2.2   Intermediate Bonding And Polarity
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2.2 Intermediate Bonding And Polarity

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2.2   Intermediate Bonding And Polarity 2.2 Intermediate Bonding And Polarity Document Transcript

  • Unit 2.2 - Intermediate bonding and polarity Electronegativity • Electronegativity is a very important word and concept for us as chemists • You need to know and understand what it means: Electonegativity is a measure of the attractive force of a specific atom in a molecule for a pair of electrons in a covalent bond. Example: Iodine (I 2 ) Iodine fluoride (IF) Measuring the electronegativity of an atom • Electronegativity is measured on the Pauling scale • The scale runs from 0 to 4 and has no units • The higher the number the ______________ the electronegativity and the _______ strongly the electrons in a bond are attracted to that atom Visualisation of electronegativites of the elements Trends in electronegativity
  • • The most electronegative element is… NB You need to • The least electronegative element is… know these trends! • Non-metals have ______________ electronegativities than metals • Electronegativity ______________ across a period • In the main block (not including the TMs) electronegativity _______________ down a group Intermediate bonding • Ionic bonding is… • Covalent bonding is… Covalent and ionic bonding are the two extremes of bonding. In practice bonds generally occur as something between the two or intermediate: Ionic bond Polar ionic bond Covalent bond How ionic/covalent is a bond? (Bond character) Deciding how much ionic and covalent character a specific bond has is easily done using this table: Difference in Percentage Percentage electronegativity ionic covalent character character Covalen
  • 0.1 0.5 99.5 0.2 1 99 0.3 2 98 0.4 4 96 0.5 6 94 0.6 9 91 0.7 12 88 0.8 15 85 0.9 19 81 1.0 22 78 1.1 26 74 1.2 30 70 1.3 34 66 1.4 39 61 1.5 43 37 1.6 47 33 1.7 51 49 1.8 55 45 1.9 59 41 2.0 63 37 2.1 67 33 2.2 70 30 2.3 74 36 2.4 76 24 2.5 79 21 2.6 82 18 2.7 84 16 2.8 86 14 2.9 88 12 3.0 89 11 3.1 91 9 3.2 92 8 Ionic Questions on bond character 1. Calculate the percentage of ionic and covalent bonding in the following compounds and add them to the diagram on the previous page: a. Lithium fluoride (LiF) b. Methane (CH4) c. Calcium bromide (CaBr2) 2. Which of the following compounds has the most covalent character? HF HCl HBr HI Polar covalent bonds • Covalent bonding involves the ________________ of electrons between two or more atoms • The electrons are not always shared equally:
  • Hydrogen (H 2 ) Hydrogen chloride (HCl) H- H H- Cl • Because chlorine is ________ electronegative than hydrogen it _____________ electrons more strongly • The electrons in the bond are pulled more closely towards ________________ distorting the bond • This results in chlorine having a partial _______________ charge (-0.17) • Hydrogen also has a partial charge, but it is _________________ (+0.17) • We indicate partial charges using the lower case Greek letter delta (δ+ or δ-) δ+ Polarity of bonds and molecules We have already seen that, depending on the difference in _______________ between the two atoms involved, the pair of ____________ in a covalent bond can be shared unequally . This leads to molecules, which have an uneven distribution of charge . We call such molecules polar molecules . Spotting polar molecules: δ- N- N H- F Nitrogen (N2) Hydrogen fluoride (HF) Methane (CH4) Water (H2O) Benzene (C6H6) Methanol (CH3OH)
  • Finding polar molecules experimentally You should now undertake the core practical (2.1), which tests several liquids to see if they are deflected by an electric field. 1. Make a table below with two columns ‘polar’ and ‘non-polar’ 2. Draw in displayed formulae for the compounds tested in the appropriate column jet of water being 3. Add partial charges (δ+ and δ-) to the polar molecules pulled to one side + + + + + + + + Measuring polarity - Dipoles and dipole moments • Polar molecules have a ______________ ‘end’ and a ________________ ‘end’ • Because of these two ‘poles’ we call them dipoles • The polarity (how polar it is) of a molecule is measured by its dipole moment • Dipole moment is given by the amount of charge separation multiplied by the distance between the centres of charge
  • • Dipole moment is measured in Debye (D) Slightly trickier examples of polar and non- polar molecules Polar Non- polar Cyclohexene Cyclohexane Trichloromethane Tetra chloromethane Boron trifluoride Quick checklist - Is it polar? Draw out the molecule Are there any electronegative atoms? Look out for O, OH, N, Cl and F Add the partial charges Is one end positive and the other negative? Watch out for symmetrical molecules, they can’t be polar!