Alpha and Beta Decay
Nuclear Reactions
1.
2.
3.
4.
5.
Occur when nuclei emit particles and/or rays.
Atoms are often converted into atoms of another element.
May involve protons, neutrons, and electrons
Associated with large energy changes.
Reaction rate is not normally affected by temperature, pressure,
or catalysts.
PROPERTIES OF RADIATION
1.
2.
3.
Alpha ()
4 He, helium nuclei

2

Blocked by paper; 6.64 x 10-24 kg

Slow moving due to mass and charge!
Beta ()
0  or 0 e, electrons

-1
-1

Blocked by metal foil; 9.11 x 10-28 kg

Fast moving

Emitted from a neutron of an unstable nucleus

Insignificant mass compared with mass of nucleus

Greater penetrating power than alpha particles
Gamma ()
0  , photons

0

Not completely blocked by lead or concrete; 0 kg

High energy electromagnetic radiation

Almost always accompanies alpha and beta radiation
Comparison of Chemical and Nuclear Reactions
Chemical Reactions
Occur when bonds are
broken or formed
Involve only valence
electrons
Associated with small
energy changes
Nuclear Reactions
Occur when nuclei
combine, split, & emit
radiation
Can involve protons,
neutrons, & electrons
Associated with large
energy changes
Atoms keeps same identity
Atoms of one element are
although they may gain,
often converted into atoms
lose, or share electrons,
of another element
and form new substances
Temperature, pressure,
concentration, and
catalysts affect reaction
rates
Temperature, pressure,
and catalysts do not
normally affect reaction
rates
Radioactivity
• Radioisotopes are isotopes that have an unstable
nucleus. They emit radiation to attain more stable
atomic configurations in a process called radioactive
decay.
• Radioactivity is the property by which an atomic nucleus
gives off alpha, beta, or gamma radiation.
• Marie Curie named the process.
• In 1898, Marie & Pierre Curie identified 2 new elements,
polonium & radium.
• The penetrating rays and particles emitted by a
radioactive source are called radiation.
Radioactivity (cont)
• The presence of too many or too few neutrons, relative
to the number of protons, leads to an unstable nucleus.
• The types of radiation are alpha (α), beta (β), or gamma
(γ).
• An unstable nucleus loses energy by emitting radiation
during the process of radioactive decay.
• Spontaneous and does not require any input of energy.
The Nucleus
• Remember that the nucleus is comprised of
protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together is
the mass of the atom.
Isotopes
• Not all atoms of the same element have the same
mass due to different numbers of neutrons in
those atoms.
• There are three naturally occurring isotopes of
uranium:
• Uranium-234
• Uranium-235
• Uranium-238
Stable Nuclei
• The shaded region in the
fig. shows what nuclides
would be stable, the socalled belt of stability.
• It is the ratio of neutrons
to protons that determines
the stability of a given
nucleus.
Predicting the mode of radioactive decay.
In general:
•neutron-rich nuclei tend to emit beta particles
•proton-rich nuclei tend to either emit positrons or
undergo electron capture
•heavy nuclei tend to emit alpha particles.
•The presence of magic numbers of nucleons and an even number of protons
and neutrons also help determine the stability of a nucleus.
Radioactive Series
• Large radioactive nuclei
cannot stabilize by
undergoing only one
nuclear transformation.
• They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead).
• Transmutation = the
reaction by which the
atomic nucleus of one
element is changed into the
nucleus of a different
element
Nuclear Equations
• For a nuclear reaction to be balanced, the sum of all the
atomic numbers and mass numbers on the right must
equal the sum of those numbers on the left.
• To figure out the unknown isotope, you need to balance
the equation.
Example
Natural Radioactive Decay
• Why
• The nucleus has many positively charged protons that are
repelling each other.
• The forces that hold the nucleus together can’t do its job and
the nucleus breaks apart.
• All elements with 84 or more protons are unstable and will
eventually undergo nuclear decay.
• How
•
•
•
•
•
Alpha particle emission
Beta particle emission
Gamma radiation emission
Positron emission (less common)
Electron capture (less common)
Alpha radiation
• A type of radiation called alpha radiation consists of
helium nuclei that have been emitted from a
radioactive source.
• These emitted particles, called alpha particles, contain
2 protons and 2 neutrons and have a double positive
charge.
Alpha Radiation (cont)
• Because of their large mass and charge, alpha particles
do not tend to travel very far and are not very
penetrating.
• They are easily stopped by a piece of paper or the surface of
skin.
• Radioisotopes that emit alpha particles are dangerous when
ingested.
Alpha Decay
= Loss of an -particle (a helium nucleus)
4
2
238
92
U
Atomic # decreases by 2
Mass # decreases by 4
# of protons decreases by 2
# of neutrons decreases by 2

He
234
90
4
2
Th + He
Alpha radiation occurs when an unstable nucleus emits a particle composed of 2 protons
and 2 neutrons. The atom giving up the alpha particle has its atomic number reduced by
two. Of course, this results in the atom becoming a different element. For example, Rn
undergoes alpha decay to Po.
Beta Particles
• A beta particle is essentially an electron that’s emitted
from the nucleus.
• A neutron is converted (decayed) into a proton &
electron…so the atomic number increases by 1 and the
electron leaves the nucleus.
• Isotopes with a high neutron/proton ratio often
undergo beta emission, because this decay allows the #
of neutrons to be decreased by one & the # of protons
to be increased by one, thus lowering the
neutron/proton ratio.
Beta Decay
= Loss of a -particle (a high energy electron)
0
−1
131
53
Atomic # increases by 1
# of protons increases by 1
# of neutrons decreases by 1
Mass # remains the same
I


0
−1
or
131
54
e
Xe
+
0
−1
e
Beta radiation occurs when an unstable nucleus emits an electron.
As the emission occurs, a neutron turns into a proton.