Unit 4 Worksheets 3Covalent structures (1) 1. Give the Lewis structure for a molecule of: (a) oxygen (d) carbon dioxide (b) nitrogen (c) tetrachloromethane (e) carbon monoxide 2. One Lewis structure for the carbonate ion, CO32–, shows one double C=O bond and two single C-O bonds. Explain why the bond lengths of all the carbon to oxygen bonds in the carbonate ion are equal. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 3. Suggest why aluminium chloride, AlCl3, readily dimerises to form Al2Cl6. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 4. The oxygen to oxygen bonds in oxygen, O2(g) and ozone, O3(g) are both broken down by ultraviolet light in the atmosphere. Suggest why higher energy ultraviolet light is required to break the oxygen to oxygen bond in oxygen gas compared to the oxygen to oxygen bond in ozone gas. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 5. Suggest a reason why diagrams of organic structures containing a phenyl ring often show the phenyl ring as a hexagon with a circle inside rather than as a hexagon with alternate double and single bonds. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 6. Explain why graphite and diamond both have very high melting points (in the region of 4000 oC) and yet diamond is a poor electrical conductor whereas graphite is a good electrical conductor. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 7. Describe the structures of both diamond and silicon and suggest why silicon has a lower melting point than diamond. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 8. Explain why silicon dioxide has high melting point (above 1600 OC) whereas carbon dioxide is a gas at room temperature and pressure. _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ Answers (Note that a line, two dots or a dot and a cross are all acceptable ways of showing a pair of electrons and different ways can be combined within the same molecule) 2. Three resonance structures for the carbonate ion can be drawn. Each contains one C=O double bond from the carbon atom to different oxygen atoms so that the average bond order for the carbon to oxygen bond is 4/3 and all the carbon to oxygen bonds are the same length and strength. 3. Aluminium chloride, AlCl3, is covalent and there are only six electrons around the central aluminium atom. When it dimerises one of the chlorine atoms from another AlCl3 molecule forms a coordinate bond with the aluminium atom so that each aluminium atom now obeys the ‘octet rule’. 4. Ozone can be drawn as two resonance hybrids in which the bond order for the O to O bond is 3/2. This means that the O to O bond in ozone is weaker than the O=O double bond in oxygen so less energy is required to break it. 5. The phenyl group is essential a substituted benzene ring. It can be shown as two resonance structures in which the double and single C to C bonds alternate. Since all six C to C bonds are of equal length and strength a circle in the phenyl ring illustrates this better than showing alternate double and single bonds which would be of different length and strength. 6. Both diamond and graphite have giant molecular structures consisting of strong covalent bonds so they have high melting points. In diamond all the C to C bonds are equal and all the electrons are held in fixed positions arranged tetrahedrally around each carbon atom. In graphite three of the C to C bonds are fixed in a hexagonal shape with bonds angels of 120o to give a flat layer. The layers are held together by weak bonds consisting of delocalised electrons. These delocalised electrons are free to move between the layers so that graphite is a good conductor of electricity. 7. Both diamond and silicon have a giant tetrahedral structure but C-C bonds are stronger than Si-Si bonds so are easier to break. 8. Silicon dioxide has giant molecular structure with strong covalent bonds resulting in a high melting point. Carbon dioxide contains individual molecules of carbon dioxide held together by weak intermolecular forces which are easily broken so it has a low melting point.
