Radioactivity : Alpha-, beta-, and gamma-Rays
The three main types of
radiation
are:-
1. large mass, positively
charged radiation (alpha a),
high speed helium nuclei, 42a,
42He,
+2
charged particle.
2. small mass, negatively
charged
radiation (beta b)
,
high speed electrons, 0-1b,
0-1e,
-1 charged
particle.
3. high energy electromagnetic
radiation (gamma g),
loss of a high energy photon from the nucleus,
no charge.
The relative penetrating
power:
a
=
1; b = 100; g = 10000.
Radioactivity:
Some heavy elements
spontaneously
emit radiation =
called
radioactive.
23392U
23592U
23892U are all
radioisotopes
of Uranium
All of these have 92 protons
and 92 electrons, and either 141,
or
143,
or 146 neutrons, respectively.
They also have different % natural abundances:
U238 is 99.3%; U235 is
0.7%
and U233 is trace.
Henri Becquerel worked with K2UO2(SO4)2.
Put it on film
and it
blackened
it.
Pierre and Marie Curie isolated
Po and Ra from pitchblende.
They
received the Nobel prize for this work in 1903.
Nuclear Equations:
Nucleons:
particles in the nucleus:
p+: proton
n0: neutron.
Mass number: the number
of p+ + n0.
Atomic number: the number
of p+, also = number of electrons
Isotopes: have the same
number of p+ and different
numbers of n0
In nuclear equations,
the
total number of nucleons is
conserved:
23892U
-----> 23490Th +
42He
radioactive decay
13153I
-----> 13154Xe + 0-1e
9943Tc
-----> X + 0-1e
Find X. 9944X,
therefore X = Ru
In the decay of
131I,
an electron is emitted. For balancing
purposes,
we assign the electron an atomic number of -1.
The total number of
nucleons
before a nuclear reaction
must be the
SAME as the total number of nucleons after
the reaction.
Types of Radioactive
Decay:
In nuclear chemistry, to
ensure conservation of nucleons,
we write all
particles with their atomic and mass numbers:
42He
and 42a
both represent alpha-radiation.
Nucleons can undergo
decay:
10n
-----> 11p + 0-1e
(beta-emission)
01e
+ 0-1e
----->
(positron annihilation)
11p
-----> 10n + 01e
(positron or beta+emission)
11p
+ 0-1e -----> 10n
(electron capture)
A positron
is a particle with the same mass as an electron
but with a
positive charge.
Patterns of Nuclear Stability:
All nuclei with 83 or more protons are radioactive.
Neutron-to-Proton Ratio
The proton has high mass
and high charge.
Therefore the
proton-proton
repulsion is large.
In the nucleus, the
protons
are very close to each other.
The cohesive forces in
the nucleus are called strong
nuclear forces. Neutrons are involved with the strong
nuclear force.
As more protons are added
(the nucleus gets heavier),
the proton-proton repulsion gets larger. Therefore, the
heavier the nucleus, the more neutrons are required for
stability.
The belt of stability
deviates
from a 1:1 neutron to proton
ratio for high atomic mass.
At Bi (83 protons), the
belt of stability ends and all nuclei
are unstable.
1. Nuclei above the
belt
of stability undergo beta- emission.
An 0-1e (beta) is ejected
from
the nucleus when a neutron
is converted into a proton. 10n
-----> 11p + 0-1e
2. Nuclei below the
belt
of stability undergo positron emission
or electron capture. Positrons are emitted when protons
are converted to neutrons. 11p
-----> 10n + 0+1e
3. Nuclei with atomic numbers
greater than 83 usually
undergo alpha-emission.
Radioactive
Disintegration
Series:
A nucleus usually
undergoes
more than one transition on its
path to stability.
The series of nuclear
reactions
accompanying this path is
the radioactive series.
Nuclei resulting from
radioactive
decay are called daughter
nuclei.
For U-238, the first
decay
is to Th-234 (alpha-decay).
The Th-234
undergoes beta- emission to Pa-234 and U-234.
U-234
undergoes
alpha-decay (several times) to Th-230,
Ra-226,
Rn-222,
Po-218, and Pb-214. Pb-214 undergoes
beta-
emission
(twice) via Bi-214 to Po-214, which
undergoes
alpha-decay to Pb-210. The Pb-210 undergoes
beta-
emission
to Bi-210 and Po-210, which decays (alpha)
to the
stable
Pb-206.