Nuclear Chemistry
Chapter 10 – Prentice Hall
Physical Science
1
Review


All the chemistry we’ve discussed so far has involved
electrons.
Questions:
1.
If element X has a molar mass of 3 g/mol and element Y has a
molar mass of 5 g/mol, what must be the molar mass of X2Y?
2.
If you tossed 128 coins in the air, about how many would you
expect to land heads-up?
3.
What do the mass number and atomic number represent?
4.
Which subatomic particles are found in the nucleus?
2
Radioactivity



Antoine Henri Becquerel (1896) experimented
with uranium salts and discovered radioactivity
Radioactivity (or nuclear decay): an unstable
nucleus emits charged particles and energy
Radioisotope: radioactive isotope - any atom
that has an unstable nucleus. Examples:
(used in Becquerel’s experiment)
 Carbon-14 (used often in radioactive dating)
 Uranium-238
3
Isotope symbology
Isotopes are named using the element
name followed by the mass number (see
examples, slide #3)
 The symbol for isotopes includes the
element symbol, the mass number and the
atomic number as follows:

Mass #
on top
Atomic #
on bottom
238
92
U
Uranium-238
14
6
C
Carbon-14
210
94
Po
Polonium-210
4
3 Types of Nuclear Radiation

Nuclear radiation: charged particles and energy
that are emitted from the nuclei of radioisotopes
Radiation
Type
Alpha
particle
Beta
particle
Gamma
ray
Symbol
a,
b,
4
2
He
0
1
g
e
Charge
Mass
(amu)
Common
Source
2+
4
Radium-226
1-
0
1
1836
0
Carbon-14
Cobalt-60
5
Alpha Decay

4
2
He
Alpha particle, a
2
protons and 2 neutrons
 Positively charged
 Same as He nucleus

Least penetrating type of nuclear radiation
 Travel
only centimeters in air
 Can be stopped by a sheet of paper or
clothing
6
Beta Decay

0
1
e
Beta particle, b
1
electron
 Negatively charged
 Produced by a neutron that decomposes into
a proton and an electron

More penetrating than a particles
 Pass
through paper
 Stopped by a thin sheet of metal
7
Gamma Decay

Gamma ray, g
 Penetrating
ray of energy
 Like X-rays and light, only very short wavelength

Most penetrating form of the three types
discussed
 Often
accompanies alpha or beta decay
 Several centimeters of lead or several meters of
concrete required to stop it
8
Writing and Balancing Nuclear
Reactions

Similar to chemical equations, but isotope symbols are
used.
Reactants → Products
4
U 234
Th

90
2 He
238
92
0
Th234
Pa

91
1 e  g
234
90

In a balanced nuclear equation:



Mass # on the left = sum of mass #s on the right
Atomic # on the left = sum of atomic #s on the right
You will need to use your PERIODIC TABLES!
9
Example: Math Skills p. 295

Write a balanced nuclear equation for the alpha decay of polonium210.

Step 1: Define reactants and products. Use letters to represent the
unknown values.
Let Z  atomic # , A  mass # , and X  chemical symbol of product isotope.
210
84


Po
4
2
He +
A
Z
X
Step 2: Write and solve equations to find unknown atomic and mass #s.
210  A  4
84  Z  2
A  210  4  206
Z  84  2  82
Step 3: Look up the element symbol on the periodic table using the
atomic #.
Atomic # 82 = Pb (Lead)

Step 4: Write the balanced nuclear equation and double-check your
solution.
210
4
206
84 Po  2 He  82 Pb
10
Effects of Nuclear Radiation

Background radiation: naturally occurring in
the environment
 Sources:
 Radioisotopes in air, water, rocks & living things
 Cosmic radiation
 Generally at safe levels


Nuclear radiation can ionize atoms. At levels
significantly above background, this can damage
DNA and proteins
Which type of nuclear radiation is the least
harmful? Which the most?
11
Detecting Nuclear Radiation

Geiger counters



Use gas-filled tubes to measure ionizing
radiation
Gas produces an electric current when
exposed to ionizing radiation
Film badges


Photographic film wrapped in paper
Film is exposed with exposure to
radiation like photographic film is
“exposed” with exposure to visible light
12
Rate of Nuclear Decay
Nuclear decay rate describes how fast
nuclear changes take place
 Unlike chemical reactions, nuclear decay
rate does NOT vary with external
conditions – it is constant for a given
radioisotope
 Half-life: the time required for half of a
radioisotope sample to decay

