Ch 9 - Atomic and Nuclear Phenomena Flashcards
What is the photoelectric effect?
the ejection of an electron from the surface of a metal in response to light
- occurs when a photon of sufficiently high energy strikes an atom with a sufficiently low work function (photon with higher energy is more likely to produce the effect)
What is the threshold frequency?
the minimum light frequency necessary to eject an electron from a given metal
- depends on the chemical composition of a material (identity of metal)
What is the work function and what does its value depend on?
- the minimum energy necessary to eject an electron from a given metal
- depends on the metal used and can be calculated by multiplying the threshold frequency by Planck’s
constant
W = h x fT
How does kinetic energy and incident energy of a photon relate?
the greater the energy of the incident photon above the work function, the more kinetic energy the ejected electron can posses
What is the magnitude of the current created by the ejected electron proportional to?
the intensity of the incident beam of light
What is the Bohr model of the atom?
states that electron energy levels are stable and discreet, corresponding to specific orbits
- an electron emits or absorbs radiation only when making a transition from one energy level to another
How can electron jump from a lower energy to a higher energy orbit?
by absorbing a photon of light of the same frequency as the energy difference between the orbits
What does an electron do when it falls from a higher energy to a lower energy orbit?
it emits a photon of light of the same frequency as the energy difference between the orbits
How does molecular structure affect absorption spectra?
may be impacted by small changes in molecular structure
When does fluorescence occur?
- when a species absorbs high frequency light and then returns to its ground state in multiple steps
- each step has less energy than the absorbed light and is within the visible range of the electromagnetic spectrum
What is the nuclear binding energy?
the amount of energy that is released when nucleons (protons and neutrons) bind together
- the more binding energy per nucleon released, the more stable the nucleus
E = mc^2
What are the 4 fundamental forces of nature?
the strong and weak nuclear forces, (which contribute to the stability of the nucleus), electrostatic forces, and gravitation
What is the mass defect?
the difference between the mass of the unbonded nucleons and the mass of the bonded nucleons within the nucleus
mass defect = mass of unbound protons + mass of unbound neutrons - mass of bonded nucleons
- the amount of mass converted to energy during nuclear fission
How do unbonded and bonded constituents of the nucleons compare to each other?
the unbonded have more energy, and, therefore, more mass than the bonded constituents
What are the differences/similarities between fusion and fission?
- fusion occurs when small nuclei combine into larger nuclei
- fission occurs when a large nucleus splits into smaller nuclei
- energy is released in both fusion and fission because the nuclei formed in both processes are more stable than the starting nuclei
What is radioactive decay?
the loss of small particles from the nucleus
- in problems, first step is to balance the number of protons (the atomic number)
What is alpha decay?
the emission of an alpha particle, which is a helium nucleus
- alpha particles do not have any electrons so they carry a charge of +2
A/Z X –> A-4/Z-2 Y + 4/2alpha
What is beta-negative decay?
the decay of a neutron into a proton, with emission of an electron and an antineutrino
A/Z X –> A/Z+1 Y + beta-
neutron converted to proton, particle emitted
What is beta-positive decay?
(positive emission/decay) the decay of a proton into a neutron, with emission of a positron and a neutrino
A/Z X –> A/Z-1 Y + beta+
proton converted to neutron, particle emitted
What is gamma decay?
the emission of a gamma ray, which converts a high energy nucleus into a more stable nucleus
A/Z X* –> A/Z X + gamma
no change, only gamma ray emitted
What is electron capture?
the absorption of an electron from the inner shell that combines with a proton in the nucleus to form a neutron
(reverse beta negative decay)
A/Z X + e- –> A/Z-1 Y
particle absorbed
What is a half life?
the amount of time required for half of a sample of radioactive nuclei to decay
(1/2)^x where x = number of half lives
What is expoenential decay?
the rate at which radioactive nuclei decay is proportional to the number of nuclei that remain
n = no e^-lambda x t
delta n/ delta t = - lambda x n
n is the number of radioactive nuclei that have not yet decayed in the sample
lambda is the decay constant
no the number of decayed nuclei at t = 0
How is the energy of a photon of light calculated?
E = hf
energy of a photon increases with frequency
How is the maximum kinetic energy of an electron in the photoelectric effect calculated?
Kmax = hf - W
- light energy causes an increase in electric potential energy in the atom, enough to allow the electron to escape
- if there is any energy left over, it cannot be destroyed, so transferred into kinetic energy in the ejected electron
Why do we only talk about electrons being ejected from metals and not protons or neutrons?
because of the weak hold that metals have on their valence electrons due to their low ionization energies
What electrical phenomenon results from the application of the photoelectric effect?
the accumulation of moving electrons creates a current during the photoelectric effect
What determines the absorption spectrum of a single atom?
the energy differences between ground state electrons and higher level electrons orbits determine the frequencies of light a particular material absorbs (its absorption spectrum)
During which electronic transitions is photon emission most common?
when electrons transition from higher energy state to a lower energy state, they will experience photon emission
What is the strong nuclear force?
one of the 4 primary forces and provides the adhesive force between the nucleons within the nucleus
How many half lives are necessary for the complete decay of a radioactive sample?
- because the amount remaining is cut in half after each half life, the portion remaining will never quite reach 0
- theoretically, all of a sample is considered to have decayed after 7-8 half lives
Which type of nuclear decay could be detected in an atomic absorption spectrum?
because gamma radiation produces electromagnetic radiation (rather than nuclear fragments) it can be detected on an atomic absorption spectrum
How does intensity correlate with photoelectric effect?
- the greater the intensity, the greater the number of incident photons and, therefore the greater the number of electrons that will be ejected from the metal surface (larger current)
- intensity of light determines the number of electrons ejected per time (current)
