Study Notes on Molecular Geometries

Central Atom Geometry Overview

  • Focus on central atoms with bonds but no lone pairs.

  • Ideal geometries are determined by the number of effective pairs around a central atom.

  • Effective pairs refer to the total number of bonding pairs (bonds formed with other atoms).

Introduction to Avogadro Software

  • Avogadro is a free, open-source molecular builder and visualization tool.

  • It allows users to visualize molecular geometries.

  • Can be downloaded from the official website (not provided in the transcript).

Ideal Geometry with Two Effective Pairs

  • Description: Central atom (A) bonded to two terminal atoms (X).

  • Angle: Effective pairs achieve maximum separation at 180 degrees.

  • Geometry Name: Linear.

  • Example: Carbon Dioxide (CO2)

    • Structure: Central carbon atom with two double bonds.

    • Bond Angle: Oxygen-Carbon-Oxygen (O-C-O) bond angle is 180 degrees.

    • Visualization: CO2 optimally displays linear structure.

  • Experiment:

    • Adjusting one bond to decrease the angle from 180 degrees.

    • Optimization shows that CO2 reverts back to the linear geometry due to stability preference.

Ideal Geometry with Three Effective Pairs

  • Description: Central atom (A) bonded to three terminal atoms.

  • Angle: Effective pairs are maximally separated at 120 degrees.

  • Geometry Name: Trigonal Planar.

  • Example: Boron Trifluoride (BF3)

    • Structure: Central boron atom with three fluorine atoms.

    • Bond Angle: Any bond angle (F-B-F) is 120 degrees.

    • Visualization: Boron trifluoride shown in Avogadro depicts a trigonal planar geometry where all atoms are coplanar.

  • Experiment:

    • Move an atom to show non-120-degree angles.

    • Optimization demonstrates it returning to trigonal planar shape with bond angles at 120 degrees.

Ideal Geometry with Four Effective Pairs

  • Description: Central atom (A) bonded to four terminal atoms.

  • Angle: Effective pairs achieve separation when bonded at 109.5 degrees.

  • Geometry Name: Tetrahedral.

    • Origin: From Greek, "tetra" means four, "hedron" means face/base, leading to the geometry having four faces.

  • Example: Methane (CH4)

    • Structure: Central carbon atom with four hydrogen atoms.

    • Bond Angle: Any H-C-H bond angle is 109.5 degrees.

    • Visualization: The 3D representation in Avogadro shows the tetrahedral structure of CH4 with hydrogen atoms in spatial arrangement.

  • Drawing Method:

    • Three atoms can be placed in the same plane.

    • Solid lines for bonds in the same plane.

    • Wedges for bonds out of the plane and dots for bonds into the plane.

    • Resulting visualization exhibits a three-dimensional tetrahedral geometry.

Ideal Geometry with Five Effective Pairs

  • Description: Central atom (A) bonded to five terminal atoms.

  • Arrangement: Terminal atoms arranged in two different planes; maximum separation is achieved through specific angles.

  • Equatorial Plane: Contains three effective pairs positioned at 120 degrees to each other.

  • Axial Plane: Contains two effective pairs at 180 degrees to each other.

  • Angle: Between axial and equatorial atoms is typically 90 degrees.

  • Geometry Name: Trigonal Bipyramid.

  • Example: Phosphorus Trichloride Dibromide (PCl3Br2)

    • Structure: Central phosphorus atom.

    • Bond Angles:

    • Chlorine atoms in the equatorial plane at 120 degrees.

    • Bromine atoms positioned axially at 180 degrees.

    • Relative angle between axial and equatorial atoms is 90 degrees.

  • Visualization: In Avogadro, it shows a trigonal bipyramidal structure and the arrangement of chlorines and bromines.

  • Drawing Method:

    • Solid lines for bonds in the same plane.

    • Wedges and dots for those above and below the central plane.

Ideal Geometry with Six Effective Pairs

  • Description: Central atom (A) bonded to six terminal atoms.

  • Angle: All effective pairs achieve maximum separation at bond angles of 90 degrees.

  • Geometry Name: Octahedral.

    • Etymology: "Octa" means eight and "hedron" refers to the faces/base, summarizing the eight faces of this geometry.

  • Example: Sulfur Hexafluoride (SF6)

    • Structure: Central sulfur atom with six fluorine atoms.

    • Bond Angle: Any bond angle (F-S-F) is 90 degrees.

  • Visualization: In Avogadro, all bonds and angles maintain 90 degrees, and the structure is symmetric in all spatial dimensions.

  • Drawing Method:

    • Solid lines for axial and equatorial pairs in the same plane.

    • Wedges and dots for the remaining bonds to convey the three-dimensional structure.

Conclusion: Practice Exercise

  • Recommendation for students to practice the concepts covered regarding molecular geometry and visualization methods using Avogadro after reviewing notes.