Download File - Chemistry with Mr. Patmos

Nuclear Chemistry
Chapter 28
Radioactivity is the process by which an
unstable atomic nucleus becomes stable.
 Radioisotopes – isotopes of elements that
have an unstable atomic nucleus

isotopes
atoms of the same element that differ in
the number of neutrons.
 Some isotopes are stable.
 Some isotopes are not stable - radioactive

Radioactive Isotopes



Which have an unstable nucleus emit
radiation to become more stable.
Radiation – the penetrating rays &
particles released by a radioactive
source.
Consists of both matter & energy.
4
The nuclear radiation emitted by an unstable
atom takes the form of:
1. alpha particles
2. beta particles
3. positrons
4. gamma rays (no mass, only energy).
5. protons
6. neutrons
6
Alpha Radiation - α
Consists of a helium nucleus
 made up of 2 protons & 2 neutrons
 When emitted by an atom, lowers the
atomic number by 2 and the mass number
by 4.

Alpha particle decay of U238 to Th234
Reactant
Product 1
Product 2
# protons
92
90
2
# neutrons
146
144
2
Atomic #
92
90
2
Mass #
238
234
4
10
Because of their large mass and charge,
alpha particles
1. Don’t travel far
2. Can be stopped by a piece of paper or
your skin
Potential Danger to Humans?
 If
swallowed, will destroy soft tissue.
Learning Check

Write the equation for the alpha decay of
222Rn.
Solution
 222Rn
+ 4He
 Rn – 86 protons + 136 neutron
 Products:



218Po
α particle – 2 protons + 2 neutrons
? element - 84 protons + 134 neutrons
? Element = at. # 84 = Po
Beta Radiation - β
Fast moving electrons formed by the
decomposition of a neutron into
1. a proton that remains in the nucleus
2. An electron which is emitted from the
nucleus
# protons increases by 1
# neutrons decreases by 1
Beta particles have less mass and less
charge than alpha particles which means
they have greater penetrating power
 Can be stopped by aluminum foil or a thin
piece of wood.

Learning Check
Write the nuclear equation for the beta
decay of 60Co.
Solution
60Co
27
60Ni
28
+
0e
1
Neutron breaks down into proton + electron
# of neutrons decreases by 1, mass # remains the
same.
Atomic # increases by 1
19
Positron Emission

A proton is converted to a neutron and a
positron.
1p
1n
+ 0e
1

0
+1
The mass number of the new nucleus is the
same , but the atomic number decreases by 1.
49Mn
25
49Cr
24
+
0e
+1
20
Learning Check

Write the equation for the positron
emission of 15O .
Solution

Write the equation for the positron emission of
15O .
15O
15N
+ 0e
8



7
+1
Proton breaks down into a neutron + positron
Atomic # decreases by 1
Mass # stays the same do to the extra neutron.
Gamma Radiation - У
Not matter
 Energy
 Often emitted along with alpha (α) or beta
(β) radiation
 Can be somewhat stopped by several
meters of concrete or centimeters of lead.

Home Assignment
Justify each of these
equations.
Why are some nuclei
unstable?
1.

Too many neutrons relative to the
number of protons
Solution: convert a neutron into a
proton and an electron (β-particle)
Beta Emission
2.

Too few neutrons relative to the number
of protons
Solution: convert a proton into a neutron
and a positron (positive electron)
Positron Emission
3.


too many protons and neutrons – too
heavy
atomic # above 83
Solution: alpha emissions – release 2
protons and 2 neutrons
α-emissions
When is an atomic
nucleus stable?
For elements 1 - 20, when the neutron to
proton ration is 1:1
 For elements 20 – 82, when there are
more neutrons than protons – ratio
increases from 1:1 up to 1.5:1 as the
atoms get larger.
 All elements above 83 are radioactive.


