Transcript Nuclear Reactions

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NUCLEAR
CHEMISTRY
Why do some atoms
undergo nuclear
changes?
ATOMIC COMPOSITION
• Protons
– + electrical charge
– mass = 1.672623 x 10-24 g
– relative mass = 1.007 atomic mass units (amu)
• Electrons
– negative electrical charge
– relative mass = 0.0005 amu
• Neutrons
– no electrical charge
– mass = 1.009 amu
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Radioactivity
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• One of the pieces of evidence for the
fact that atoms are made of smaller
particles came from the work of
Marie Curie (1876-1934).
• She discovered radioactivity,
the spontaneous disintegration of
some elements into smaller pieces.
Radioactivity
Emission of particles and/or
energy due to a change in the
nucleus of an atom.
 Nuclear Radiation also called
Ionizing radiation
 Measure with Geiger Counter

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Geiger Counter:
Radiation detection
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Isotopes
• Hydrogen:
–
–
–
1
1H,
protium
2 H, deuterium
1
3 H, tritium
1
(radioactive)
• Helium, 42He
• Lithium, 63Li
and 73Li
• Boron, 105B
and 115B
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• Except for 11H the mass
number is always at least 2
x atomic number.
• Repulsive forces between
protons must be
moderated by neutrons.
Isotopes
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11B
10B
• Same element (Z) but different mass
number (A).
• Boron-10 has 5 p and 5 n:
10
• Boron-11 has 5 p and 6 n:
11
5B
5B
25.1 Nuclear Radiation >
Why do unstable isotopes
undergo nuclear reactions?
Unstable isotopes undergo nuclear
reactions so that they may be changed, or
transformed, into stable isotopes.
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Stability
of Nuclei
• Heaviest naturally
occurring non-radioactive
isotope is 209Bi with 83
protons and 126 neutrons
• There are 83 x 126 =
10,458 possible isotopes.
Why so few actually exist?
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Band of Stability
and Radioactive
Decay
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Isotopes with a n/p
ratio, below or above
the band of stability
decay, by various
nuclear reactions.
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Stability
of Nuclei
• Out of > 300 stable isotopes:
N
Even
Odd
Even
157
52
Odd
50
5
Z
19
9F
31 P
15
2
1
H, 63Li, 105B, 147N, 18073Ta
25.1 Nuclear Radiation >
Types of Radiation
Types of Radiation
What are three types of
nuclear radiation?
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25.1 Nuclear Radiation >
Types of Radiation
Radiation is emitted during
radioactive decay.
Three types of nuclear radiation are
alpha radiation, beta radiation, and
gamma radiation.
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Types of NUCLEAR Radiation
25.1 Nuclear Radiation >
Alpha Radiation
Types of Radiation
Some radioactive sources emit helium nuclei,
which are also called alpha particles.
• Each alpha particle contains two protons
and two neutrons and has a double positive
charge.
• An alpha particle is written He or a.
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2
– The electric charge is usually omitted.
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25.1 Nuclear Radiation >
Alpha Radiation
238
92
U
Uranium-238
Radioactive
decay
234
90
Th
+
Thorium-234
4
2
He (a emission)
Alpha particle
The radioisotope uranium-238 emits alpha
radiation and is transformed into another
radioisotope, thorium-234.
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25.1 Nuclear
Radiation >
Alpha
Radiation
When an atom loses an alpha particle, the
atomic number of the product is lowered by
two and its mass number is lowered by four.
238
92
U →
234
90
Th +
4
2
He
• In a balanced nuclear equation, the sum of the mass numbers
(superscripts) on the right must equal the sum on the left.
• The same is true for the atomic numbers (subscripts).
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25.1 Nuclear
Radiation >
Alpha
Radiation
Because of their large mass and charge, alpha
particles do not travel very far and are not
very penetrating.
• A sheet of paper or the surface of your skin can
stop them.
– But radioisotopes that emit alpha particles
can cause harm when ingested.
– Once inside the body, the particles don’t
have to travel far to penetrate soft tissue.
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25.1 Nuclear Radiation >
Beta Radiation
An electron resulting from the breaking
apart of a neutron in an atom is called a
beta particle.
• The neutron breaks apart into a proton, which
remains in the nucleus, and a fast-moving
electron, which is released.
1
0
n
Neutron
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→
1
1
p +
Proton
0
–1
e
Electron
(beta particle)
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25.1 Nuclear Radiation >
Beta Radiation
1
0
n
Neutron
→
1
1
p
Proton
+
0
–1
e
Electron
(beta particle)
• The –1 represents the charge on the electron.
• The 0 represents the extremely small mass of
the electron compared to the mass of a
proton.
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25.1 Nuclear Radiation >
Beta Radiation
14
6
C
→
Carbon-14
(radioactive)
14
7
N
+
Nitrogen-14
(stable)
0
–1
e (b emission)
Beta particle
• The nitrogen-14 atom has the same mass number as carbon-14,
but its atomic number has increased by 1.
• It contains an additional proton and one fewer neutron.
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25.1 Nuclear Radiation >
Beta Radiation
A beta particle has less charge than an alpha
particle and much less mass than an alpha
particle.
• Thus, beta particles are more
penetrating than alpha particles.
– Beta particles can pass through paper but are
stopped by aluminum foil or thin pieces of
wood.
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25.1 Nuclear Radiation >
Beta Radiation
Because of their opposite charges, alpha and beta radiation can be
separated by an electric field.
• Alpha particles move toward the negative plate.
• Beta particles move toward the positive plate.
• Gamma rays are not deflected.
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25.1 Nuclear Radiation >
Gamma Radiation
A high-energy photon emitted by a radioisotope
is called a gamma ray.
• The high-energy photons are a form of electromagnetic radiation.
• Gamma rays are emitted during radioactive decay.
230
90
Th
→
Thorium-230
234
90
Th
Thorium-234
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226
88
Ra +
Radium-226
→
234
91
Pa +
Protactinium234
4
2
He + g
Alpha
particle
0
–1
Gamma
ray
e + g
Beta
particle
Gamma
ray
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25.1 Nuclear Radiation >
Types of Radiation
Gamma Radiation
Gamma rays have no mass and no electrical
charge.
• Emission of gamma radiation does not alter the atomic
number or mass number of an atom.
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25.1 Nuclear Radiation >
Gamma Radiation
Gamma rays are extremely penetrating making
them dangerous.
• Gamma rays pass easily through paper, wood, and the human body.
• They can be stopped, although not completely, by several meters of
concrete or several centimeters of lead.
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Penetrating Ability
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25.1 Nuclear Radiation >
CHEMISTRY
& YOU
Gamma rays can be dangerous because
of their penetrating power. What property
determines the relative penetrating power
of electromagnetic radiation?
The wavelength and energy of electromagnetic
radiation determine its relative penetrating
power. Gamma rays have a shorter wavelength
and higher energy than X-rays or visible light.
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25.1 Nuclear Radiation >
Interpret Data
Characteristics of Some Types of Radiation
Type
Consists of
Alpha
radiation
Alpha particles
(helium nuclei)
Beta
radiation
Beta particles
(electrons)
Gamma
radiation
High-energy
electromagnetic
radiation
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Symbol
4
2
a, He
b,
g
0
–1
e
Charge
2+
1–
0
Mass
(amu)
Common
source
Penetrating
power
4
Radium226
Low
(0.05 mm
body tissue)
1/1837
Carbon14
Moderate
(4 mm body
tissue)
Cobalt-60
Very high
(penetrates
body easily)
0
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25.1 Nuclear Radiation >
Which process involves a radioactive
nucleus releasing a high-speed
electron?
A. oxidation
B. alpha emission
C
C. beta emission
D. gamma radiation
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25.1 Nuclear Radiation >
Radioactivity
How do nuclear
reactions differ from
chemical reactions?
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25.1 Nuclear Radiation >
Radioactivity
Unlike chemical reactions,
nuclear reactions are NOT
affected by changes in
temperature, pressure, or the
presence of catalysts.
Nuclear reactions of a given
radioisotope cannot be slowed
down, sped up, or stopped.
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Nuclear Reactions
• Ernest Rutherford found Ra forms Rn gas
when emitting an alpha particle.
• 1902—Rutherford and Soddy proposed
radioactivity is the result of the natural
change of the isotope of one element into
an isotope of a different element.
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Nuclear Reactions
orTransmutations


