Post on 16-May-2018
Chapter 18: Radioactivity
& Nuclear Reactions
Radioactivity
Types of Radiation
Nuclear Reactions
Half Life
Radioactivity
The word radioactivity
was first used in the 1890s by
Marie Curie.
She used the word radioactivity to describe
the property of certain substances to give
off invisible “radiations” that could be
detected by films.
Radioactivity
We now know that
radioactivity comes from the
nucleus of the atom.
If the nucleus has too many
neutrons, or is unstable for
any other reason, the atom
undergoes radioactive
decay.
Radioactivity
Unstable atoms will continue to undergo
radioactive decay until they form stable atoms.
(Change into atoms of another element.)
The word decay means to "break down.”
Radioactive decay gives off energy.
Nuclear Reactions and Energy
A nuclear reaction is any process that changes
the nucleus of an atom.
Radioactive decay is one form of nuclear
reaction.
Types of Radiation
Scientists quickly learned that
there were 3 different kinds of
radiation given off by radioactive
materials.
The scientists called them “rays”
because the radiation carried
energy and moved in straight lines,
like light rays.
Types of Radiation In Alpha decay, the nucleus ejects two protons and
two neutrons.
Beta decay occurs when a neutron in the nucleus
splits into a proton and an electron.
Gamma decay is not truly a decay reaction in the
sense that the nucleus becomes something different.
Alpha Radiation
It decayed to an alpha
particle and U-235. This is the original
“unstable” atom.
Number of Protons Number of Protons
An alpha particle is
equivalent to a helium-4
nucleus
This is an
example
of a nuclear
equation.
Fusion Reactions
A fusion reaction is a
nuclear reaction that
combines, or fuses, two
smaller nuclei into a larger
nucleus.
It is difficult to make fusion
reactions occur because
positively charged nuclei
repel each other.
Fission reactions
A fission reaction splits up
a large nucleus into smaller
pieces.
A fission reaction typically
happens when a neutron
hits a nucleus with enough
energy to make the nucleus
unstable.
Examples of fusion and fission.
FISSION Fission is used to produce energy for nuclear bombs, but we also use fission peacefully everyday to produce energy in nuclear power plants
FUSION Fusion is the process that takes place in stars like our Sun. Whenever we feel the warmth of the Sun and see by its light, we are observing the products of fusion.
Half-Life • The “half-life” (h) is the time it takes for
half the atoms of a radioactive substance to decay.
• For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left ….
10,000 (50%)
5,000 (25%)
2,500 (12.5%)
1 hour (one lifetime) ?
2 hours (two lifetimes) ?
3 hours (three lifetimes) ?
Time #atoms
remaining
% of atoms
remaining
1. What is the percentage of radioactive
nuclei left after 3 half-lives pass?
Half-Life Practice
2. The half-life of Rn-222 is 3.823 day. If the original mass of a sample of this isotope is 0.20 g, how much of it remains after 7.646 days?
Notice how the decays are fast and furious at the beginning and slow down over time
Half-Life Graph
What is the half life of this isotope?
Decay Chain