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Molecular shapes

Molecular shapes. Balls and sticks. Valence shell electron pair repulsion. Lewis dot structure provides 2D sketch of the distribution of the valence electrons among bonds between atoms and lone pairs; it provides no information about molecular shape

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Molecular shapes

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  1. Molecular shapes Balls and sticks

  2. Valence shell electron pair repulsion • Lewis dot structure provides 2D sketch of the distribution of the valence electrons among bonds between atoms and lone pairs; it provides no information about molecular shape • First approach to this problem is to consider repulsion between groups of electrons (charge clouds)

  3. Electron groups (clouds) minimize potential energy • Valence shell electron pair repulsion (VSEPR) • Identify all of the groups of charge: non-bonding pairs or bonds (multiples count as one) • Distribute them about the central atom to minimize potential energy (maximum separation) • This specifies the electronic geometry also sometimes called molecular geometry

  4. Choices are limited • Groups of charge range from 2 – 6 • Only one electronic geometry in each case • More than one molecular shape follows from electronic geometry depending on number of lone pairs • One surprise: the lone pairs occupy more space than the bonded atoms (with very few exceptions) • Manifested in bond angles (examples follow) • Molecular shape selection (particularly in trigonal bipyramid)

  5. Two groups: linear • Except for BeH2, all cases with two groups involve multiple bonds

  6. S O O Three groups: trigonal planar • Two possibilities for central atoms with complete octets: • Trigonal planar (H2CO) • Bent (SO2) • BCl3 provides example of trigonal planar with three single bonds • B is satisfied with 6 electrons

  7. Four groups: tetrahedral • Three possibilities: • No lone pairs (CH4) - tetrahedral • One lone pair (NH3) – trigonal pyramid • Two lone pairs (H2O) – bent • Note: • H-N-H angle 107° • H-O-H angle 104.5° • Tetrahedral angle 109.5°

  8. Representations of the tetrahedron

  9. Molecules with multiple centers • A central atom is any atom with more than one atom bonded to it • Perform exercise individually for each atom • Electronic geometry and molecular shape will refer only to the atoms/lone pairs immediately attached to that atom

  10. Polar or non-polar? That is the question. • A molecule is polar if the centers of positive and negative charge do not coincide. • How do we determine this? • Rigorous approach needs consideration of symmetry and mathematical calculations • Approximate approach considers arrangements of bonds

  11. How many polar bonds? • Bond is polar if electronegativity difference greater than 0.4 • Zero bonds: always nonpolar • One bond: always polar • Two or more bonds: may or may not… • Consider the molecular shape • Do individual bond polarities cancel? • If yes, nonpolar. If no, polar

  12. Two bonds • Equal bonds oppose (linear) • Nonpolar (CO2) • Unequal bonds oppose (linear) • Polar (HCN) • Equal bonds do not oppose (bent) • Polar (H2O)

  13. Three bonds • Equal bonds oppose in trigonal planar arrangement • Nonpolar • Unequal bonds in trigonal planar arrangement • Polar

  14. Gets more complicated • Planar or pyramidal? • Depends on number of groups of charge • BCl3 is trigonal planar – nonpolar • NCl3 is trigonal pyramidal – polar • Four bonds works better with models

  15. Roadmap to polarity • Establish skeleton of molecule • Determine Lewis dot structure using S = N – A • Determine electronic geometry using VSEPR • Identify molecular geometry from molecular • Count number of polar bonds • Perform polarity analysis using rules described above

  16. Important properties related to polarity • Solubility: polar molecules dissolve in polar solvents; nonpolar molecules dissolve in nonpolar solvents • Oil (nonpolar) and water (polar) don’t mix • Ammonia (polar) dissolves in water • Melting and boiling points • Polar substances have high intermolecular forces: • Melting and boiling points are much higher than with nonpolar substances (H2O is a liquid, CO2 is a gas)

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