CHEM 18 Module 1

• Wave is a disturbance that transmits energy into a medium
• Characteristics of a Wave

  1. Wavelength
  2. Amplitude
  3. Frequency
  4. Velocity
• The wavelength (λ) is the distance between two successive points on a wave, measured perpendicular to its direction of travel.
• The frequency (f) is the number of waves passing through a point per unit time.
• The amplitude (A) is the maximum displacement from equilibrium or rest position.
• The velocity (v) is the speed at which the wave travels through the medium.
• Period (T): The period is the amount of time it takes for one complete cycle of a wave to pass by a given point.
• In-phase: When two waves have the same phase angle, they are said to be in phase with each other.
• EMR has a constant velocity of $3.0x10^8$m/s in a vacuum
• Important Properties of EMR Waves

  1. Diffraction
  2. Interference
  3. Reflection
  4. Refraction
• Light can also bend when passing from water to air because of different refractive indices.
• Diffraction involves the change in direction of waves as they pass through a slit around the barrier
• Interference is a combination of two or more EMR waves to form a resultant wave.
• Interference is a combination than can be in-phase or out-of-phase
• Constructive interference occurs when two waves are in phase, resulting in an increase in amplitude
• Destructive interference occurs when two waves are out of phase, resulting in a decrease in amplitude
• Electromagnetic radiation travels in straight lines until it encounters matter
• Visible light has a range of wavelengths from about 400 nm (violet) to 750 nm (red).
• Reflection involves the change in direction of waves as they bounce off a barrier
• Refraction involves the change in direction of waves as they pass through a medium to another medium
• Diffusion is the movement of particles from high concentration to low concentration due to random motion.
• The Line Spectra is where each element has its own spectral fingerprint
• Light as a Particle: Three Phenomena that cannot be explained by Classical Theory

  1. Blackbody Radiation
  2. Photoelectric Effect
  3. Emission Spectra
• Blackbody Radiation is the radiation emitted by heated objects
• Classical theory:

  1. The intensity of radiation emitted increases indefinitely
  2. No limitations on the energy of a system
• Photoelectric effect is when light hits metal surface and electrons are ejected
• Electron emission depends only on frequency not intensity
• In photoelectric effect, there is no threshold frequency (no minimum energy)
• In photoelectric effect, the maximum kinetic energy depends on the frequency of incident photons
• In emission spectra, atoms emit only certain wavelengths of light
• Intensity does not affect electron emission
• Emission spectra is the spectrum of wavelengths or frequencies of light emitted from an atom, molecule, or ion when it returns to its ground state from an excited state.
• Wavelength of black body radiation decreases as temperature increases
• Planck's constant is h = 6.62 x 10^-34 J s
• Emission spectrum can be used to identify elements
• Absorption spectra occurs when an atom absorbs specific frequencies of light
• Atoms absorb light at characteristic frequencies
• Each shell can hold up to two electrons with opposite spins.
• The Bohr model describes electrons orbiting around the nucleus at specific distances called shells.
• Quantum Theory:

  1. Difference between two allowed energies has a specific value.
  2. Energy is limited into specific sets of discrete values.