Chem matters ch7_covalent_bonding

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Chem matters ch7_covalent_bonding

  1. 1. Chemistry Matters – Chapter 7Covalent Bonding and Metallic Bonding 1
  2. 2. Why do Covalent Bonds form?When non-metals react with one another, theiratoms share valence electrons.The bond formed between atoms that share electronsis called a covalent bond.After bonding, each atom attains the electronicconfiguration of a noble gas. 2
  3. 3. What are Molecules?When atoms combine by sharing electrons,molecules are formed. A molecule is a group of two or more non-metal atoms held together by covalent bonds. Covalent bonds, like ionic bonds, are STRONG bonds. 3
  4. 4. Molecules of ElementsMany non-metallic elements exist asmolecules made up of two or more identicalatoms which bond by sharing electrons inorder to become stable.Examples:Hydrogen (H2), chlorine (Cl2), oxygen (O2),nitrogen (N2), fluorine (F2), bromine (Br2). 4
  5. 5. Example 1: Two hydrogen atoms share a pair of electrons so thatHydrogen (H2) each atom has two electrons in its valence shell. As a result, bothA hydrogen atom has atoms have a stable dupletone valence electron. configuration.H (1) H (1) H – H or H2 Single covalent bond consisting of 2 shared 5 electrons.
  6. 6. HydrogenThe sharing of two electrons forms a singlecovalent bond.A single covalent bond or a single bond is usuallyrepresented by a single line ‘–’ in the structural formula. A structural formula shows A molecular formula only how the atoms are arranged shows the number of each and the bond between them. kind of atom in a molecule. 6
  7. 7. Example 2:Chlorine A chlorine atom has seven valence electrons. To attain an octet structure, two chlorine atoms combine to share a pair of electrons (two electrons). 7
  8. 8. ChlorineLike hydrogen, chlorine has a single covalent bond.Q. How many electrons are shared in a singlecovalent bond? Two 8
  9. 9. OxygenAn oxygen atom has It needs two more electrons tosix valence electrons. form a stable octet structure. Each oxygen atom shares two of its electrons with another oxygen atom. 9
  10. 10. Oxygen4 electrons are shared between two oxygen atoms.Such a bond is called a double covalent bond. O=O A double bond is represented by ‘==’ in a structural formula. Q. How many electrons are shared in a double covalent bond? 10 Four electrons (two pairs of electrons)
  11. 11. Let’s Practice!Q. What can you deduce about the bondingin this particle? • It is a molecule in which the atoms are bonded by a single covalent bond. 11
  12. 12. Let’s Practice!Q. The valency of an atom is the number of electrons from each atom that is shared. What is the valency of each atom here? One. 12
  13. 13. Let’s Practice!Q. How many electrons are shared the a single covalent bond? Two. Examples of this kind of molecule – Cl2, F2, Br2, I2. 13
  14. 14. Let’s Practice!Q. Give the names and molecular formulae of molecules of the type shown here. Fluorine (F2), chlorine (Cl2), bromine (Br2), iodine (I2). (Group VII elements) 14
  15. 15. Molecules of Compounds(Molecules are always covalent, not ionic!)When 2 or more different types of atoms formcovalent bonds, a molecular compound orcovalent compound is formed.Water (H2O), methane (CH4),, ammonia (NH3)and carbon dioxide (CO2)are examples ofcovalent compounds. 15
  16. 16. Example 1: WaterWater is formed by the reaction of hydrogen withoxygen such that all three atoms attain noble gasconfigurations.Each water molecule contains two single covalent bonds. 16
  17. 17. Water molecule (H2O) + → The hydrogen atoms are now electronically like helium and the oxygen atom like neon. Two hydrogen – oxygen (H – O) 105°. single covalent bonds are formed. The water molecule is V-shaped, with the H-O-H bond angle of 105°. 17
  18. 18. Example 2: MethaneMethane (CH4) contains carbon and hydrogen.In a methane molecule, the carbon atom has an octetconfiguration while each hydrogen atom has a dupletconfiguration. Methane has four single covalent bonds. 18