Natural Decay
Spontaneous breakdown
of unstable nuclei.

Called
Radioisotopes
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NATURAL Decay:
DECAY = Fall apart
Alpha DECAY
Beta DECAY
Positon DECAY
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25.1 Nuclear Radiation >
Radioactivity
Radioactive decay is a spontaneous process
that does not require an input of energy.
• If the product of a nuclear reaction is
unstable, it will decay too.
• The process continues until unstable
isotopes of one element are changed, into
stable isotopes of a different element.
• Stable isotopes are not radioactive.
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Nuclear Reactions
• Alpha emission
Note that mass number (A) goes down by 4 and atomic
number (Z) goes down by 2.
Nucleons = PROTON OR NEUTRON
(particle in the nucleus)
Nucleons are rearranged but conserved
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Nuclear Reactions
• Beta emission
Note that mass number (A) is unchanged and
atomic number (Z) goes up by 1.
How does this happen?
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Other Types of Nuclear Reactions
Positron (0+1b): a positive electron
207
207
K-capture: the capture of an electron from the first or
K shell
An electron and proton combine to form a neutron.
0 e + 1 p --> 1 n
-1
1
0
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Radioactive Decay Series
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Writing Nuclear Reactions

Parent element = Reactant

Daughter element = Product

Radioactivity = radiation
produced
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Writing Nuclear Reactions
Law of Conservation of Mass
 L. of C. of Charge
 Isotopic Notation
 Mass # and Nuclear Charge

14
6C
=
14
7N
+
0
-1e
42
43
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25.1 Nuclear Radiation >
Key Concepts
Unlike chemical reactions, nuclear
reactions are not affected by changes in
temperature, pressure, or the presence of
catalysts. Also, nuclear reactions of a
given radioisotope cannot be slowed
down, sped up, or stopped.
Three types of nuclear radiation are alpha
radiation, beta radiation, and gamma
radiation.
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