Lesson 19

To-Do List

  • Continue reading Chapter 4
  • Chapter 3 HW due today (3/5) by 11:55pm
  • Chapter 3 Quiz Friday (3/7)
  • Exam 2 in less than 1 week (3/11)
  • Bring Calculators & Clickers

Molecular Geometry and VSEPR Theory

  • Question: An unknown molecule with the formula AB3AB_3 has a trigonal pyramidal geometry. How many electron groups (as defined by VSEPR theory) are on the central atom (A)?
  • Answer: 4 electron groups. Trigonal pyramidal geometry corresponds to a tetrahedral electron domain geometry with 3 bonding pairs and 1 lone pair.

Bond Angles and Molecular Geometry

  • Question: What is the bond angle in XeO2XeO_2?
  • Answer: Less than 109.5°. The molecule has a tetrahedral electron domain geometry and a bent molecular geometry.

Molecular Geometry of IF4+1IF_4^{+1}

  • Question: Which of the following is correct for IF4+1IF_4^{+1}?
  • Answer: None of the provided options are correct.
    • Electron domain geometry: Trigonal bipyramidal
    • Molecular geometry: See-saw
    • Bond angles: Approximately 120° and 90°

Polarity and Dipole Moments

  • Polar bonds can exist in non-polar molecules if the molecular geometry cancels out the bond dipoles.
  • A dipole moment exists when there is a separation of positive and negative charge within a molecule.

Electronegativity and Polarity

  • Electronegativity values for various elements are provided to assess bond polarity.
  • Polar Molecule: A molecule with a net dipole moment (region of positive and region of negative charge).

Ammonia (NH3) Polarity

  • The molecule NH3NH_3 is polar.

Silicon Tetrachloride (SiCl4) Polarity

  • SiCl4SiCl_4 is a nonpolar molecule with polar bonds.

Polarity Rules

  • Nonpolar Molecules:
    • Symmetrical shape.
    • Same atoms around the central atom.
    • No lone pairs on the central atom.
  • Polar Molecules:
    • Asymmetrical shape.
    • Different atoms around the central atom.
    • Lone pairs on the central atom.
  • Exceptions:
    • Square pyramidal: Polar due to distortion from unshared electrons.
    • Square planar: Nonpolar because electron pairs are opposite, resulting in no distortion.

Polarity Based on Attached Atoms

  • If all atoms bound to the central atom are the same, the molecule might be nonpolar.
  • Different atoms around the central atom usually indicate a polar molecule.

Identifying Polar Covalent Molecules

  • Question: Which of the following is a polar covalent molecule?
  • Answer: PCl3PCl_3 (Phosphorus trichloride) - Trigonal pyramidal geometry, resulting in a net dipole moment.

Ion Polarity

Question: Determine if the ion ClO3−1ClO_3^{-1} is polar. Which side will have a partial positive charge?
*
Answer: (b) δ+.

Determine if the molecule PH3PH_3 is polar

Polar slightly

Impact of Unshared Electrons on Molecular Polarity

  • Unshared electrons influence the degree of polarity.
  • Example:
    • Molecule with a dipole moment of 1.46 (more polar)
    • Molecule with a dipole moment of 0.23 (less polar)