13
Rates of Nuclear Decay (cont’d)
Nuclear Decay Rate
Radioisotope Remaining (%)
100
80
60
40
20
0
0
1
2
3
4
5
Time (# of Half-lives)
14
Rates of Nuclear Decay (cont’d)



Different radioisotopes have
different half-lives
To determine how many halflives have elapsed for a
sample, divide the total time
of decay by the half-life
Known decay rates are used
in radioactive dating
Radioisotope
Half-life
Radon-222
3.82 days
Iodine-131
8.07 days
Carbon-14
5730 years
Thorium-230
75,200 years
Uranium-238
4.47x109 years
15
Radiocarbon dating

Carbon-14 exists naturally in the
atmosphere at a fairly constant ratio to C-12
CO2 absorbed while living
(including some C-14)
As C-14 decays,
it’s replaced by
C-14 absorbed
from atmosphere
Tree dies – no more
CO2 absorbed to
replace decaying C14
Age of fossil
determines by
comparing C-14/C-12
ratio in fossil to
atmospheric ratio
16
Radiocarbon dating (cont’d)
Used for objects less that 50,000 years old
 For older objects, must use different
isotope with longer half-life
 What isotopes would work well to date a
rock formation that is thought to be close
to a trillion years old?

17
Artificial Transmutation
Transmutation: conversion of atoms of
one element into atoms of another
 Alchemists have attempted this for
hundreds of years (but not through nuclear
chemistry)
 First artificial transmutation: Ernest
Rutherford (1919) turned nitrogen into
oxygen-17

18
Artificial Transmutation (cont’d)

Transmutation achieved by bombarding atomic
nuclei with high-energy particles
 Protons, neutrons or alpha particles
 Example: Ernest Rutherford’s transmutation
used
which particle?
14
7

N  24He 178 O 11H
Transuranium elements
 Many
produced by artificial transmutation of a lighter
element
 All are radioactive
19
Nuclear Forces

Strong nuclear forces:
Electric forces:
The strong nuclear force
attracts protons and neutrons.

Stronger than electric forces
over short distances
 Decreases with distance (like
gravity)


Electric repulsions push
protons apart.
When a nucleus is large
enough, the electric forces can
overcome the strong nuclear
forces.
Small nucleus
Proton from a
small nucleus

Nuclei are unstable at this
point.
 Any atom with 83 or more
protons is unstable – and,
therefore, radioactive.
Large nucleus
Proton from a
large nucleus
20
Fission

Fission: splitting of nucleus into two smaller
parts
 Lise
Meitner, Fritz Strassman and Otto Hahn’s
experiments (1939) first demonstrated nuclear fission.
 A small amount of the original mass is converted into
91
a lot of energy
36 Kr
((
Neutron
))
Energy
((
((
235
92
U
236
92
U
(very unstable)
142
56
Ba
21
Fission (cont’d)

About how much energy was released from 6.2 kg of
Plutonium-239 in the second atomic bomb explosion?
(Note: Only about 1 kg underwent fission – the rest was
scattered.)
m  1kg
c  3.0 108
m
s
2
m

E  mc2  1kg  3.0 108   9 1016 J
s

This quantity = 2.5 x
1010 kWh, or enough
energy to power my
house for over 3.6
million years!
22
Fission and Chain Reactions

Fission can result in a chain reaction.
 Neutrons
released from the first reaction can
trigger another reaction, and so on – similar to
a rumor spreading.
Ba
Kr
U
91
36
Neutron
142
56
91
36
235
92
Energy
+
+
Kr
91
36
Energy
Energy
235
92
235
92
U
+
142
56
+
Ba
+
+
U
91
36
Ba
Energy
235
92
142
56
Kr
U
+
142
56
Kr
+
Ba
+
23
Chain Reactions (cont’d)

For a chain reaction to happen, each split
nucleus must produce at least one neutron with
enough energy to split another nucleus
 This
only happens when a specific mass of
fissionable material is available – called the critical
mass.
 Controlled chain reactions are used to generate
electricity in nuclear power plants.
 Uncontrolled chain reactions are used in nuclear
weapons
24
Nuclear Fusion

Fusion: nuclei of two atoms combine
 The
sun and other stars are powered by
fusion of H into He
 Requires extremely HIGH temperatures
 What state is matter in at such high
temperatures? PLASMA
2
1
H
+
3
1
H
4
2
Fusion
He
+
1
0
n
ENERGY
(17.6 MeV)
25