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Nuclear Chemistry
Chapter 10 – Prentice Hall Physical Science
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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?
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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: Uranium-238 (used in Becquerel’s experiment) Carbon-14 (used often in radioactive dating)
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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:
U23892
Uranium-238
C146
Carbon-14
Po21094
Polonium-210
Mass # on top
Atomic # on bottom
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3 Types of Nuclear Radiation
Nuclear radiation: charged particles and energy that are emitted from the nuclei of radioisotopes
Radiation Type
Symbol ChargeMass (amu)
Common Source
Alpha particle
, 2+ 4 Radium-226
Beta particle
, 1- Carbon-14
Gamma ray
0 0 Cobalt-60
He42
e01 1836
1
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Alpha Decay
Alpha particle, 2 protons and 2 neutronsPositively chargedSame as He nucleus
Least penetrating type of nuclear radiationTravel only centimeters in airCan be stopped by a sheet of paper or
clothing
He42
7
Beta Decay
Beta particle, 1 electronNegatively chargedProduced by a neutron that decomposes into
a proton and an electron More penetrating than particles
Pass through paperStopped by a thin sheet of metal
e01
8
Gamma Decay
Gamma ray, 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
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Similar to chemical equations, but isotope symbols are used.
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!
Writing and Balancing Nuclear Reactions
ePaTh
HeThU
01
23491
23490
42
23490
23892
Reactants → Products
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Example: Math Skills p. 295 Write a balanced nuclear equation for the alpha decay of polonium-
210. Step 1: Define reactants and products. Use letters to represent the
unknown values.
Step 2: Write and solve equations to find unknown atomic and mass #s.
Step 3: Look up the element symbol on the periodic table using the atomic #.
Step 4: Write the balanced nuclear equation and double-check your solution.
Po21084 He4
2 XAZ
isotope.product of symbol chemical X and ,# mass A ,# atomic Let Z
+
2064210
4210
A
A
82284
284
Z
Z
Atomic # 82 = Pb (Lead)
PbHePo 20682
42
21084
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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?
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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
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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
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Rates of Nuclear Decay (cont’d)
Nuclear Decay Rate
0
20
40
60
80
100
0 1 2 3 4 5
Time (# of Half-lives)
Rad
iois
oto
pe
Rem
ain
ing
(%
)
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Rates of Nuclear Decay (cont’d)
Different radioisotopes have different half-lives
To determine how many half-lives 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
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Radiocarbon dating
Carbon-14 exists naturally in the atmosphere at a fairly constant ratio to C-12
As C-14 decays, it’s replaced by C-14 absorbed
from atmosphere
Tree dies – no more CO2 absorbed to
replace decaying C-14
CO2 absorbed while living (including some C-14)
Age of fossil determines by
comparing C-14/C-12 ratio in fossil to
atmospheric ratio
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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?
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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
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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?
Transuranium elements Many produced by artificial transmutation of a lighter
element All are radioactive
HOHeN 11
178
42
147
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Nuclear 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. Nuclei are unstable at this point. Any atom with 83 or more
protons is unstable – and, therefore, radioactive.
Small nucleusProton from a small nucleus
Proton from a large nucleus
Strong nuclear forces:
Electric forces:
Large nucleus
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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
a lot of energyNeutron ((
((
(( ))
((
((
))
))
U23592 U236
92
(very unstable)
Energy
Kr9136
Ba14256
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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.)
J109s
m100.3kg1
s
m100.3
kg1
162
82
8
mcE
c
m This quantity = 2.5 x 1010 kWh, or enough energy to power my house for over 3.6 million years!
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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.
Neutron
U23592
Energy
Kr9136
Ba14256
U23592
U23592
U23592
Energy
Kr9136 Ba142
56
+ + +
Energy
Kr9136 Ba142
56
+ + +
Energy
Kr9136 Ba142
56
+ + +
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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
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Nuclear Fusion
Fusion: nuclei of two atoms combineThe sun and other stars are powered by
fusion of H into HeRequires extremely HIGH temperaturesWhat state is matter in at such high
temperatures? PLASMA
Fusion
+ + ENERGY (17.6 MeV)
H21
H31 He4
2 n10
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