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Radioactivity and
Nuclear Reactions
Ch 9.1-9.2, 9.4
Nucleus and the Strong Force
Protons and neutrons are packed tightly together
Two positives normally repel each other, so why don’t the
protons in the nucleus repel?
Strong force = one of four basic forces that causes protons
and neutrons to be attracted to each other
100 times stronger than electric force
Short-range force, so it weakens with distance
Small vs Large Nuclei
Protons and neutrons are held together less tightly in
large nuclei. Why?
Small nuclei have few protons, so the repulsive force
on a proton due to other protons is small
In a large nuclei, the attractive strong force is
exerted only by the nearest neighbors. All the
protons exert repulsive forces making the repulsive
force large.
Radioactivity
In many nuclei, the strong force keeps the nucleus
together (STABLE)
When it can’t, the nucleus can decay and give off
matter and energy in a process of radioactivity
Larger nuclei tend to be unstable – all nuclei
containing more than 83 protons are radioactive
All elements with more than 92 protons are synthetic
and decay soon after they are created (UNSTABLE)
Stable and Unstable Nuclei
Smaller elements neutron to proton ratio is 1:1 to be
stable isotopes
Heavier elements neutron to proton ratio is 3:2 to be
stable isotopes
Nuclei of any isotopes that differ much from these
ratios are unstable, whether heavy or light
Nuclear Radiation
When an unstable nucleus decays, particles and
energy are emitted from the decaying nucleus
Alpha Particles – (2 p and 2 n lost) massive,
comparatively speaking; loses energy quickly; can’t
pass through paper; changes the element
(transmutation); mass changes; can damage the body
Beta Particles – (n turns into p and emits e) e emitted
from n; transmutation changes the element; mass
doesn’t change; much faster and penetrating; damage
body
Gamma Rays – electromagnetic waves that carry
energy; most penetrating form; cause less damage to
biological molecules
At a glance…
Radioactive
Half-Life
Some radioisotopes decay in less than a second,
while others take millions of years
Half-life: the amount of time it takes for half the
nuclei in a sample of the isotope to decay
Radioactive Half-Life cont
Ch 21.3: Absolute-Age
Dating of Rocks
Relative-age dating vs. Absolute-Age Dating
Relative-age dating: compares past geologic events
based on the observed order of strata in rock record
Absolute-age dating: determines actual age of a rock,
fossil, or other object
Radioactive Decay
Radioisotopes are found in igneous and metamorphic
rocks, some fossils, and organic remains
Emission of radioactive particles and the resulting
change into other elements over time is called
radioactive decay
This decay stays constant regardless of the
environment, pressure, temperature, or any other
physical changes
So, these atomic particles become accurate
indicators of the absolute age of an object
I love you Mrs.
Sjuts! 
Radioactive Dating
Fossils and rocks can be dating using radioactive
isotopes
Amounts of the radioisotope and its daughter
nucleus are measured in a sample
Then, the number of half-lives that need to pass to
give the measured amounts of the isotope are
calculated
The number of half-lives is the amount of time that
has passed since the isotope began to decay AND
usually is the same as the age of the object.
Carbon Dating
The radioactive isotope C-14 is often used to find the
ages of once living objects
It is naturally found in most all living things
An atom of C-14 eventually will decay into N-14 with a
half-life of 5,730 years
By measuring the amount of C-14 in a sample and
comparing it to the amount of C-12, scientists can
determine the approx age of plants and animals that
lived within the last 50,000 years
Uranium Dating
Some rocks contain uranium, which has two
radioactive isotopes with long half-lives, both
decaying into isotopes of lead
By comparing the uranium isotope and the daughter
nuclei the number of half-lives since the rock was
formed can be calculated
U-235  0.7 billion years
U-238  4.5 billion years
Ch 9.4 Nuclear
Reactions
Nuclear Fission – the process of splitting a nucleus
into two nuclei with smaller masses
Chain reaction – ongoing series of fission reactions
Critical mass – the amount of fissionable material
required so that each fission reaction produce
approximately one more fission reaction
Nuclear Fusion – two nuclei with low masses are
combined to form one nucleus of larger mass
Nuclear Fission
Large elements need a TON of energy in order to hold their
nucleus together.
When the large nucleus is split into smaller nuclei, those smaller
nuclei don’t require as much energy to stay together…
So, that leftover energy is released!
Atomic bomb – used in Hiroshima and Nagasaki
Fission - Chain Reaction
Nuclear Fission: Pros and Cons
Nuclear Meltdown
Cooper Nuclear Station near Brownville, NE
Fort Calhoun Nuclear Generating System between Ft. Calhoun and
Blair
Nuclear Fusion
Need very high temperatures in order to overcome
the repulsive forces. Sun's Fusion
Scientists cannot control fusion reactions for the
purpose of power.
We can, however, use it to make nuclear weapons.
Large ones. Hydorgen Bomb - Fusion
Nuclear Decay vs.
Nuclear Reactions
Decay happens spontaneously
Reactions are controlled and self-sustaining and
release much more energy
Nuclear Reaction:
Plutonium
Pu-239 Used to make nuclear weapons like the one
dropped on Nagasaki in 1945