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Interactions of Light and Matter

  • Molecules behave differently when exposed to different types of electromagnetic radiation.

  • Types of electromagnetic radiation and their effects:

    • UV Radiation:

    • Breaks chemical bonds in molecules, leading to potential molecular fragmentation or dissociation.

    • Infrared Radiation (IR):

    • Causes vibrations within the molecules, specifically involving stretching and bending of bonds.

    • Microwave Radiation:

    • Induces molecular rotations, leading to changes in molecular orientation and alignment.

Electronegativity

  • Definition: Electronegativity is a measure of an atom’s ability to attract bonded electrons in a chemical bond.

  • Trends in Electronegativity:

    • Increases from the bottom to the top of a group in the periodic table.

    • Increases from left to right across a period in the periodic table.

    • Conceptual analogy: Similar to a "tug-of-war" for electrons between two atoms in a chemical bond.

    • The atom that exerts a stronger pull for the electrons is characterized by a higher electronegativity value.

Electronegativity in Chemical Bonds

  • In a chemical bond:

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

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

  • Example: Oxygen (O) vs Carbon (C):

    • Oxygen is located to the right of carbon on the periodic table, making it more electronegative.

    • In an oxygen-carbon (C-O) bond, the electrons are pulled closer to the oxygen atom, leading to:

    • A partial negative charge on the oxygen side of the bond.

    • A corresponding partial positive charge on the carbon side of the bond.

  • Impact of Molecular Geometry on Charge Distribution:

    • As bonds within a molecule are stretched or bent, the distribution of charges will change accordingly.

    • If the geometry and direction of the stretching/bending cancel out the charge redistribution, the molecule will not absorb infrared radiation.

Molecular Vibrations and Infrared (IR) Absorption

  • Illustration of molecular vibrations for carbon dioxide (CO₂):

    • Each representation resembles a spring, embodying the C=O bond in the molecule.

    • Cases of Molecular Vibrations:

    • (a) No net change in electron distribution (continuous dipole) – Result: No absorption of IR radiation.

    • (b, c, d) Net change in dipole – Result: Absorption of IR radiation occurs due to alterations in charge distribution caused by molecular vibrations.

  • Molecular Geometry Influences on IR Absorption:

    • The alignment and geometric configuration of molecular vibrations play a pivotal role in determining whether infrared radiation is absorbed.