Ch 9 - Atomic and Nuclear Phenomena

Question 1

What is the photoelectric effect?

Answer 1

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)

Question 2

What is the threshold frequency?

Answer 2

the minimum light frequency necessary to eject an electron from a given metal
- depends on the chemical composition of a material (identity of metal)

Question 3

What is the work function and what does its value depend on?

Answer 3

• 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

Question 4

How does kinetic energy and incident energy of a photon relate?

Answer 4

the greater the energy of the incident photon above the work function, the more kinetic energy the ejected electron can posses

Question 5

What is the magnitude of the current created by the ejected electron proportional to?

Answer 5

the intensity of the incident beam of light

Question 6

What is the Bohr model of the atom?

Answer 6

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

Question 7

How can electron jump from a lower energy to a higher energy orbit?

Answer 7

by absorbing a photon of light of the same frequency as the energy difference between the orbits

Question 8

What does an electron do when it falls from a higher energy to a lower energy orbit?

Answer 8

it emits a photon of light of the same frequency as the energy difference between the orbits

Question 9

How does molecular structure affect absorption spectra?

Answer 9

may be impacted by small changes in molecular structure

Question 10

When does fluorescence occur?

Answer 10

• 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

Question 11

What is the nuclear binding energy?

Answer 11

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

Question 12

What are the 4 fundamental forces of nature?

Answer 12

the strong and weak nuclear forces, (which contribute to the stability of the nucleus), electrostatic forces, and gravitation

Question 13

What is the mass defect?

Answer 13

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

Question 14

How do unbonded and bonded constituents of the nucleons compare to each other?

Answer 14

the unbonded have more energy, and, therefore, more mass than the bonded constituents

Question 15

What are the differences/similarities between fusion and fission?

Answer 15

• 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

Question 16

What is radioactive decay?

Answer 16

the loss of small particles from the nucleus

- in problems, first step is to balance the number of protons (the atomic number)

Question 17

What is alpha decay?

Answer 17

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

Question 18

What is beta-negative decay?

Answer 18

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

Question 19

What is beta-positive decay?

Answer 19

(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

Question 20

What is gamma decay?

Answer 20

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

Question 21

What is electron capture?

Answer 21

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

Question 22

What is a half life?

Answer 22

the amount of time required for half of a sample of radioactive nuclei to decay
(1/2)^x where x = number of half lives

Question 23

What is expoenential decay?

Answer 23

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

Question 24

How is the energy of a photon of light calculated?

Answer 24

E = hf

energy of a photon increases with frequency

Question 25

How is the maximum kinetic energy of an electron in the photoelectric effect calculated?

Answer 25

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

Question 26

Why do we only talk about electrons being ejected from metals and not protons or neutrons?

Answer 26

because of the weak hold that metals have on their valence electrons due to their low ionization energies

Question 27

What electrical phenomenon results from the application of the photoelectric effect?

Answer 27

the accumulation of moving electrons creates a current during the photoelectric effect

Question 28

What determines the absorption spectrum of a single atom?

Answer 28

the energy differences between ground state electrons and higher level electrons orbits determine the frequencies of light a particular material absorbs (its absorption spectrum)

Question 29

During which electronic transitions is photon emission most common?

Answer 29

when electrons transition from higher energy state to a lower energy state, they will experience photon emission

Question 30

What is the strong nuclear force?

Answer 30

one of the 4 primary forces and provides the adhesive force between the nucleons within the nucleus

Question 31

How many half lives are necessary for the complete decay of a radioactive sample?

Answer 31

• 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

Question 32

Which type of nuclear decay could be detected in an atomic absorption spectrum?

Answer 32

because gamma radiation produces electromagnetic radiation (rather than nuclear fragments) it can be detected on an atomic absorption spectrum

Question 33

How does intensity correlate with photoelectric effect?

Answer 33

• 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)