Molecular Geometry and Polarity

Molecular Geometry

  • Lewis structures are essential for determining molecular geometry, angles, and polarity.
  • The central atom dictates the geometry.
  • AXE formula: A is the central atom, X is the number of atoms bonded to the central atom, and E is the number of lone pairs on the central atom.

Electron Domain Families

  • Five basic families based on electron domains:
    • Two electron domains: linear.
    • Three electron domains: trigonal planar.
    • Four electron domains: tetrahedral.
    • Five electron domains: trigonal bipyramidal.
    • Six electron domains: octahedral.
  • Electron domain geometry: a fancy name for each of the families.
  • Each family has an ideal angle:
    • Linear: 180°
    • Trigonal planar: 120°
    • Tetrahedral: 109.5°
    • Trigonal bipyramidal: 90° and 120°
    • Octahedral: 90° and 180°

Trigonal Bipyramidal Family

  • Central atom (A) with three atoms (X) in a trigonal planar arrangement (120° apart).
  • Two additional electron domains (Y) above and below the plane.
  • Angle between Y and X: 90°
  • Angle between X and A: 120°
  • Angle between Y and Y: 180°
  • Lone pairs are placed in the X positions first.
  • Lone pairs push other atoms closer, reducing ideal angles.

Impact of Lone Pairs on Bond Angles

  • Lone pairs are more electronegative and push bonded atoms together, reducing bond angles.
  • More lone pairs lead to greater reduction in bond angles.
  • Exceptions where lone pairs cancel out (starred):
    • Linear with multiple lone pairs.
    • Square planar.
    • Octahedral with opposing lone pairs.

Influence of Different Electron Groups

  • Different electron groups change ideal angles.
  • Example: Carbon bonded to three hydrogens and a fluorine.
    • The angle C-F is greater than 109.5°.
    • The angle C-H is smaller than 109.5°.

Polarity

  • Bond polarity: difference in electronegativity (EN) between two atoms.
  • Molecular polarity: assessment of all polar bonds in the molecule.
  • Even with polar bonds, a molecule can be nonpolar if the bonds cancel out (zero dipole).
  • Example: CO_2 has two polar bonds, but the molecule is nonpolar.

Rules for Predicting Polarity

  • If the central atom has exactly one lone pair, the molecule must be polar. Note: Polarity can depend on the atoms bonded to the central atom. Some shapes must be polar, no matter what the atoms are, based solely on the position of their lone pairs of electrons. The shapes that must be polar are:
    • Trigonal pyramidal.
    • Bent.
    • Seesaw.
    • T-shaped.
    • Square pyramidal.
  • If all surrounding domains are the same element, the molecule must be nonpolar.
  • If exactly one surrounding atom is unique, the molecule is polar.
    • Example: CH_3F is polar because fluorine is unique.
  • All hydrocarbons are nonpolar.

Practice Problems

Problem 21

  • Determine the EDG (electron domain geometry) and molecular geometry (MG) of IF_3.
  • Lewis Structure:
    • Iodine is the central atom bonded to three fluorine atoms.
    • Total valence electrons: 7 (I) + 3 * 7 (F) = 28
    • Six electrons for single bonds leaves 22.
    • Fluorine requires 6 electrons each to obey the octet rule, total 18, leaving 4 for central iodine.
    • Iodine accepts 4 electrons because it can.
  • Electron Domains: 5 (3 atoms + 2 lone pairs)
    • EDG: Trigonal Bipyramidal
  • AXE Formula: AX3E2
    • MG: T-shaped

Problem 22

  • Rank the following molecules in increasing bond angles: BeF2, CF4, PF3, SF2
  • BeF_2
    • Lewis Structure: Beryllium in the middle, bonded to two fluorines.
    • Valence electrons: 2 (Be) + 2 * 7 (F) = 16
    • Minus the four for two single bonds leaves 12.
    • Fluorine requires 12 valence electrons, puts us at zero.
    • Be is an exception to the octet rule. Only wants the four.
    • AX2 format, linear family.
    • EDG: Linear, 180°
  • CF_4
    • AX4 formula.
    • EDG: Tetrahedral, 109.5°
  • PF_3
    • Lewis structure looks like this with three atoms and a lone pair on central phosphorus.
    • AX_3E formula.
    • EDG: Tetrahedral
    • Ideal angle: 109.5°
  • SF_2
    • Central atom has two lone pairs of electrons and two atoms surrounding it.
    • AX2E2 formula.
    • Four electron domains, also tetrahedral.
    • Ideal angle: 109.5°
  • Lone pair electrons push the other atoms together making them even closer.
    • The angle right here between our fluorines will actually be a lot less than 109.5 degrees.
  • From smallest to largest, SF2, PF3, CF4, BeF2.

Problem 23

*Rank from greatest to least polarity CH4, CF4, CBr4, CCl4
*Polarity comes from the change in electronegativity.
*Fluorine is the most electronegative followed by chlorine then bromine then hydrogen.
*So, the order from the largest to smallest polarity is
CF4, CBr4, CCl_4.
*Carbon and hydrogen are Non polar.

Problem 24

*Molecule XF4 has seesaw molecular geometry. What could element X be? *Figure out what we have from having the fact that is seesaw in shape. *For seesaw the correct geometry is supposed to be ax4e1. *The bonds to fluorine always have a single bond. *Each fluorine needs 6 lone pairs surrounding them. *And the x atom will always need one lone pair. *Working backwards we see there is 34 electrons to make this Lewis Structure. *Take a look at XF4 to tally up the electrons.
*For central X you do not know so going to call it X.
*Then add it together for flourine electrons at 7 x 4 for 34 electrons.
*From this we get for X to be 6.
*Look at the options to see what has a valence electron of 6, we see it being group 6 that we need.
*Selenium Boom, oxygen boom these are in group 6.
*Period 2 cannot violate the octet wall.
*This element violates the octet wall, with Selenium being the correct answer.

Problem 25

  • Which of the following molecules are polar?
  • They are all hydrocarbons, and all hydrocarbons are nonpolar.
  • So, the answer is none of them are.

Problem 26

*Rank the following compounds on a scale from Nonpolar to ionic.
*Nonpolar is the least and ionic being the most.
*See ionic is just greatly polarized.
*First compound N_2 which is a pure elemental will be nonpolar.
*Second compound is NaCl metal nonmetal, this is ionic.
*Third and fourth compounds is No and HCl, both are molecular compounds but also are compounds of polar.
*To rank the polarity look a how far apart the compound is, where the greatest is the one that is more polar, and that leads to HCl being more polar than No