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The Atom, Light Emission, and the Quantum. Models. Impossible to know what atoms “look like” since they’re so small. Instead, use models (conceptual, graphical and or mathematical representation). Classic model of the atom.
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Models • Impossible to know what atoms “look like” since they’re so small. • Instead, use models (conceptual, graphical and or mathematical representation)
Classic model of the atom • Planetary model. Very outdated, and not an accurate representation of the atom. • Still useful for understanding certain processes, like light emission
Light Quanta • Is light a particle or wave? • Wave behaviors of light include reflection, refraction, diffraction, and interference • Light also displays particle behavior (example later)
Light Quanta • Light occurs in elemental quantized packets of energy, called photons. A quantum is a unit--the smallest amount of something • Think-The mass of a gold ring is the mass of a single gold atom multiplied by the number of atoms in the ring.
The energy of the emitted photon is related to the frequency, given by the equation E = hf (proposed by Max Planck) • h = Planck’s constant (6.6 x 10-34 Js) • Constant in nature of Energy to frequency • Sets a limit on the smallness of things • Gives the smallest amount of energy that can be converted to light with frequency f • So light is emitted from an atom as a stream of photons, each photon with a frequency and energy of hf
Photoelectric Effect • Einstein found support for quantum theory of light in the “Photoelectric Effect” • The ejection of electrons from certain metals when light falls upon them. (these metals are “photosensitive”) • High frequency light, even when dim, can eject electrons from a photosensitive surface • Low frequency light, even if very bright, may not be able to eject electrons from the surface
Photoelectric Effect • Einstein explained by thinking of light in terms of photons, instead of continuous waves • The number of photons in a beam controls the brightness. The frequency of the light controls the energy of individual photons
Waves as particles • So what is light? A particle? A wave? A particle that waves as it goes by? • Light shows properties of both • This is known as Wave-Particle Duality
Particles as Waves • Recall Thomas Young’s double slit experiment that showed an interference pattern
Shoot beams of photons (dim light) • An interference pattern forms photon by photon on the screen • Each single photon has wave and particle properties. Different aspects show at different times • A photon behaves as a particle when it is being emitted or absorbed by detectors, and behaves as a wave in traveling to a source to a detector
Particles as waves • So if light can show wave and particle properties, what about other forms of matter? • Turns out that all bits of matter have wave properties • Explained by Louis de Broglie • Wavelength = h/momentum
Check your understanding • Does a 0.5 kg baseball moving 10 m/s have a wavelength?
Electron waves • You usually think of electrons as negatively charged particles • In “Bohring Spectra”, you saw that electrons have specific energy levels. • This is best understood by considering the wave properties of an electron
Orbital radius of electron • Whole number integers of de Broglie wavelengths are needed for an orbital radius to be possible. • Will the above radius work?
Section 27.1 A Particle Model of Waves Radiation from IncandescentBodies • When the dimmer control is used to increase the voltage to the bulb, the temperature of the glowing filament increases. • As a result, the color changes from deep red to orange to yellow and finally, to white.
Section 27.1 A Particle Model of Waves Radiation from Incandescent Bodies • This color change occurs because the higher-temperature filament emits higher-frequency radiation. • The higher-frequency radiation comes from the higher-frequency end of the visible spectrum (the violet end) and results in the filament appearing to be whiter.
Section 27.1 A Particle Model of Waves Radiation from Incandescent Bodies • What would you expect to see if you viewed the glowing filament through a diffraction grating? • When viewed in this way, all of the colors of the rainbow would be visible. • The bulb also emits infrared radiation that you would not see. • A plot of the intensity of the light emitted from a hot body over a range of frequencies is known as an emission spectrum.
Section 27.1 A Particle Model of Waves Radiation from Incandescent Bodies • Emission spectra of the incandescent body at temperatures of 4000 K, 5800 K, and 8000 K are shown in the figure. Note that at each temperature, there is a frequency at which the maximum amount of energy is emitted.
The Bohr Model of the Atom Quantized Energy • As shown in the figure below, the quantization of energy in atoms can be likened to a flight of stairs with decreasing-height steps. • To go up the stairs, you must move from one step to the next—it is impossible to stop at a midpoint between steps.
The Bohr Model of the Atom Energy of an Atom • The change in energy of the atom equals the energy of the emitted photon.
The Bohr Model of the Atom Energy and Electron Transitions • Some of hydrogen’s energy levels and the possible energy level transitions that it can undergo are shown in the figure at right. • Note that an excited hydrogen atom can emit electromagnetic energy in the infrared, visible, or ultraviolet range depending on the transition that occurs.
The Bohr Model of the Atom Energy and Electron Transitions • Ultraviolet light is emitted when the atom drops into its ground state from any excited state. • The four visible lines in the hydrogen spectrum are produced when the atom drops from the n = 3 or higher energy state into the n= 2 energy state.
