Overview
This lecture covers the interaction between atoms and various forms of electromagnetic radiation, focusing on how electronic structure relates to absorption and emission of light, spectrum analysis, and core calculations involving wavelength, frequency, and energy.
Electromagnetic Spectrum & Spectroscopy
- Electromagnetic radiation is energy traveling at the speed of light with oscillating electric and magnetic fields.
- Visible light is a small segment of the electromagnetic spectrum, which also includes gamma rays, X-rays, UV, infrared, microwaves, and radio waves.
- Spectroscopy studies how substances absorb and emit electromagnetic radiation to understand atomic electronic structure.
- White light produces a continuous spectrum; gases like hydrogen and helium produce a line (discrete) spectrum.
Wavelength, Frequency, and Energy
- Wavelength (λ, lambda) is the distance between wave peaks and is measured in meters, nanometers, etc.
- Frequency (ν, nu) is the number of cycles per second, measured in hertz (Hz or s⁻¹).
- Speed of light (c) is 2.998 × 10⁸ m/s.
- Wavelength and frequency are inversely proportional (c = λν).
- Long wavelengths have low frequency and energy; short wavelengths have high frequency and energy.
Photons and Atomic Spectra
- A photon is a packet of electromagnetic energy.
- Electrons absorb energy and jump to higher levels; when returning to lower levels, they emit energy as photons.
- Energy is quantized, so electrons can only occupy specific energy levels, resulting in discrete line spectra for elements.
- Line spectra act as unique fingerprints for elements and allow identification of substances (e.g., in stars).
Key Equations and Relationships
- Speed of light: c = λν (wavelength × frequency)
- Energy of a photon: E = hν (Planck's constant × frequency)
- Energy can also be expressed as E = hc/λ.
- Planck’s constant (h) is 6.626 × 10⁻³⁴ J·s.
- Knowing one equation allows derivation of others using algebra.
Example Calculation: Radio Signal
- To find wavelength: λ = c/ν (e.g., 90.1 MHz radio signal gives λ = 3.3 m).
- To find energy: E = hν, then convert J/photon to kJ/mole using Avogadro's number (e.g., 3.60 × 10⁻⁵ kJ/mol for the example).
Key Terms & Definitions
- Electromagnetic Spectrum — Range of electromagnetic radiation types ordered by wavelength/frequency.
- Spectroscopy — Study of matter’s interaction with electromagnetic radiation.
- Photon — A quantized unit of electromagnetic energy.
- Wavelength (λ) — Distance between successive wave peaks.
- Frequency (ν) — Number of wave cycles per second.
- Planck’s Constant (h) — Fundamental constant (6.626 × 10⁻³⁴ J·s).
- Line Spectrum — Discrete lines of color emitted by energized elements.
- Continuous Spectrum — Blended, uninterrupted range of colors (seen with white light).
Action Items / Next Steps
- Memorize Planck’s constant (h) and speed of light (c).
- Practice using the equations: c = λν, E = hν, and E = hc/λ.
- Be able to convert between units (e.g., MHz to Hz, J to kJ, per photon to per mole).
- Review the electromagnetic spectrum and identify regions by wavelength and energy.