Unit 9: Nuclear Chemistry
Key Terms
Radioactivity
radioactivity
Rate of Nuclear Decay
half-life
Fission and Fusion
strong nuclear force
radioisotope
fission
nuclear radiation
chain reaction
alpha particle
critical mass
beta particle
fusion
gamma ray
plasma
background radiation
Essential Questions

What is nuclear fission? What is nuclear fusion?
Strong nuclear force: the attractive force between protons and neutrons. About 100x greater than the electric
force that repels protons it quickly weakens as protons and neutrons get further apart.
A nucleus becomes unstable, or radioactive, when the strong nuclear force can no longer overcome the
repulsive electric forces on the protons. All nuclei with more than 83 protons are radioactive.
fission – the splitting of an atomic nucleus into two smaller parts.
fusion – the process by which the nuclei of two atoms combine to form a larger nucleus.

What is alpha, beta and gamma radiation?
Radioactivity is the process in which an unstable atomic nucleus emits charged particles and energy.
These particles are referred to as Nuclear Radiation:
Alpha – (  )positively charges particle made up of two protons and two neutrons (same as a He nucleus)
- mass # on left (238) = sum of mass # on right (234+4)
- atomic # on left (92) = sum of atomic # on right (90+2)
- least penetrating type of nuclear radiation
- travel no more than a few centimeters in air
- can be stopped by a sheet of paper or clothing
Beta – (  ) an electron emitted by an unstable nucleus.
Why would a nucleus emit an electron? ANS: during Beta Decay, a neutron decomposes into a
proton and an electron. The proton stays trapped in the nucleus, while the electron is
released.
- the electron emitted has a very small mass (essentially 0). Dues to its small mass and
high speed, beta particles penetrate further than alpha.
- Can be stopped by a thin sheet of metal.
Gamma – (  ) a penetrating ray of energy emitted by an unstable nucleus.
- has no moss and no charge. Like X-rays and visual light, gamma rays are energy waves that travel
through space at the speed of light.
- during gamma decay, the atomic number and mass number stay the same, but the energy of the
nucleus decreases.
- it often accompanies alpha or beta decay.
- it can take several centimeters of lead or several meters of concrete to stop gamma radiation.

What happens in a radioactive decay?
When the nucleus of an atom possesses either too many or too few neutrons compared to the number of
protons it becomes unstable. These are called radioactive isotopes. Unstable nuclei split up in a process called
radioactive decay and emit radioactive radiation.

How are nuclear reactions balanced?
(example on page 295)

What is the difference between a chemical reaction and a nuclear reaction?
Chemical reaction – atoms are rearranged in a chemical reaction and the same number of each type of
atom are present before and after the reaction.
Nuclear Reaction – the nucleus changes during a nuclear reaction and that nuclear reactions convert a
small amount of mass to a huge amount of energy.

How can radioactive half-life be used to date fossils?
EXAPMLE OF SCIENCE CHANGING: it was thought that early Americans were people from Siberia from the
Bearing Straight about 13,000 years ago. In the 1990’s, an archeological site discovered tools, charcoal, and
animal bones that were at least 15,000 yrs old. Some were 17,000.
- Because most materials contain at least trace amounts of radioisotopes, scientists can estimate how
old they are based on rates of nuclear decay.
Every radioisotope decays at a specific rate that can be expressed as a “half-life”.
Half-Life : the time required for one half of a sample of a radioisotope to decay.

What is a molecular clock? How are molecular clocks used to show evolutionary relationships?
The molecular clock (based on the molecular clock hypothesis (MCH)) is a technique in molecular
evolution that uses fossil constraints and rates of molecular change to deduce the time in geologic
history when two species or other taxa diverged. It is used to estimate the time of occurrence of events
called speciation or radiation. The molecular data used for such calculations are usually nucleotide
sequences for DNA or amino acid sequences for proteins. It is sometimes called a gene clock or
evolutionary clock.
Radioactive Dating :
- Carbon reacts with oxygen in the atmosphere to make carbondioxide.
- plants absorb carbon dioxide during photosynthesis.
- when a plant or animal dies, it no longer absorbs Carbon.
- from this point on, the organisms carbon-14 levels decrease as the radioactive carbon decays.
- living organisms maintain the same ration of Carbon-14 to carbon-12 as in the
atmosphere.
- scientist compare the amount of Carbon–14 in a sample to the amount in an item with a
known age (tree age, they have rings for years)
- can be used to date any carbon-containing objects less than 50,000 yrs old.
- older objects contain too little carbon-14 to be measurable.
DATING OLDER OBJECTS:
- scientist measure the amount of radioisotope with linger half-lives than carbon-14.
- Geologists use potassium-40, uranium-235, and uranium-238 to date rock formations.
- the older the rock, the lower are the levels of the radioisotopes present.
Carbon-14
5,730 years
Uranium-238 4,470,000,000 years
Potassium-40 1,280,000,000 years