Chemistry Feb 26 lecture

Overview of Intramolecular Forces

• Intramolecular forces are the forces that hold atoms together within a molecule.

• Covalent bonds are a primary type of intramolecular force that involve the sharing of electrons between atoms.

Electronegativity and Bonding

• Electronegativity refers to the tendency of an atom to attract electrons within a bond.

• When two identical atoms bond together (e.g., O2 or F2), they have the same electronegativity, resulting in an equal sharing of electrons.

• This equal sharing means that there is no dipole moment, and thus no polarity in the bond.

Characteristics of Molecules

• A molecule's asymmetry in shape and composition can create a dipole:

  • A molecule is asymmetrical in composition if it has different types of atoms connected (e.g., HF).

  • A molecule can be symmetrical in shape (i.e., linear) but still have differences in composition, leading to polarity.

Example: HF (Hydrogen Fluoride)

• HF has different atoms contributing to the bond:

  • Electronegativity: High difference leads to a dipole.

  • Shape: The molecular structure is asymmetrical, creating a dipole across the molecule.

Example: Ammonia (NH3)

• NH3 features three hydrogen atoms bonded to a nitrogen atom:

  • Dipoles: Each N-H bond has a dipole due to nitrogen's high electronegativity.

  • Shape: The structure is asymmetrical in shape, leading to a net dipole moment across the molecule.

Trigonal Planar Configuration

• Molecules in a trigonal planar shape, like Boron Trifluoride (BF3):

  • Electronegativity: Similar to previous examples with Fluorine.

  • Shape: Trigonal planar results in a symmetrical shape and composition, so no net dipole moment exists despite bond dipoles.

Bond Dipoles as Vectors

• Bond dipoles can be conceptually treated as vectors:

  • Vectors represent the direction and magnitude of bond polarity, influencing the overall molecular dipole.

Summary

• Intramolecular forces are essential for understanding molecular structure and behavior.

• Electronegativity differences create polar bonds, and the molecular geometry determines the presence of net dipoles.