CHEM 1061 Chemical Principles

Light as a Wave

  • Electromagnetic Radiation consists of various types:

    • Radio Waves

    • Micro Waves

    • Infrared

    • Visible Light

    • Ultraviolet

    • X-rays

    • Gamma Rays

  • Models of Light

    • Light can be described both as a particle and a wave.

Wave Properties

  • Definitions:

    • Wavelength ( πœ†): distance between two identical points (measured in meters).

    • Frequency (𝑓): number of wavefronts passing a point per second (Hertz).

    • Relationship: Speed of Light (c):

      • c = πœ† Γ— 𝑓

      • c = 3.00 Γ— 10^8 m/s

  • Examples of Distance Calculation:

    • Distance between objects 10^1 m apart = 10 m

    • Distance between objects 10^2 m apart = 100 m

X-rays

  • Properties of X-rays:

    • Wavelength comparable to the size of atoms (approximately in nm)

    • Example: Determine wavelength of an X-ray with frequency of 3.0 Γ— 10^18 Hz:

      • Calculate wavelength:

        • πœ† = c/𝑓 = 1.0 Γ— 10^-10 m

Light as a Wave (Part 2)

Wavelength and Frequency Comparisons

  • Longest Wavelength: Infrared

  • Highest Frequency & Energy: X-rays

Predictions About Electron Emission from UV Light

  • Increasing Intensity: number of electrons emitted increases

  • Keeping Intensity Same but Increasing Wavelength: number of electrons emitted decreases when wavelength is increased toward yellow; no electrons emitted if threshold is not met.

Light as a Particle (Part 2)

Summary of the Photoelectric Effect

  • Frequency Dependency:

    • Light frequency must exceed threshold frequency for electrons to be emitted.

    • Increasing intensity leads to more electrons emitted if frequency is above the threshold.

    • No emission occurs if frequency is below threshold, regardless of intensity.

Particle Properties of Light

  • Photon Energy (E = h𝑓):

    • Planck's constant (h = 6.626 x 10^-34 Js) quantizes light energy.

Electromagnetic Radiation as a Particle

Photoelectric Effect Fundamentals

  • Energy transfer occurs when photons hit a metal surface and can eject electrons.

  • Factors influencing photoelectric effect:

    • Wavelength (Ξ») determines if electrons are ejected and their speed.

    • Intensity affects the number of electrons ejected, if any.

Quantum Mechanics

  • Wave-Particle Duality:

    • All matter exhibits both wave and particle characteristics.

  • de Broglie's Equation:

    • Wavelength (πœ†) = h/mv

  • Importance for minute particles (e.g., electrons, atoms)

Energy Calculations in Electromagnetic Spectrum

Example Calculation for Photon Energy

  • For X-ray Photon:

    • Frequency of 4.0 Γ— 10^18 Hz. Energy determined as:

    • E = h𝑓 = (6.626 x 10^-34 Js)(4.0 Γ— 10^18 Hz) = 2.65 Γ— 10^-15 J

Atomic Structure

Bohr Model vs. Rutherford Model

  • Rutherford Model: Proposes electrons orbit nucleus like planets, but it cannot explain atomic absorption or emission spectra correctly.

  • Bohr Model: Electrons move in defined orbits at quantized energy levels; successful in explaining hydrogen's emission and absorption spectrum.

Absorption & Emission Spectroscopy

  • Atoms emit light, creating atomic emission spectra, unique to each element.

  • Absorption spectra show light absorbed by atoms at specific wavelengths.

The Process of Spectroscopy

  • Can be used to analyze how different materials absorb or emit light, leading to a better understanding of atomic structure.

  • Light from different elements is characterized by specific wavelengths, indicating quantum behavior.

Conclusion: Wave-Particle Duality in Matter

  • de Broglie hypothesis: All matter exhibits wave properties, particularly on an atomic scale, affecting behavior significantly.