Nuclear Chemistry Flashcards
From alpha decay to half-life calculations, use these cards to master the topic of nuclear phenomena as tested in most introductory undergrad chemistry courses and even on the AP Chemistry exam.
Define:
nuclear force
The nuclear force holds protons and neutrons together in the nucleus.
In very large molecules, or ones with an over-abundance of neutrons, the nuclei can become unstable and decay.
Define:
alpha particle and alpha decay
An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons (without the electrons).
Alpha decay is a nuclear decay and occurs when a nucleus emits an alpha particle. The daughter nucleus of alpha decay will have an atomic number 2 less and mass number 4 less than the parent nucleus.
What will the daughter atom be when uranium 238 undergoes a single alpha decay?
23490Th
In single alpha decay, an alpha particle is emitted. To identify the daughter nucleus, therefore, simply subtract 2 from the parent nucleus’ atomic number, and 4 from its mass. Subtracting 2 protons from uranium (92) shifts it to thorium (90).
Under what conditions would alpha decay be an isotope’s preferred form of radioactive decay?
Alpha decay is typical only in large nuclei (atomic number 60 or greater).
Many of the most famous radioactive elements, such as radium (88), uranium (92), and plutonium (94), decay primarily via alpha decay.
Define:
beta particle and beta decay
A beta particle is simply an electron, a massless negatively-charged particle.
Beta decay is a nuclear decay and occurs when a nucleus emits a beta particle. The daughter nucleus of beta decay will have an atomic number 1 more* and mass number identical to the parent nucleus.
*When the electron is ejected, total charge must remain constant, so a neutron is converted to a new proton.
What will the daughter atom be when carbon 14 undergoes a single beta decay?
147N
In single beta decay, a beta particle (electron) is emitted. To identify the daughter nucleus, therefore, simply add 1 to the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one larger than carbon is nitrogen.
Under what conditions would beta decay be an isotope’s preferred form of radioactive decay?
Beta decay is typical in smaller nuclei, when the isotope’s mass number is greater than the element’s atomic weight.
Ex: The carbon 16 isotope, which is heavier than carbon’s atomic weight of 12.011, decays via beta decay.
Define:
positron and positron decay
A positron is an anti-electron, a massless positively-charged particle.
Positron decay is a nuclear decay and occurs when a nucleus emits a positron. The daughter nucleus of positron decay will have an atomic number 1 less* and mass number identical to the parent nucleus.
*When the positron is ejected, total charge must remain constant, so a proton is converted to a new neutron.
What will the daughter atom be when carbon 11 undergoes positron decay?
115B
In positron decay, a positron is emitted. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with an atomic number one less than carbon is boron.
Define:
electron capture
Electron capture is a nuclear decay and occurs when a nucleus captures one of its own electrons. The electron merges with a proton to form a neutron. The daughter nucleus of electron capture will have an atomic number 1 less and mass number identical to the parent nucleus.
What will the daughter atom be when beryllium 7 decays via electron capture?
73Li
In electron capture, an electron merges with a proton, forming a neutron. To identify the daughter nucleus, therefore, simply subtract 1 from the parent nucleus’ atomic number, without changing its mass. The element with atomic number one less than beryllium is lithium.
Under what conditions would positron emission or electron capture be an isotope’s preferred form of radioactive decay?
Both positron emission and electron capture are typical when the isotope’s mass number is less than the element’s atomic weight or when there is a superabundance of protons.
Ex: The carbon 11 isotope, which is lighter than carbon’s atomic weight of 12.011, decays via positron emission.
Define:
gamma ray and gamma emission
A gamma ray is a high energy photon, a massless and uncharged particle.
Gamma emission is a nuclear decay and occurs when a nucleus emits a gamma ray. The daughter nucleus of gamma emission is simply lower energy, hence will have identical atomic and mass numbers to the parent nucleus.
Define:
half-life of a radioactive isotope
A radioactive isotope’s half-life is the time it takes for exactly one-half of the individual atoms in a population of the isotope to undergo radioactive decay.
After one half-life elapses, exactly one half of the original amount of the parent nuclei will still be present; the remainder will have decayed into the daughter nuclei.
If radioactive isotope X has a half-life of 1 hour, and 3 hours elapse, what proportion of the original amount of X will remain?
One-eighth of the original amount will remain after 3 hours.
For each half-life that elapses, one half of the remaining isotopes will decay. So after one hour, 1/2 the original population remains. During the second hour, 1/2 of the remaining half (1/4 more) decays, leaving behind one quarter of the original population. In the third hour, 1/2 of the remaining quarter (1/8 more) decays, leaving behind one eighth of the original.
