Ch. 9: Atomic and Nuclear Phenomena Flashcards
defn: photoelectric effect
when light of a sufficiently high frequency (typically, blue to ultraviolet light) is incident on a metal in a vacuum, the metal atoms emit electrons
defn: current (in context of photoelectric effect)
electrons liberated from the metal by the photoelectric effect will produce a net charge flow per unit time (current)
provided the light beam’s frequency is above the threshold frequency of the metal, light beams of greater intensity produce larger or smaller current?
larger!
if the light beam has higher intensity, what does that say about the number of photons per unit time that fall on an electrode, and the number of electrons per unit time liberated from the metal?
higher intensity light beam = greater # of photons falling on electrode = greater # electrons liberated from the metal
defn: threshold frequency (fT)
the minimum frequency of light that causes ejection of electrons
what does the threshold frequency depend on?
the type of metal being exposed to the radiation
the photoelectric effect is an “all-or-nothing” response, so what happens if the frequency of the incident photon is less than the threshold frequency?
the frequency of the incident photon is greater than the threshold frequency?
if the frequency of the incident photon is less than the threshold frequency, then no electron will be ejected because the photons do not have sufficient energy to dislodge the electron from its atom
if the frequency of the incident photon is greater than the threshold frequency, then an electron will be ejected, and the maximum kinetic energy of the ejected electron will be equal to the difference between hf and hfT (the work function)
defn: photons
an integral number of light quanta
do waves with higher frequency have shorter or longer wavelengths? higher or lower energy? what about waves with lower frequency?
what color/type of wave is each near?
HIGH FREQUENCY = short wavelengths = high energy = blue and ultraviolet end
LOW frequency = long wavelengths = low energy = red and infrared end
what happens to the electron if: the frequency of a photon of light incident on a metal is AT the threshold frequency? ABOVE the threshold frequency?
AT: electron barely escapes from the metal
ABOVE: photon has more than enough energy to eject a single electron, the excess energy will be converted to kinetic energy in the ejected electron
defn: work function
the minimum energy required to eject an electron
when is Kmax achieved?
when all possible energy from the photon is transferred to the ejected electron
func: infrared (IR) spectroscopy
used to determine chemical structure because different bonds will absorb different wavelengths of light
func: UV-Vis spectroscopy
takes IR spectroscopy one step further, looking at the absorption of light in the visible and ultraviolet range
what are two ways that absorption spectra can be represented?
- a color bar with peak areas of absorption represented by black lines
- a graph with the absolute absorption as a function of wavelength
what happens if one excites a fluorescent substance with ultraviolet radiation?
it will begin to glow with visible light
what are 3 examples of a fluorescent substance?
- ruby
- emerald
- the phosphors found in fluorescent lights
what happens after the electron is excited to a higher energy state by ultraviolet radiation? + char of what happens
the electron in the fluorescent substance returns to its original state in two or more steps
each step involves less energy, so at each step, a photon is emitted with a lower frequency (longer wavelength) than the absorbed ultraviolet photon
what are the criteria for the wavelength of the emitted photon to be seen as light of the particular color responding to that wavelength?
if the wavelength of the emitted photo is within the visible range of the electromagnetic spectrum
what causes the wide range of colors of fluorescent lights (whitish green office lighting to glaring neon)?
it is a result of the distinct multi-step emission spectra of different fluorescent materials
defn: mass defect
the difference between the actual mass of every nucleus and the assumed mass of the nucleus (as the sum of the masses of all the protons and neutrons within it)
what causes the mass defect?
it is a result of matter that has been converted into energy
what attracts protons and neutrons to each other?
the strong nuclear force
(this is strong enough to more than compensate for the repulsive electromagnetic force between the protons)
what is an asset and a limitation of the strong nuclear force?
it is the strongest of the four fundamental forces BUT
it only acts over extremely short distances, less than a few times the diameter of a proton or neutron (the nucleons have to get very close together in order for the strong nuclear force to hold them together)
defn: nucleon
proton or neutron
what must happen before the mass defect can become apparent? why does this happen?