  19. 19. Methane, CH4 19
  20. 20. Example 3: Ammonia, NH3Q. How many electrons does each hydrogen atom needto become stable? 1Q. How many electrons does each nitrogen atom need tobecome stable? 3 Ammonia molecule, NH3 (with 20 3 single covalent N-H bonds)
  21. 21. Example 4: Carbon dioxide (CO2) 8p 6p 8pEach carbon Eachatom shares oxygentwo electrons 8p 6p 8p atomeach with shares twotwo oxygen electrons.atoms. Two double covalent (C=O) bonds 21
  22. 22. Different ways of representing the carbon dioxidemolecule.A carbon dioxide molecule (CO2) contains twodouble covalent bonds. Each double bond has 4shared electrons or 2 pairs of shared electrons. 22
  23. 23. Example 4: Carbon dioxide (CO2) Each carbon atom shares two electrons each with two oxygen atoms.Each oxygen atom shares two electrons. 23
  24. 24. Structures of Covalent Substances • Simple (small) There are 2 molecules forms in which covalent (2) Giant structuressubstances exist: 24
  25. 25. Simple (small) Covalent MoleculesAlso known as simple molecularstructures.Most covalent substances exist in this forme.g.Hydrogen (H2), Oxygen (O2), nitrogen (N2),chlorine (N2), Iodine (I2), Methane (CH4), water(H2O), carbon dioxide (CO2), Hydrogenchloride (HCl), ammonia (NH3). 25
  26. 26. Ex 1: Simple Covalent Molecules Methane (CH4) In a molecule of methane, CH4, the four C–H covalent bonds are strong. However, weak van der Waals’ forces between methane molecules hold them together loosely. Therefore, methane exists as a gas at room temperature and pressure. 26
  27. 27. Simple Covalent Molecules have low melting & boiling points Little energy is needed to overcome the weak forces between molecules. (In melting or boiling which are PHYSICAL CHANGES, no chemical bonds are broken; e.g. in methane C-H bonds are NOT broken). 27
  28. 28. Ex 2: Simple Covalent Molecules Iodine (I2) Within each iodine molecule, the iodine atoms are held together by strong covalent bonds. Between the iodine molecules, there are only weak van der Waals’ forces holding the molecules together. Weak force 28
  29. 29. Q. Why does iodine sublime easily when heated gently? Sublimation is a physical change and only bonds between iodine molecule are broken. The intermolecular (van der Waals’ forces are weak, so little energy is needed to overcome them.Weak force between molecules 29
  30. 30. Physical Properties of SimpleMolecular (Covalent) Compounds1. Volatile – low melting and boiling points2. Soluble in organic solvents; most areinsoluble in water. 3. Cannot conduct electricity in any state.(Some exceptions are: carbon (graphite), ammonia,ethanol, sugar, chlorine, hydrogen chloride, sulphurdioxide.) 30
  31. 31. Q. Why are many covalent substancesliquids or gases at room temperature? This is because of the weak van der Waal’s forces between the molecules, so little energy is needed to overcome them during changes of state. 31
  32. 32. Q. Why do covalent substances not conduct electricity even when molten or in aqueous solution? Covalent substances contain neutral molecules. There are no mobile, charged ions or electrons to conduct electricity. (Exceptions: solid Carbon, in the form of graphite, conducts electricity. Hydrogen chloride, sulfur dioxide and ammonia react with water to form solutions that conduct electricity. ) 32
  33. 33. Giant Molecular (Covalent) Structures Examples: Diamond Silicon(IV) oxideGraphite (silicon dioxide) 33
  34. 34. Giant Covalent (Molecular) Structures Tetrahedral structureEx 1: DiamondIt is a form (allotrope) ofcarbon.Each carbon atomis bonded to 4 othercarbon atoms bystrong covalentbonds. 34
  35. 35. How to draw the diamond structure. 35
  36. 36. Physical properties of diamond1. Diamond is a very hard substance. It is not easily scratched or worn out.2. It has high melting point (3500°C) and high boiling point (4800 °C).3. Diamond does not conduct electricity.4. It is insoluble in water. 36
