Explain what the VSEPR Theory is Title: Shapes of Molecules Date: 09/04/2019 GET YOUR PLANNER OUT! We are Learning To: Draw molecules in 3D Explain what the VSEPR Theory is Apply VSEPR to predict the shape of molecules Starter: 1. Copy Title, Date and WALT 2. What 3D shapes are you familiar with?

3D shapes in 2D form H represents a bond in the plane of the paper C represents a bond in a direction behind the plane of the paper H H H represents a bond in a direction in front of the plane of the paper

VSEPR Theory The shape of a molecule is determined by the number of groups of electrons around the central atom. The 'groups' might be a non-bonding or bonding pair of electrons, a double pair of bonding electrons etc. The electron 'groupings' repel to make the angles between them as wide as possible.

two groups of electrons around central atom Usually two bonding pairs of electrons. LINEAR shape bond angle 180 BeH2 and CO2 are examples Draw a dot cross diagram to be sure How is CO2 slightly different?

three groups of electrons around a central atom three bonding pairs TRIGONAL PLANAR shape bond angle exactly 120 BH3 and BF3 are examples Draw a dot cross diagram to be sure

H H C C H H Planar Molecules Ethene is a good example of a planar molecule. remember – it’s the number of groups of electrons that determine the shape not the number of electron pairs. H H C C H H To predict the shape of a molecule you need to: Count the number of bonding pairs of electrons Count the number of non-bonding (lone) pairs of electrons

four groups of electrons around the central atom four bonding pairs TETRAHEDRAL shape bond angle exactly 109.5 CH4 is an example Draw a dot cross diagram to be sure

Effect of lone pairs on bond angles Boardworks AS Chemistry Structure and Shape Effect of lone pairs on bond angles Teacher notes It may be worth pointing out to students that the dotted line represents a bond extending behind the plane of the screen, and the wedge-shaped bond represents a bond extending in front of the plane of the screen.

four groups of electrons around the central atom three bonding pairs & one lone pair TRIGONAL PYRAMIDAL shape bond angle approx. 107 (lone pair has an ‘extra’ repulsive push) NH3 is an example Draw a dot cross diagram to be sure Why is ammonia not tetrahedral?

four groups of electrons around the central atom two bonding pairs & two lone pairs V-SHAPED or BENT shape bond angle approx. 104.5 (lone pairs have an ‘extra’ repulsive push) H2O is an example Draw a dot cross diagram to be sure

six groups of electrons around central atom 6 bonding pairs OCTAHEDRAL shape bond angles 90 SF6 is an example Draw a dot cross diagram to be sure

Shapes of ions. We can use the same ideas to work out the shapes of ions as we use for molecules. We need to include any extra electrons that have been added or taken away. Extension – If NH3 has a Trigonal pyramidal shape (109o) what shape would the NH4+ ion have?

Summary of electron repulsion theory: Construct “dot and cross” diagram Count bonding pairs and lone pairs and add them up! (double bonds count as one) If four bonding pairs: 109o If three bonding: 120o If two bonding: 180o

3 Complete the following table using the textbook to help you: Build models of common molecules Complete the following table using the textbook to help you: Number electron pairs Bond angle Example Shape 4 (4 bonding) (3 bonding 1 non-bonding) (2 bonding 2 non-bonding) 1. 2. 3. Tetrahedral Trigonal Pyramidal V-Shaped / Bent 3 Planar triangular 2 Linear 3. Homework – Complete EL2.2 – a bit of cut and stick!

Activity EL2.2; Shapes and Bond angles;

Formula of molecule/ion Dot-cross diagram Shape Bond Angle BCl3 Trigonal Planar 120 BeCl2 Linear 180 CH4 SiH4 NH4+ Tetrahedral 109 H2O SCl2 V-shaped 109(ish) approx 104.5 NH3 Trigonal pyramidal 109(ish) approx 107 SCl6 Octahedral 90 PCl5 Trigonal bipyramidal 90 and 120

Shapes of molecules activity Boardworks AS Chemistry Structure and Shape Shapes of molecules activity

Explain what the VSEPR Theory is Title: Shapes of Molecules Date: 09/04/2019 GET YOUR PLANNER OUT! We are Learning To: Draw molecules in 3D Explain what the VSEPR Theory is Apply VSEPR to predict the shape of molecules Starter: 1. Copy Title, Date and WALT 2. What 3D shapes are you familiar with?