the bonded system is at a lower energy level than the unbonded constituents and this difference in energy must be radiated away in the form of heat, light, or other electromagnetic radiation
defn + func: binding energy
that difference in energy between the bonded system and the unbonded constituents
allows the nucleons to bind together in the nucleus
what element contains the most stable nucleus?
iron
char + func: weak nuclear force
contributes to the stability of the nucleus
about 1 millionth the strength of the strong nuclear force
what group of forces do the strong and weak nuclear force belong to?
the four fundamental forces of nature
what are the other two of the four fundamental forces of nature?
electrostatic forces and gravitation
what are 3 types of nuclear reactions and what is one commonality between them?
- fusion
- fission
- radioactive decay
involve either the combining or splitting the nuclei of atoms
defn: isotopic notation
a way of writing elements where they are preceded by their atomic number as a subscript and mass number as a superscript
defn: atomic number vs. mass number
ATOMIC number (Z) = the number of protons in the nucleus
MASS number (A) = the number of protons plus neutrons
defn + example (2): fusion
occurs when small nuclei combine to form a larger nucleus
ex: how stars power themselves, fusion power plants
defn + char: fission
a process by which a large nucleus splits into smaller nuclei
rarely occurs spontaneously
how can fission be induced?
through the absorption of a low energy neutron
what types of fission reactions are of special interest? why?
the fission reactions that release more neutrons because these other neutrons will cause a chain reaction in which other nearby atoms can undergo fission which in turn releases more neutrons, continuing the chain reaction
this type of induced fission reactions power most commercial nuclear power plants
defn: radioactive decay
a naturally occurring spontaneous decay of certain nuclei accompanied by the emission of specific particles
what is the result when the parent nucleus x undergoes unclear decay? + equation?
forms daughter nucleus Y + emitted decay particle
A,ZX –> A’,Z’Y + emitted decay particle
what is important when balancing nuclear reactions?
the sum of the atomic numbers and the sum of the mass numbers must be the same on both sides of the equation
defn + eqn: alpha decay
the emission of an alpha-particle which is a 4,2He nucleus that consists of two protons two neutrons and zero electrons
A,ZX –> (A-4),(Z-2)Y + 4,2alpha
char (4): alpha particle
very massive compared to the beta particle and carries double the charge
interact with matter very easily –> do not penetrate shielding very extensively
defn: beta decay
the emission of a beta-particle (an electron given the symbol e- or beta-)
how are electrons emitted, since they do not reside in the nucleus?
they are emitted by the nucleus when a neutron decays into a proton, a beta-particle, and an antineutrino
why is the beta radiation from radioactive decay more penetrating than alpha radiation?
because an electron is singly charged and 1836 times lighter than a proton
defn + aka + char: induced beta decay
aka: positron emission
a positron is released, which has the same mass as an electron, but carries a positive charge (symbol e+ or beta+)
a neutrino is still emitted
process + eqn: beta- decay
a neutron is converted into a proton and a B- particle is emitted
A,ZX –> A,(Z+1)Y + B-
process + eqn: beta+ decay
a proton is converted into a neutron and a B+ particle is emitted
A,ZX –> A,(Z-1)Y + B+
defn + char + eqn: gamma decay
the emission of gamma rays (high-energy, high-frequency photons)
gamma rays carry no charge and lower the energy of the parent nucleus without changing the mass number or atomic number
A,ZX* –> A,ZX + gamma
what does the * in the gamma decay equation represent?
the high-energy state of the parent nucleus
process + char + eqn: electron capture
how can we think about this in relation to other types of radioactive decay?
certain unstable radionuclides are capable of capturing an inner electron that combines with a proton to form a neutron, while releasing a neutrino
a rare process that is best thought of as the reverse of B- decay
A,ZX + e- –> A,(Z-1)Y
defn: half-life (T0.5)
the time it takes for half of the sample to decay
in each subsequent half-life, one-half of the remaining sample decays so that the remaining amount asymptomatically approaches 0
