lecture recording on 20 February 2025 at 14.39.57 PM

Molecular Geometry and Shapes

  • Understanding molecular geometry is essential for predicting the shape of molecules based on their bonding and lone pairs of electrons.

Common Geometrical Shapes

  • Linear:

    • Shape occurs when a central atom is connected to two other atoms with no lone pairs.

    • Bond angle: 180°.

  • Trigonal Planar:

    • Formed when a central atom is connected to three atoms and has no lone pairs.

    • Bond angle: 120°.

  • Tetrahedral:

    • Central atom connected to four other atoms, with no lone pairs.

    • Bond angle: 109.5°.

  • Trigonal Bipyramidal:

    • One central atom surrounded by five other atoms, requiring consideration of equatorial and axial positions.

    • Bond angles: 120° (equatorial) and 90° (axial).

  • Octahedral:

    • Central atom is bonded to six atoms.

    • Bond angles: 90°.

Variations of Shapes Due to Lone Pairs

  • Trigonal Pyramid:

    • Occurs when one of the positions in tetrahedral geometry is occupied by a lone pair.

    • The shape is trigonal pyramidal with bond angles reduced from 109.5° due to lone pair repulsion.

  • Bent Shape:

    • Result of two bonds and two lone pairs on the central atom; bond angle is less than 109.5°.

  • Seesaw:

    • Derivable from trigonal bipyramidal by replacing one equatorial bond with a lone pair.

    • Features a mix of 90° and 120° bond angles.

Complex Molecules and Central Atoms

  • Breaking Down Larger Molecules:

    • Larger molecules can often be broken down into simpler units or central atoms to determine geometrical arrangement.

    • Each central atom's electron distribution should be analyzed based on bonding and lone pairs.

Electron Counting and Structure Drawing

  • For a given molecule, first, count the total number of valence electrons available from its constituent atoms.

  • Construct a basic skeletal structure, using the least electronegative atom as the central atom.

Example: Sulfur Tetrachloride (SCl4)

  • Electron Count:

    • Sulfur contributes 6 electrons. Each Chlorine (4 total) contributes 7 electrons (28 total).

    • Total: 34 valence electrons.

  • Skeletal Structure Design:

    • Sulfur as the central atom connected to four Chlorine atoms, consuming 8 electrons.

    • Distribute remaining electrons to satisfy the octet rule for terminal Chlorine atoms.

Electronegativity and Polarity in Molecules

  • Understanding Bond Polarities:

    • Polar bonds can occur between atoms of different electronegativities, but the overall polarity of the molecule depends on geometry.

  • Symmetrical Molecules:

    • Molecules with symmetrical geometries can have polar bonds without being polar overall due to cancellation of dipole moments.

Summary of Key Principles

  • The molecular shape is greatly influenced by bonding pairs versus lone pairs of electrons.

  • Regular practices and understanding geometries and electron distribution aid in accurately predicting the shape and properties of a molecule.

  • Recognizing how lone pairs affect angles and shapes is crucial for understanding molecular behavior and reactivity.