride (BCl 3 ) Number of pairs = Trigonal planar Methane (CH 4 ) Number of pairs = Phosphorous(V) chloride (PCl 5 ) Number of pairs = Trigonal bipyramidal Sulphur(VI) fluoride (SF 6 ) Number of pairs = Octahedral N.B. You need to know the names of these shapes, be able to draw them and remember the bond angles The effect of lone pairs • Some molecules have electrons around the central atom that are not involved in bonding • These lone pairs have a big effect on the shape of a molecule Consider ammonia (NH3):
  3. 3. Dot-cross diagram Electron density Shape • The central nitrogen atom in ammonia is surrounded by ___ pairs of electrons • ____ of these pairs make up covalent bonds with hydrogen atoms • There is ____ lone pair of electrons • The geometry of ammonia is therefore based on a ________________ shape • The bond angles in a tetrahedral molecule should be ________ • However, in ammonia the bond angle is 107.8º Ammonia is actually a trigonal pyramid because… • The electron density of lone pairs is closer to the nucleus of the central atom than that of bonding pairs • Lone pairs exert a stronger repulsive force than bonding pairs Think… Possible exam question? Could you recall the name for and explain the shape of an ammonia molecule? Shapes of ions Predictions for the shapes of ions follow the same basic VSEPR rules: 1. Sketch a dot-cross diagram 2. Count the pairs of electrons , be sure to take into account any charges 3. Look out for any lone-pairs 4. Decide on the shape according to the number of pairs… e.g. formation of the dimethylaminium cation Dimethylamine Dimethylaminium ion Some more ions:
  4. 4. Nitrate (NO 3 - ) Sulphate (SO 4 2- ) Sulphite (SO 3 2- ) 3 electron pairs Tetrahedral Try this fiendish example: Q ~ What shape is the iodine tetrafluoride anion (IF4-) Hint: Start with a dot-cross diagram! Shapes of organic molecules Alkanes • Methane is the simplest alkane it has a ___________________ geometry • In fact all carbons in alkanes are tetrahedral • We can represent the three-dimensional shapes of these molecules as shown on the right Alkenes • All alkenes contain at least one carbon-carbon _______________ bond • This means that molecules like ethene (right) are flat or planar to use the correct scientific term Alcohols • Simple alcohols are similar to alkanes but include a hydroxyl (OH) group • In methanol (right) there is __________________ geometry around the carbon with _____ H-C-H bond angles and the OH group has a bent linear shape similar to ____________ with a C-O-H bond angle of around 104º
  5. 5. Carboxylic acids • All carboxylic acids contain the carboxyl group (COOH) • The carboxyl group involves a carbon-oxygen ____________ bond • As a result the carboxyl group is planar Halogenoalkanes • Halogenoalkanes, like chloromethane, have ___ bonding pairs of electrons • This means that they all have a ______________ shape Aldehydes and Ketones • The central carbon atom in both ketones and aldehydes has ___ atoms bonded to it • They are __________ with ____ bond angles Allotropes of carbon Aldehyde group Ketone group Allotropes are different forms of the same element that exist in the same physical state. Carbon occurs naturally as several different allotropes: 1. Diamond Properties and uses: Bonding and structure: 2. Graphite Properties and uses: Bonding and structure:
  6. 6. 3. Fullerenes The first fullerene, discovered at the University of Brighton by Harry Kroto (now Sir Harry Kroto and a Nobel Prize winner for his work!), was the football-like C60. Many other forms have been discovered since including larger spheres and tubes . Fullerenes, like graphite, have free electrons and can therefore conduct electricity and are soluble in many organic solvents. A great deal of research is still focussed on fullerenes due to their many potential uses in nanotechnology.