Atomic and Nuclear Phenomena Flashcards
photoelectric effect
- when a sufficiently high frequency light (usually blue-UV) is incident on a metal in a vacuum, the metal atoms emit electrons
- these freed electrons will then produce a net charge flor per unit time, a current
- greater light beam intensity (amplitude) above threshold produce larger current, greater photons fall in electrode and greater liberated electrons
threshold frequency
- fT
- depends on metal type
energy of a photon
E = hf
h: 6.626 x 10^-34 J•s
kinetic energy of ejected electrons
Kmax = hf - W
- only achieved when all possible energy from the photon is transferred to the ejected electron
Work function of an electron
W = hfT
fluorescence
- upon excitation of a fluorescent substance w UV radiation it will begin to glow in visible light
- when electrons returns to original state in 2 or more steps, each step involves less energy and thus the photon emitted has lower frequency than UV absorbed
mass defect of the nucleus
E = mc^2
mass defect is the result of…
- matter that has been converted to energy
- a very small amount of mass will yield a huge amount of energy
- 1 TJ = 10^12 J
nucleons and strong nuclear force
protons + neutrons
- attracted to one another by a strong nuclear force, compensates for the repulsive electromagnetic force between protons
- acts over extremely short distances
binding energy
- bonded system at a lower energy level than unbonded
- allows the nucleons to bind together in the nucleus
- the amount of mass that is transformed into dissipated energy will be a measurable fraction of the initial total mass
- binding energy per nucleon peaks at iron (most stable nucleus)
- intermediate sized nuclei are more stable than very large or small nuclei
weak nuclear force
- also contributes to the stability of the nucleus
- but it’s about one millionth a strong as the strong nuclear force
fusion
- small nuclei combine to form a larger nucleus
fission
- large nucleus split into smaller nuclei
- rarely occurs spontaneously
- absorption of low energy neutron, fission can be induced in certain nuclei
- of special interest are fission reactions that release more neutrons because these other neutrons will cause a chain reaction in which other nearby atoms can undergo fission; this in then releases more neutrons, continuing the chain rxn
radioactive decay
- spontaneous decay of certain nuclei accompanied by the emission of specific particles
balancing nuclear reactions…
sum if atomic numbers must be same on both sides
alpha decay
- emission of an a-particle, which is
4
2 He - do not penetrate shielding
- very massive compared to beta particle and carrie’s double the charge
beta decay
- emission of a B-particle
- an electron e-
- emitted when a neutron decays into a p+, a B-particle, and an antineutrino
(A Z) X ~> (A [Z+1]) Y + B-
- in some cases of induced decay (positron emission), positron released e+ as well as a neutrino
(A Z) X ~> (A [Z-1]) Y + B+
gamma decay
- emission of gamma rays (high freq photons)
- carry no charge
- simply lower energy of parent nucleus
(A Z) X* ~> (A Z) X + g
electron capture
- reverse of B- decay
- unstable radionuclides are capable of capturing an inner electrons that combines with a proton to form a neutron
(A Z) X + e- ~> (A [Z-1]) Y
half-life T1/2
- time it takes for half of a radioactive sample to decay
half life 4 years, 12 years later
12/4 = 3
(1/2)^3 = 1/8 of the sample remains
rate of nuclei decay
delta n/delta t = - (decay constant) • n
n: number of radioactive nuclei that have not yet decayed in a sample
exponential decay
n = n0 e ^ (- decay constant • t)
e: 2.72
exponential decay rearranged to give decay constant
decay constant = ln2/half-life = 0.693/half-life
