Nuclear Chemistry
Chapter 4
Most isotopes with atomic number
19 or lower are stable.
But elements with atomic number 20 or higher
typically have one or more unstable isotopes.
An unstable isotope has a ratio of neutrons to protons that make it unstable.
An unstable nucleus is radioactive: it emits radiation in
order to become more stable.
Radiation has different forms
 – particles
 – particles
+ (positrons)
 rays (electromagnetic radiation)
When a positron and an electron
collide their mass are converted into
energy in the form of gamma rays.
0 e + 020 γ
−1
+1
0
When radiation hits a molecule it can knock
off an electron forming an unstable ion.
In radioactive decay a nucleus
spontaneously decays by emitting
radiation.
In -decay the mass number of the
nucleus decreases by 4 and the
atomic number decreases by 2
while an -particle is emitted.
221 Fr  217 At + 4 He
87
85
2
214 Po  ? + 4 He
84
2
214 Po  210 Pb+ 4 He
84
82
2
In b-decay the mass number of the radioactive
nucleus and the daughter nucleus are the
same. The atomic number of the daughter
nucleus increases by 1.
51 Cr  ? + 0 e
24
−1
51 Cr  51 Mn + 0 e
24
25
−1
In positron emission the mass number of the
radioactive nucleus and the daughter nucleus
are the same. The atomic number of the
daughter nucleus decreases by 1.
24 Al  24 Mg+ 0 e
13
12
+1
What is the balanced nuclear
equation for xenon-118, which
undergoes positron emission?
118 Xe  118 I+ 0 e
54
53 +1
There are not many gamma
emitters, but one of them,
technetium (Tc), is used in radiology.
An unstable isotope of technetium (metastable
technetium)forms a more stable nucleus by
gamma emission.
99 m Tc  99 Tc+ 0 γ
43
43
0
The half-life (t1/2) of an isotope is the
time it takes for one half of a sample to
decay.
131 I  131 Xe+ 0 e
53
54
−1
C-15 has a t1/2 of 2.4 seconds. How much
of a 240 g sample of C-15 will remain after
12 seconds?
7.5 g
In nuclear fission atomic energy is
generated by splitting an atom.
This can cause a chain reaction.
In nuclear fusion two small nuclei
combine to form a larger nucleus
(+ energy).