Shapes of Molecules 24-25 - 1. Lewis Model and VSEPR (2).pdf

1a. The Lewis model What is this? 1 The language of chemistry 2 Why is water non-linear? 3  Is shape important? Yes!!! The properties of a covalent compound are influenced...

1a. The Lewis model What is this? 1 The language of chemistry 2 Why is water non-linear? 3  Is shape important? Yes!!! The properties of a covalent compound are influenced by the shape of its molecules. If water was linear, life would not exist!! 21/09/2024 4  Lewis model – localised bonding approach Localized bonding approach – A bond is formed due to electron sharing between two atoms The electronic structure of a molecule is represented by the sum of all bonding electron pairs and lone electron pairs – Lewis Diagram Octet rule: atoms combine so that in a molecule each atom has eight electrons in its valence shell Only valence electrons used in bonding 21/09/2024 5  Lewis model – localised bonding approach 21/09/2024 6  Lewis model for methane, ammonia and water Methane (CH4) 4 bond pairs 6C - 1s2, 2s2, 2p2 0 lone pairs 1H – 1s1 Ammonia (NH3) 7N 3 bond pairs – 1s2, 2s2, 2p3 1 lone pair 1H – 1s1 Water (H2O) 2 bond pairs 8O – 1s2, 2s2, 2p6 2 lone pairs 1H – 1s1 Atoms once in the formula – central Metals – usually central O, H - peripheral 21/09/2024 7  Lewis model for water, ammonia and methane In water - total of 8 valence electrons (ignore the two 1s electrons on oxygen as they lie very low in energy – core electrons). There are 4 pairs of electrons but only two pairs are needed for covalent bonding and thus we have 2 lone pairs The best way (lowest energy) to place the 4 electron pairs around the central atom (oxygen) is in the form of a tetrahedron – for example as is also seen for methane (and ammonia) 21/09/2024 8  Lewis model - multiple bonds Unsaturated systems – No. of valence electrons not sufficient for single bonded diagrams, use of multiple bond is required Carbon dioxide (CO2) Nitrous oxide aka laughing gas (N2O) 6C- 1s2, 2s2, 2p2 7N – 1s2, 2s2, 2p3 8O – 1s2, 2s2, 2p6 8O – 1s2, 2s2, 2p6 21/09/2024 9  Lewis model - multiple bonds Can share more than two electrons between two atoms –multiple bonding e.g. in CO2 we also have 8 valence electrons but now these are used to form two  (sigma) and two  (pi) bonding pairs. Consider the -electron pairs when determining shapes of molecules as -electron pairs lie along the same axes as -electrons. Best way (lowest energy) to place the 2 electron pairs around central atom (carbon) is opposite one another – linear – also seen in N2O (laughing gas) 21/09/2024 10  1b. Valence Shell Electron Pair Repulsion Theory Lewis structures – electron dot structures in 2D (flat) BUT, molecules are 3D Why do molecules take on 3D shapes instead of being flat? Valence Shell Electron Pair Repulsion theory “because electron pairs repel one another, molecules adjust their shapes so that the valence electron pairs are as far apart from another as possible.” 21/09/2024 11  1b. Valence Shell Electron Pair Repulsion Theory Predicting the shapes of more complex molecules can be more difficult. One way of approaching this problem is to use the valence shell electron pair repulsion (VSEPR) theory It is based on a localised bonding model and makes three assumptions regarding the nature of covalent bonds 1. The atoms in a molecule are held together by pairs of electrons – bonding pairs 2. Some atoms in the molecule have pairs of electrons that are not used in bonding – lone pairs 3. Electron pairs repel each other and, on each atom, adopt positions as far apart from one another as possible 4. A lone pair takes up more space around the central atom than a bonding pair 21/09/2024 12  1b. Valence Shell Electron Pair Repulsion Theory How to determine molecule shape? Draw electron dot or structural formula Count the number of bonding and non-bonding pairs of electrons around the central atom (number of places electrons are found) Multiple (double, triple) bonds count as one “location” or “region” Apply the correct geometry predicted by VSEPR Theory based on the number of bonding and non-bonding electron pairs 21/09/2024 13  Ideal shapes – lowest energy conformations The shapes of molecules in which the electron pairs are held as far apart as possible are shown below for AXn (n = 2-7); 21/09/2024 14  VSEPR – worked example CH4 21/09/2024 15  VSEPR – worked example NF3 While each fluorine also has three lone pairs, we can ignore these as they are not used in the bonding and thus halide chemistry is very similar (structurally) to that of hydrogen 21/09/2024 16  VSEPR – worked example PF5 21/09/2024 17  VSEPR – worked example [PF6]- 21/09/2024 18  VSEPR – worked example SeO3 With oxygen we now need to have two added electrons i.e. in a sigma and pi –bonds. Thus each oxygen donates 2 electrons (the other four are two lone pairs on each oxygen) and needs to have 2 electron pairs The -bonding pairs lie along the same axes as the s-bonds and thus we ignore them when establishing the coordination geometry 21/09/2024 19  VSEPR – limitations 1 delocalised bonding VSEPR is based on two-centre two-electron bond (localised model) it necessarily will not work when the bonding becomes more delocalised in nature (see next section) 21/09/2024 20  VSEPR – limitations 2 transition metals With transition metals we have d-orbitals in the valence shell – there are NINE orbitals in the valence shell of a transition metal (1 x s, 3 x p, 5 x d) 21/09/2024 21