Radioactive Decay
What is Radioactivity?
Quick Review
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Nucleus contains protons
and neutrons
Electron circles the
nucleus in orbits
Proton: +1 charge,
mass number (MN) of 1
Neutron: 0 charge,
MN of 1
Electron: -1 charge,
MN nearly zero
Standard Atomic Notation (SAN)
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Mass Number
9
5
B
Atomic Symbol
Atomic Number (number of protons)
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SAN tells us all we
need to know
about an atom
For boron,
# of protons = 5
# of electrons = 5
Mass number = 9
# neutrons = 4
Try these…
Use your periodic table to state the following for each
atom:
 Atomic number
 # of protons
 # of electrons
 rounded atomic mass
 # of neutrons
 Standard Atomic Notation
1) barium
2) uranium
3) chlorine
Answers
Barium
Uranium
Chlorine
Atomic number
56
92
17
# of protons
56
92
17
# of electrons
56
92
17
Mass number
137
238
35
# of neutrons
81
146
18
Standard
Atomic Notation
137
56
Ba
238
92
U
35
17
Cl
What about these?
16
8
O
and
17
8
O
and
18
8
O
All three are forms of the oxygen atom found in nature but each
have different numbers of neutrons.
16
8
17
8
18
8
O
has 8 protons and 8 neutrons
O
has 8 protons but 9 neutrons
O
has 8 protons and 10 neutrons
Isotopes can also be
18
written as Oxygen-18 for 8
O
They are isotopes
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Isotopes are the same atoms with different mass numbers
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The different masses in the three oxygen isotopes are from
extra neutrons in the nucleus
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Isotopes appear in all the same compounds as the element
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Isotopes have different but similar properties, e.g., ordinary
water (water with
atoms) boils at 1000 C but heavy water
(water with
atoms) boils at 101.420 C
A new type of reaction
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Isotopes mean that the nucleus has changed (added
neutrons)
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This is new because in chemical reactions the nuclei of the
reactants and products remain unchanged
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A reaction that changes the nucleus of an atom is called a
nuclear reaction
Unstable nuclei
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Elements with atomic number <83 [except for technectium
(43) and promethium (61)] have one or more stable isotopes
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All elements above atomic number 83 have isotopes with
unstable nuclei
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The nuclei of these elements spontaneously disintegrate
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The spontaneous disintegration of a nucleus is called
radioactivity
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The unstable nucleus that disintegrates is said to be
radioactive
Nuclear disintegration
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When a nucleus disintegrates it can emit:
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alpha particles
beta particles (electrons or positrons)
gamma rays
energy
The new nucleus that remains may be radioactive or stable
Alpha particles
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Given the Greek symbol for alpha,  (first letter of the
Greek alphabet)
Fast moving, high energy
Relatively heavy, loses energy quickly
Stopped by a piece of paper
Travel 2.5 cm in air
Penetrate skin only 0.3 mm
Consist of 2 protons and 2 neutrons
Rutherford found an alpha particle to be a helium atom,
Alpha Decay
When an unstable nucleus
emits an alpha particle it
undergoes alpha decay
• The resulting new nucleus is
an isotope
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Example: Uranium-238
undergoes alpha decay
Uranium-238 decays to make a
new isotope - thorium and an
alpha particle
U
238
92
234
90
Th
4

2
He
Alpha particle
* Notice the conservation of mass holds (no particles are destroyed).
Nuclear Equation
Beta particles
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Given the Greek symbol for beta, β (second letter of the
Greek alphabet)
Very fast
Very light
Stopped by 0.1 mm of lead
Can travel 4.5 m in air
Penetrate skin 17 mm
Beta particles may be positively or negatively charged
Beta Decay
Two types of beta decay are known:
β – Decay
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Neutron in the nucleus is converted into a proton
Atomic number increases by 1 (new element)
During this conversion an electron and an antineutrino are
ejected from the nucleus
Also known as electron emission
An example of β- decay is:
234
90
Th
234
91
Pa
0
 -1
e
__

v
Electron Antineutrino
Beta Decay
β + Decay (positron emission)
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Proton in the nucleus is converted into a neutron
Atomic number decreases by 1 (new element)
During this conversion a positron and a neutrino are ejected
from the nucleus
Also known as positron emission
An example of β+ decay is:
22
11
Na
22
10
Ne
0

1
e

Positron
v
Neutrino
Gamma Rays
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Given the Greek symbol for gamma, γ (third letter of the Greek
alphabet)
Type of ionizing electromagnetic radiation
Most penetrating form
Highest frequency and shortest wavelength in the electromagnetic
spectrum
Produced when the nucleus of an atom is in an excited state and
then releases energy, becoming more stable
When a nucleus emits an  or β particle, the daughter nucleus is
sometimes left in an excited state. It can then jump down to a
lower level by emitting a gamma ray
Nuclear Reaction Equations
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Nuclear reactions are just like regular chemical reactions
Mass is conserved
Nuclear reactions show the starting material, the type of radiation
given off and the end products
Example:
Radioactive atom
U
new radioactive element
238
234
92
90
Uranium atom
234
90
Th
Thorium atom
Th

4

New isotope
234
Pa
91
New isotope
particle emitted
2
He
Alpha particle

0
e
-1
Electron