Lecture 3 Wk 1

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• All images sourced from: Blackman, Bottle, Schmid, Mocerino, and Wille, Chemistry, 2012 (John Wiley & Sons) ISBN: 9780470810866

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• Overview of Lecture Topics:

  • Light

  • Atomic structure

  • Elements

  • Electronic structure

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• Learning Outcomes:

  • Understand the relationship between frequency, wavelength, and energy.

  • Recognize the wave and particle natures of light.

  • Understand that light is a form of electromagnetic radiation.

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• Light as a Source of Energy:

  • Absorption and emission of light by atoms/molecules critical for understanding their structure.

  • Light carries energy.

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• Two Theories of Light:

  1. Light as a wave.

  2. Light as a beam of particles.

  • Historical perspective:

    • Newton (1687) supported particle theory.

    • Huygens (1690) supported wave theory.

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• Thomas Young’s Double-Slit Experiment:

  • Early 19th-century evidence favored wave theory.

  • Light produced a diffraction pattern typical of waves.

  • Inquiry into what oscillates in a light wave.

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• Electromagnetic Waves:

  • James Clerk Maxwell (1873) defined light as an electromagnetic wave.

  • Oscillating electric and magnetic fields propagate as light.

  • Verified by Heinrich Hertz (1887).

  • Speed of electromagnetic wave: 3 x 10^8 m/s.

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• Electromagnetic Radiation Spectrum:

  • Wavelength (nm) distribution:

    • Gamma rays

    • X-rays

    • Ultraviolet

    • Visible

    • Infrared

    • Microwaves

    • Radio frequency

  • Visible light occupies a narrow band in this spectrum: 400-750 nm.

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• The Wave Nature of Light:

  • Wavelength (λ): Distance between identical points on a wave.

  • Frequency (ν): Number of wave crests passing a point per unit time.

  • Amplitude (A): Height of the wave; related to intensity/brightness.

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• Properties of Electromagnetic Waves:

  • All travel at the same speed (speed of light in vacuum: c = 2.998 × 10^8 m/s).

  • Relationship: c = λν (wavelength and frequency correlation).

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• Origins of Quantum Theory:

  • Early 20th century brought a shift in understanding energy and light.

  • Blackbody Radiation phenomenon exhibited discrepancies with classical theories (e.g., ultraviolet catastrophe).

  • Max Planck (1900) introduced energy quantization: E = hν

    • (ν = frequency of oscillation, h = Planck’s constant).

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• Further Developments in Quantum Theory:

  • Despite quantization, light was still viewed as a wave by most physicists, including Planck.

  • Photoelectric Effect: Light can eject electrons from metals only if frequency is above a specific threshold.

    • Classical view: energy proportional to amplitude (wave theory) vs. light's energy proportional to frequency (particle view).

  • Einstein (1905) proposed light has both wave and particle natures (photons).

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• The Particle Nature of Light:

  • All electromagnetic radiation travels at the same speed, but energy varies with frequency.

  • Higher frequency = faster arriving photons = higher energy.

  • Formula: E = hν (Planck's relation).

  • Constant: h = 6.626 x 10−34 Js.

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• Wave-Particle Duality:

  • Light exhibits both wave behaviors (as shown in Young's double-slit experiment) and particle behaviors (photoelectric effect).

  • Chemists utilize the model (wave or particle) that best fits experimental data.

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• Final Learning Outcomes:

  • Understand the relationship c = λν and E = hν.

  • Evidence for both wave and particle behavior demonstrated by experiments.

  • Recognize oscillation of electric and magnetic fields in wave model of light.

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• Preparation for Next Lecture:

  • Question: Comparing red and blue light in terms of frequency, wavelength, speed, and energy.

  • (a) Which has higher frequency? (b) Longer wavelength? (c) Faster speed? (d) Greater energy?