When the neutron to proton ratio in atoms
are plotted on a graph, the stable nuclei
are located in the region called the band
of stability.
Half Life
Half-Life
The half-life of a radioisotope is the
time for the radiation level to decrease
(decay) to one-half of the original
value.
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Decay Curve
What is the
half life of I131 ?
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
Radioisotopes that are
 Naturally occurring tend to have
long half-lives.
 Used in nuclear medicine have
short half-lives.
Radioisotope
Naturally 14C
Occurring 40K
226Ra
238U
Half-life
5730 yr
1.3 x 109 yr
1600 yr
4.5 x 109 yr
Used
Medically
28 days
8 days
46 days
6.0 hr
51Cr
131I
59Fe
99mTc
Half-Life Calculations


In one half-life, 40 mg of a radioisotope
decays to 20 mg.
After two half-lives, 10 mg of radioisotope
remain.
40 mg x
1
2
x 1 = 10 mg
2
1 half-life
Initial
40 mg
2 half-lives
20 mg
10 mg
Learning Check
The half life of 123I is 13 hr. How
much of a 64 mg sample of 123I
remains active after 26 hours?
1) 32 mg
2) 16 mg
3) 8 mg
Solution
2) 16 mg
STEP 1 Given 64 g; 26 hr; 13 hr/half-life
STEP 2 Plan
26 hours
STEP 3 Equalities
Number of half-lives
1 half-life = 13 hr
STEP 4 Set Up Problem
Number of half-lives = 26 hr x 1 half-life = 2 half-lives
13 hr
64 mg
32 mg
13 hr
16 mg
13 hr
Medical Applications
Radioisotopes with short half- lives are used in
nuclear medicine because they

Have the same chemistry in the body as
the nonradioactive atoms.

Give off radiation that exposes a
photographic plate (scan) giving an
image of an organ.
Thyroid scan
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin
Cummings
Learning Check
Which of the following radioisotopes
are most likely to be used in nuclear
medicine?
1)
40K
half-life 1.3 x 109 years
2)
42K
half-life 12 hours
3)
131I
half-life 8 days
Solution
Which of the following radioisotopes
are most likely to be used in nuclear
medicine?
Radioisotopes with short half-lives are
used in nuclear medicine.
2)
42K
half-life 12 hours
3)
131I
half-life 8 days
Transmutation Reactions
Transmutation – the conversion of an
atom of one element to an atom of
another element.
 Example: radioactive decay



1.
2.
3.
Transmutations can also occur when high
energy particles bombard the nucleus of
an atom.
These particles may be
Protons
Neutrons
Alpha particles
Learning Check
What radioactive isotope is produced when a
neutron bombards 59Co and causes the
emission of an alpha particle?
59Co
27
+
1n
0
? +
4He
2
54
Solution
Sum of mass numbers
60
=
60
59Co
27
+
1n
0
56Mn
25
+
4H
2
e
27
=
27
Sum of atomic numbers
55
Transuranium elements
Have atomic numbers above 92
 undergo transmutation
 Do not occur in nature
 Have been synthesized in nuclear reactors
or particle accelerators
 Are radioactive

Fission & Fusion
Nuclear Fission
A large nucleus is bombarded with a
small particle (often a neutron).
 The nucleus splits into smaller nuclei
and several neutrons.
 Large amounts of energy are released
(often in the form of heat).
58
When a neutron bombards
235U,

an unstable nucleus of
(splits).

Smaller nuclei are produced such as
142Ba.

Neutrons are released to bombard more 235U.
1n + 235U
236U
91Kr + 142Ba + 3 1n
0
92
(236/92)
92
(236/92)
236U
36
undergoes fission
56
91Kr
and
0
(236/92)
59
Learning Check
Supply the missing atomic symbol to complete
the equation for the following nuclear fission
reaction.
1n
0
+
235U
92
137Te
52
+ ?X + 2 1n + energy
?
0
61
Solution
1n
0
+
235U
92
137Te
52
+
97Zr
40
+ 2 1n + energy
0
62
Nuclear Fusion




Occurs at extremely high temperatures
(100 000 000°C).
Combines small nuclei (usually hydrogen) into
larger nuclei.
Releases large amounts of energy.
Occurs continuously in the sun and stars.
63
Learning Check
Indicate if each of the following describes
1) nuclear fission
or 2) nuclear fusion
___ A. A nucleus splits.
___ B. Large amounts of energy are released.
___ C. Small nuclei form larger nuclei.
___ D. Hydrogen nuclei react.
___ E. Several neutrons are released.
64
Solution
Indicate if each of the following is
1) nuclear fission
or 2) nuclear fusion
1
A. A nucleus splits.
1, 2 B. Large amounts of energy are released.
2
C. Small nuclei form larger nuclei.
2
D. Hydrogen nuclei react.
1
E. Several neutrons are released.
65