Presentation on theme: "How is VSEPR theory used to predict molecular structure?"— Presentation transcript:
1 How is VSEPR theory used to predict molecular structure? Three key ideas that are described in Valence Shell Electron Pair Repulsion theory (VSEPR theory) are:Repulsion between electron pairs causes molecular shapes to adjust so that the valence-electron pairs stay as far apart as possible.Repulsion also occurs between unshared pairs of electrons and bonding pairs.Double and triple bonds are viewed as single bonds.You can use VSEPR theory to predict the three-dimensional structure of molecules, including linear, trigonal planar, and tetrahedral shapes.(contd.)
2 Methane has a tetrahedral shape. Methane (CH4) is an example of a molecule that has a tetrahedral shape. Methane has four bonding electron pairs that are farthest apart when the angle between the central carbon and its attached hydrogen atoms is 109.5º.Three ways to represent the tetrahedral shape of methane are shown here.SLIDE 2:• Flush left lines 2 thru 5. TECH CHECK: SEE NOTE ON SLIDE 1.• Is the boldfaced caption below the shapes too small? See also Slides 2 thru 6. TECH CHECK: CAPTION LABEL COMES FROM PROTOTYPE (12 PT ARIEL). PLS ADJUST IN ALL PLACES IF THIS IS WRONG.Methane has a tetrahedral shape.(contd.)
3 Ammonia has a pyramidal shape. Ammonia (NH3) is also surrounded by four pairs of valence electrons, but one of these pairs is an unshared pair. These electrons are held closely to the nitrogen atom. They strongly repel the bonding pairs, pushing them closer together than they would be in a molecule such as methane. The angle formed by any two hydrogen atoms and the nitrogen atom is 107º. The resulting shape is known as pyramidal.Ammonia has a pyramidal shape.(contd.)
4 Water is an example of a bent molecule. Carbon dioxide is linear. Water (H2O) and carbon dioxide (CO2) show how unshared pairs of electrons can affect the shape of a molecule made of three atoms.A water molecule is bent because the two unshared pairs of electrons on oxygen repel the bonding electrons. The atoms form a bond angle of 105º, as shown below in Row a.In contrast, the carbon dioxide molecule has no unshared electron pairs. It is linear, that is, the atoms form an angle of 180º, as shown below in Row b.Water is an example of a bent molecule. Carbon dioxide is linear.(contd.)
5 The carbonate ion is trigonal planar. The carbonate ion (CO32–) consists of a central carbon atom surrounded by three oxygen atoms. One electron dot formula that may be used to represent the ion is shown to the right.The double bond between one oxygen atom and the central carbon is regarded as a single bond. The three areas of bonding pairs stay as far apart as possible, leading to a molecular shape that is flat and triangular, or trigonal planar. The bond angles all are 120º.The carbonate ion is trigonal planar.(contd.)
6 Shown here are common molecular shapes that may be predicted using VSEPR theory. 1. Predict Use Valence Shell Electron Pair Repulsion (VSEPR) theory to predict the molecular structures of SiO2 and CF4.