Sec. 18.2 Notes—Nuclear Charge
Objectives:
1. Predict the particles and electromagnetic waves
produced by different types of radioactive decay, and
write equations for nuclear decays.
2. Identify examples of nuclear fission, and describe
potential benefits and hazards of its use.
3. Describe nuclear fusion and its potential as an energy
source.
Vocabulary: radioactivity, beta particle, gamma ray, nuclear
fission, chain reaction, critical mass, nuclear fusion
Radioactive Decay
Radioactivity is the process by which an unstable nucleus
will spontaneously break apart and emit one or more
particles and energy in the form of electromagnetic radiation.
Radioactivity is also referred to as radioactive decay.
Stabilizing Nuclei by Converting Neutrons into Protons
If a particular isotope has too many neutrons (large N/Z
ratio), the nucleus will decay and emit radiation. A neutron
may decay and emit a high-energy electron called a beta
particle (β particle).
1
0𝑛
→
1
+1𝑝
+
0
−1𝑒
or
1
0𝑛
→
1
+1𝑝
+ 𝛽
Because this process changes a neutron into a proton, the
atomic number of the nucleus increases by one and the
product becomes a different element. The mass number does
not change.
14
6𝐶
→
14
7𝑁
+
0
−1𝑒
Stabilizing Nuclei by converting Protons into Neutrons
An element that is unstable due to too many protons can
become more stable by converting protons into neutrons
through a process called electron capture. In this process
gamma rays (Greek symbol γ) are released.
1
+1𝑝
+
0
−1𝑒
→ 10𝑛 + 𝛾
In this process, the unstable nucleus loses a proton and
becomes the element with the next lower atomic number.
The mass number does not change.
51
24𝐶𝑟
+
0
−1𝑒
→
51
23𝑉
+ 𝛾
Stabilizing Nuclei by Positron Emission
1
+1𝑝
→ 10𝑛 +
0
+1𝑒
When a proton changes into a neutron and emits a positron,
the atomic number decreases by one but the mass number
remains the same.
51
24𝐶𝑟
→
49
23𝑉
+
0
+1𝑒
The positron is the opposite or antiparticle of an electron.
Once created, a positron will instantly collide with an
electron, in a process called annihilation of matter, to create
gamma ray.
0
+1𝑒
+
0
−1𝑒
→ 2𝛾
Gamma rays from electron-positron annihilation have
characteristic wavelength that can be used to identify nuclei
that decay by positron emission. Such gamma radiation has
been detected coming from the center of the Milky Way
galaxy.
Stabilizing Nuclei by Losing Alpha Particles
An unstable nucleus that has an N/Z number that is much
larger than 1 can decay by emitting an alpha particle.
238
92𝑈
→
234
90𝑇ℎ
+ 42𝐻𝑒
No elements with atomic numbers greater than 83 or mass
numbers greater than 209 are stable, and many of these
unstable become stable through alpha decay.
Many heavy nuclei go through a series of reactions called a
series before they reach a stable state.
Uranium-238 decays to lead-206 through a decay series that
includes alpha and beta decay. Each isotope in the decay
series has a specific half-life that determines how long it
exists before it decays to something else. Eventually,
uranium-238 decays to the stable isotope lead-206 in a
process that takes millions of years.
Notice that when writing nuclear equations the sum of the
mass numbers (superscripts) on one side always equals the
sum on the other side. The same holds true for the atomic
numbers (subscripts). Remember that, when the atomic
number changes, the identity of the element changes.
Nuclear Fission
Radioactivity can be of two types:
natural radioactivity (occurring in
nature) or induced or artificial
radioactivity (brought about by
particle bombardment). The
reactions studied so far in which
the nucleus decays by adding or losing particles are natural
radioactivity. Nuclear fission occurs when the nucleus splits
into two or more fragments in a process that produces
additional neutrons and a lot of energy. Most fission
reactions happen artificially or are induced by bombarding
nuclei with neutrons.
The following equation and figure show what happens when
a uranium-235 isotope is bombarded with a neutron. The
reaction produces two or three neutrons. Each of these
neutrons can then cause the fission of another uranium-235
nucleus. If this is allowed to continue, a process called a
chain reaction can occur. A characteristic of a chain reaction
is that the particle that starts the reaction—in this case a
neutron—is also produced from the reaction. A minimum
quantity of radioactive material, called a critical mass, is
needed to keep the chain reaction going.
235
92𝑈
→
93
36𝐾𝑟
+
140
56𝐵𝑎
+ 3 10𝑛
Chain reactions Occur in Nuclear Reactors
Fission reactions are used to generate electrical energy in
nuclear power plants. Fission reactions produce a lot of
energy: 1 gram of uranium-235 can generate as much energy
as the combustion of 2700 kg of coal. Uranium-235 and
plutonium-239 are the main radioactive isotopes used in
these reactors.
In a nuclear reactor core, the fuel rods are surrounded by a
moderator, a substance that slows down neutrons. Control
rods. Made from cadmium, the control rods are used to
adjust the rate of the chain reactions. They can absorb some
of the free neutrons produced by fission, and moving them in
or out of the core can slow or speed up the reaction. As
shown in the figure, water is heated by the energy released
from the controlled fission. This water turns to steam and the
pressurized steam drives a turbine to produce electrical
energy. Therefore, a nuclear power plant is very similar to a
coal fired power plant. All that is different is the source of
heat used to heat the water.
Nuclear Fusion
Nuclear fusion occurs when small nuclei combine, or fuse, to
form a larger, more stable nucleus. In the process, mass is
converted into a large amount of energy is release as the new
nucleus forms. Fusion releases greater amounts of energy
than fission.
Fusion powers stars, like our Sun. Four hydrogen nuclei fuse
to form a single helium-4 nucleus.
4 11𝐻 → 42𝐻𝑒 + 2 +10𝑒 + 𝑒𝑛𝑒𝑟𝑔𝑦
Very high temperatures are needed to bring the nuclei
together. The temperature of the sun’s core, where some of
the fusion occurs, is about 1.5 x 107 oC (or 15 million
degrees centigrade).