  37. 37. Q. Why is Diamond hard? A crystal of diamond contains millions of carbon atoms joined by strong covalent bonds. A large amount of energy is required to break these strong covalent bonds. This explains why diamond is so hard. 37
  38. 38. Q. Why is the melting point of diamond high? A large amount of energy is required to break the millions of strong covalent bonds so that the atoms are separated. 38
  39. 39. Q. Why diamond does not conduct electricity? All the valence electrons of the carbon atoms are used for bonding. There are therefore no free electrons that are available move through the structure to conduct electricity. 39
  40. 40. Uses of diamondSynthetic diamonds produced under highpressures and temperatures are used at thetips of drills and other cutting tools.They are used for drilling, grinding andpolishing very hard surfaces. 40
  41. 41. GraphiteGraphite is another form (allotrope) of carbon. It is made of layers of carbon atoms. 41
  42. 42. Graphite - Structure Within each layer, each carbon atom forms strong covalent bonds (C – C) with three other carbon atoms. The atoms form rings of six carbon atoms that are joined together to form two- dimensional flat layers.Each layer is a giant molecule. 42
  43. 43. Graphite - Structure The layers of carbon atoms are held together by weak van der Waals’ forces. Strong covalent bond (C-C) between carbon atoms 43
  44. 44. Why does graphite have high melting and boiling points? In graphite, the (C-C) bonds within each layer are strong and difficult to break.Hence, graphite has high melting and boiling points. 44
  45. 45. Why is graphite soft and slippery? The forces of attraction between the layers of carbon are very weak. The layers can slide over each other. This explains why graphite is soft and slippery. 45
  46. 46. Why is graphite able to conduct electricity? In graphite, each carbon atom has one outer (valence) shell electron that is not used to form covalent bonds, and can move through the layer to conduct electricity! 46
  47. 47. Why is graphite able to conduct electricity? These electrons are delocalised (free) , that is, they can move along the layers from one carbon atom to the next when graphite is connected to a battery.Hence, graphite is a good conductor of electricity. 47
  48. 48. Uses of graphite1. As a dry lubricant.2. As brushes for electric motors (to reducefriction).3. To make inert electrodes for electrolysis.4. Baked with clay to make pencil lead. 48
  49. 49. Silicon (IV) oxide Sand is actually silicon (IV) oxide. It is also commonly known as silica. The formula of silicon (IV) oxide is SiO2. It is also known as silicon dioxide. 49
  50. 50. Silicon (IV) oxide - structure Each silicon atom is bonded to four oxygen atoms in a tetrahedral structure and each oxygen atom is bonded to two silicon atoms. 50
  51. 51. Why does Silicon (IV) oxidehave a high melting point?This is because the silicon andoxygen atoms are all held togetherby strong covalent bonds in a giantcovalent (molecular) structure. 51
  52. 52. Writing the formulae of covalent compoundsEx 1:What is the chemical formula of carbon dioxide?We assume the first element named (carbon)contains only one atom unless otherwise stated.The word ‘dioxide’ means ‘two oxygen atoms’. So the formula is CO2. 52
  53. 53. Ex 2: What is the chemical formula of dinitrogen monoxide?Dinitrogen means 2 nitrogen atoms.Monoxide means 1 oxygen atom. So the formula is N2O. 53
  54. 54. Ex 3: What is the chemical formula of dinitrogen tetroxide?Dinitrogen means 2 nitrogen atoms.Tetroxide means 4 oxygen atoms. So the formula is N2O4. 54
  55. 55. Formulae of covalent substances are not always so easily derived.Chemical formulae of some common covalentsubstances which cannot be derived from theirnames include: ammonia (NH3), hydrogen peroxide (H2O2), methane (CH4), ozone (O3), water (H2O). 55

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