Interactions of Light and Matter and Electronegativity

Interactions of Light and Matter

  • Molecules exhibit different behaviors when subjected to various types of electromagnetic radiation, which includes:

    • Ultraviolet (UV) Radiation:

    • UV radiation leads to the breaking of chemical bonds within molecules.

    • Infrared (IR) Radiation:

    • IR radiation induces vibrations within molecules, specifically.

      • Vibrational Actions:

      • Stretching of bonds (increases distance between atoms),

      • Bending of bonds (angles between bonds change).

    • Microwave Radiation:

    • Microwaves cause molecular rotations, affecting the spatial orientation of molecules.

Electronegativity

  • Definition: Electronegativity quantifies an atom's ability to attract bonded electrons.

  • Trends in Electronegativity:

    • Electronegativity tends to increase:

    • As one moves from the bottom to the top of a group in the periodic table.

    • As one moves from left to right across a period in the periodic table.

  • Analogy: Consider electronegativity to be like a "tug-of-war" for electrons between two atoms bonded together.

  • Comparative Strength:

    • The atom with higher electronegativity is deemed the "stronger" atom within the bond.

Charges in Bonds

  • In a bond involving two atoms with differing electronegativities:

    • The more electronegative atom acquires a partial negative charge (denoted as δ−).

    • The less electronegative atom corresponds with a partial positive charge (denoted as δ+).

  • Example:

    • In a bond between oxygen and carbon:

    • Oxygen is more electronegative than carbon, as evidenced by its position to the right of carbon in the periodic table.

    • Therefore, in an oxygen-carbon bond, the electrons are pulled closer to the oxygen, resulting in a partial negative charge on that side of the bond.

Molecular Geometry and Infrared (IR) Absorption

  • Impact of Molecular Vibrations:

    • Each molecular vibration alters the charge distribution within the molecule.

  • Key Concept:

    • If the stretching or bending of bonds in a molecule leads to an imbalance in charge distribution, this creates a net change in dipole moment, rendering the molecule capable of absorbing IR radiation.

  • Illustration:

    • Molecular Vibrations of Carbon Dioxide (CO2):

    • Sprung representations depict C=O bonds within CO2.

    • Scenarios include:

      • (a): No net change in electron distribution, leading to no IR absorption.

      • (b, c, d): Net change in dipole occurs and thus, IR radiation is absorbed.