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Chemistry 1011

Chemistry 1011. TOPIC Physical Properties of Matter TEXT REFERENCE Masterton and Hurley Chapter 9. Review. YOU ARE EXPECTED TO BE ABLE TO:

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Chemistry 1011

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  1. Chemistry 1011 TOPIC Physical Properties of Matter TEXT REFERENCE Masterton and Hurley Chapter 9 Chemistry 1011 Slot 5

  2. Review YOU ARE EXPECTED TO BE ABLE TO: • Draw electron dot diagrams to represent bonding in molecules and ions; define electronegativity; determine whether a covalent bond will possess a dipole moment; determine the geometric shape of molecules using VSEPR theory; determine whether a given covalent molecule will possess a net dipole moment. (REVIEW FROM CHAPTER 7) Chemistry 1011 Slot 5

  3. REVIEW • In order to understand intermolecular forces and their effects, you need to be able to: • Draw electron dot diagrams to represent bonding in molecules and ions; • define electronegativity; • determine whether a covalent bond will possess a dipole moment; • determine the geometric shape of molecules using VSEPR theory; • determine whether a given covalent molecule will possess a net dipole moment Chemistry 1011 Slot 5

  4. REVIEW: Drawing Electron Dot Diagrams • Electron dots represent valence electrons • In simple molecules, elements share pairs of valence electrons to form covalent bonds. • Atoms other than hydrogen tend to from bonds until they are surrounded by eight electrons – the octet rule e.g. CH4, NH3, H2O, HF • A non-metal element normally forms a number of covalent bonds equal to 8 – the number of valence electrons • Double bonds, triple bonds are formed when two or three covalent bonds exist between a pair of atoms Chemistry 1011 Slot 5

  5. REVIEW: Steps for Writing Electron Dot Diagrams • Identify the central atom • Draw an electron dot diagram for the molecule showing only single bonds • Determine the number of bonds in the molecule #bonds = ½ (#atoms x 8 – actual # valence electrons) • Add any required double or triple bonds • Draw in any remaining unshared pairs of electrons Chemistry 1011 Slot 5

  6. Electron Dot Diagram for NF3 • N is Central atom • Actual # valence electrons = (3 x 7) + 5 = 26 • Maximum # valence electrons = 4 x 8 = 32 • Difference = 32 – 26 = 6; \ 3 bonds • Distribute remaining electrons Chemistry 1011 Slot 5

  7. Electron Dot Diagram for NO2- • N is Central atom • Actual # valence electrons = (2 x 6) + 5 + 1 = 18 • Maximum # valence electrons = 3 x 8 = 24 • Difference = 24 – 18 = 6; \ 3 bonds • There must be a double bond • Distribute remaining electrons Chemistry 1011 Slot 5

  8. Electron Dot Diagram for BF3 • Boron trifluoride is electron deficient • The octet rule does not apply • Boron is the central atom • Actual # valence electrons = (3 x 7) + 3 = 24 • Maximum # valence electrons = 4 x 8 = 32 • Difference = 32 – 24 = 8; 4 bonds should be present, but only 3 are possible Chemistry 1011 Slot 5

  9. Electron Deficient Molecules • F • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • In a few molecules, there are less than 8 electrons around the central atom: F – Be – F F – B – F Chemistry 1011 Slot 5

  10. Expansion of the Valence Level - Electron Dot Diagram for SF6 • Sulfur (3s2, 3p4)has 6 valence electrons and would normally form 2 bonds • However, two valence electrons can be promoted to the 3d orbitals (3s1, 3p3 ,3d2) • In this way, sulfur can form additional bonds Chemistry 1011 Slot 5

  11. REVIEW: Electronegativity • The electronegativity of an element is a measure of the ability of an atom of the element to attract a shared electron pair in competition with an atom of a different element • Electronegativities vary in much the same way as ionization energies • They DECREASE going DOWN a Group • They INCREASE going ACROSS a period from left to right E.g. F>O>N>C and F>Cl>Br>I Chemistry 1011 Slot 5

  12. REVIEW: Bond Dipole Moments • The valence electrons that make up a covalent bond between two atoms of the same element will be shared equally • When bonds form between different elements, with different electronegativities, the bonding valence electrons will be attracted more towards one atom than the other • The result is the existence of a bond dipole • E.g. HF Chemistry 1011 Slot 5

  13. REVIEW: VSEPR Theory and Molecular Shapes • The pairs of electrons that surround the central atom repel each other and arrange themselves in space in such a way that they are as far apart as possible Chemistry 1011 Slot 5

  14. Molecular Shapes • Four bonding pairs – CH4 • Three bonding pairs, one non-bonding pair – NH3 • Two bonding pairs, two non-bonding pairs – H2O • Electron deficient - three bonding pairs only – BF3 • Expanded valence level – six bonding pairs - SF6 • Two bonding “pairs” – CO2 Chemistry 1011 Slot 5

  15. Geometries of AX2-AX6 molecules Chemistry 1011 Slot 5

  16. Geometries of AX2-AX6 molecules Chemistry 1011 Slot 5

  17. REVIEW: Molecular Dipole Moments • Unsymmetrical molecules with polar bonds will have a molecular dipole • HF • H2O • Symmetrical molecules with polar bonds will not have a molecular dipole • BF3 • SF6 Chemistry 1011 Slot 5

  18. Steps in Determining Molecular Polarity • Electron dot diagram • Bond dipoles? • Molecular shape • Symmetrical or non-symmetrical molecule? • Molecular dipole? Chemistry 1011 Slot 5

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