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 ( 𝜆): 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 = 𝜆 × 𝑓

    • c = 3.00 × 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 × 10^18 Hz:

    • Calculate wavelength:

      • 𝜆 = c/𝑓 = 1.0 × 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 (λ) 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 (𝜆) = 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 × 10^18 Hz. Energy determined as:

  • E = h𝑓 = (6.626 x 10^-34 Js)(4.0 × 10^18 Hz) = 2.65 × 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.