Chapter 21: Nuclear Chemistry
Review from Ch 2:
Nucleons: particles that reside in
nucleus
Neutrons
Protons
All atoms of a given element have the
same # of protons - this is the atomic
number
Isotopes: atoms of a given element with
different mass numbers (# of neutrons);
mass # = total # of nucleons
e.g.,
238
92 U
superscript: mass number
subscript: atomic number
- how many neutrons does 238
92 U have?
Isotopes that are radioactive: radioisotopes
239
Nuclear Reactions
Most nuclei found in nature are stable and remain
intact indefinitely
Radionuclides (or radioisotopes) are unstable
Spontaneously emit particles and electromagnetic
radiation
This can transform the unstable nucleus into a
stable one - the emitted radiation carries off extra
energy
e.g., decay of uranium-238 by spontaneous emission of particles:
-particle: 42 He
The nuclear equation for this process is
238
4
234
92 U2 He  90Th
Note that both mass number and atomic number are
conserved on both sides of the equation
When a nucleus spontaneously decomposes in this way, it
is said to have undergone radioactive decay
Note that radioactive properties are independent of the
state of chemical combination of an atom – we are not
concerned with whether the atom is in the form of an
element or in a compound
240
Types of Radioactive Decay
Unstable nuclei may be transformed into more stable ones
by emission of radiation in the form of:
Alpha (particles
4
2 He
nuclei
small mass (~10-24g)
not very penetrating
e.g. write a balanced nuclear equation for the alpha decay
of radium-226
(Beta)  particles:
0
1e
or
0
1β
100x more penetrating than -particles
electrons (not orbital electrons)
originate in nucleus
extremely small mass
-emission converts a neutron into a proton
(atomic number increases by 1)
e.g.,  - decay of iodine-131
241
gamma ( radiation
very high-energy photons
extremely penetrating (10,000 x more than alpha
particle)
generally not shown in nuclear equations
Positron emission:
0
1e
same mass as e-, opposite charge
positron emission converts a
proton to a neutron
atomic number decreases by 1
e.g., carbon-11 decays by positron emission:
Electron capture
capture of an inner-shell e- by the nucleus
converts a proton into a neutron
electron appears as a reactant
e.g., rubidium-81 undergoes electron capture:
242
How can we predict if a nucleus is unstable and how it
might decay?
General guidelines:
neutron-to-proton (n/p) ratio:
stable nuclei with low atomic numbers (up to 20) have
approximately equal numbers of neutrons and
protons
For nuclei higher atomic numbers, # of neutrons
necessary to create a stable nucleus increases more
rapidly than does # of protons
We plot # of neutrons v. number of protons:
n/p ratio where stable nuclei reside is called the 'belt
of stability'
belt of stability ends at atomic number 83: all nuclei
with atomic number  84 are radioactive
243
The type of decay which a given radioisotope will undergo
depends largely on its n/p ratio
Three situations:
n/p > 1 (above belt of stability)
lower n/p by -emission (turns n  p)
n/p < 1: positron emission (light) or e- capture
(heavier elements)
converts p  n
-decay: primarily in nuclei with atomic number 
84
emission decreases both n & p
E.g., predict the mode of decay for
Boron-8
Iodine-137
Cerium-133
244
Other Guidelines:
nuclei with 2, 8, 20, 28, 50, or 82 protons
or
2, 8, 20, 28, 50, 82 or 126 neutrons
are more stable than nuclei with other numbers
nuclei with even numbers of both protons and neutrons
are more stable than those with odd numbers of nucleons
245
Problems du Jour
Each of the following nuclei undergoes either beta or positron
emission. Predict the type of emission for each.
66
32 Ge
Iodine-137
One of the nuclides in each of the following pairs is radioactive.
Predict which is radioactive, and which is stable, and explain.
39 40
19 K,19 K
Nickel-58 and Nickel-65
246