Lecture 10
Professor Hicks
Inorganic Chemistry (CHE152)
Copyright © 2009 Charles Hicks
Becquerel rays
• Henry Becquerel observed
that salts of uranium emit a
radiation that exposed
photographic film in the dark
• constant rate of emission
• did not depend on chemical
state of the substance
Henry Becquerel
Copyright © 2009 Charles Hicks
Becquerel rays  radioactivity
• Marie Curie discovered elements, polonium and radium
• they also emit radiation that can be seen on film
• rate of the emission from all these elements also unaffected
by the chemical state of the matter or temperature
radioactive sample
other chemical
reactions
oxidize
reduce
rate radiation comes out of the
sample does not change when
element undergoes reactions
Marie Curie
Copyright © 2009 Charles Hicks
1
Rutherford’s Experiment
++++++++++++
g
b

radioactive
sample
-------------charged plates
radioactive elements can emit three types of radiation:
• alpha () particles – heavy, positively charged
• beta (b) particles – light, negatively charged
• gamma (g) particles - uncharged
Copyright © 2009 Charles Hicks
Penetrating ability of radioactive particles
pieces of lead
radioactive sample

lead box
g
b
0.01 mm
1 mm
100 mm
Copyright © 2009 Charles Hicks
review
The Nucleus
• protons and neutrons
• protons have equal and
opposite charge of electron
+1.6 x10-19 C
10-15 m
+
N N
N
+ N + +
N + N
• neutrons uncharged
• neutrons and protons both
called nucleons
• nuclear radii range 2-15 fm
(atom ~ 10-10 m)
nucleus
+
= proton
N
= neutron
Copyright © 2009 Charles Hicks
2
review
Atomic Mass Units (amu)
• based on mass protons, neutrons
• proton and neutron each have
about the same mass = 1.0 amu
• 6.02 x1023 amu = 1.0 gram
• 6.02 x 1023 called
Avogadro's number (NA)
+
= proton = 1.0 amu
N
= neutron = 1.0 amu
e- = electron = 1/1836 amu
(5.4 x 10 -4 amu)
NNNN
NN
NNNN
NNN
NNNNN N
NNN
NNNNNN
N
NNNNNN
NNNN
N
N NNNNN
NNN
N
NNN
NNN
NNNNN
N
N
N
N
N NNNNN
NNN
N N
NNNN
NNN N
N NN
N NNN
N NNN NN NN
N NNN NNN NN
NN
= 1.0 g
6.02 x 1023 amu
Copyright © 2009 Charles Hicks
review
Isotopes
+
N N
+ N + +
N + N
+
N N
N
+ N + +
N + N
boron-10
both are boron (5 protons)
different # neutrons
different isotopes of boron
boron-11
• same # protons
= an isotope
different # neutron
• isotopes are versions of an
element with different # neutrons
Copyright © 2009 Charles Hicks
more on isotopes
+
N N
+ N + +
N + N
boron-10
+
N N
N
+ N + +
N + N
review
two isotopes of boron
boron-11
• elements in nature are mixtures
of different isotopes
• isotopes have different masses
• nuclei of some isotopes are unstable
 break down in nuclear reactions
Copyright © 2009 Charles Hicks
3
review
Isotope notation
mass number (A)
= # protons
+ # neutrons
How many neutrons?
mass number - # protons= # neutrons
= A-Z
=13 – 6 = 7 neutrons
13
6
C
element symbol
nuclear charge (Z)
# protons
pronounced “carbon 13”
can also be written
“carbon-13” or C-13
Copyright © 2009 Charles Hicks
review
atomic mass and periodic table
# protons
atomic mass
weighted average
of isotope masses
natural Sn is a mixture of:
115
114
Sn 116
50 Sn
50 Sn 50
118
117
Sn 119
50 Sn
50 Sn 50
122
124
120
50 Sn
50 Sn
50 Sn
Copyright © 2009 Charles Hicks
Radioactivity comes from nuclear reactions
• radioactive isotopes have unstable nuclei
radioactive isotopes  stable isotopes
• , b or g radiation released
• more stable nucleus formed
•  particles = high speed helium nuclei
• b particles = high speed electrons
• g radiation= high energy electromagnetic radiation
“light”
Copyright © 2009 Charles Hicks
4
The Positron
• positively charged
“anti-electron”
• same mass, opposite charge
of electron
• predicted to exist by
Paul Dirac
Paul Dirac
e-
p+
Copyright © 2009 Charles Hicks
Modes of nuclear decay
• alpha () emission
nucleus breaks up   particles emitted
• beta emission
neutron  proton + electron  b particles emitted
• gamma emission
nucleus shifts into a more stable arrangement
without changing N or P
• positron emission
proton  neutron + positron  positron emitted
• electron capture
proton reacts with electron
proton + electron neutron
Copyright © 2009 Charles Hicks
Nuclear Symbols
Particle
proton
Nuclear
Symbol
1
1
H 11p
neutron
1
0
0
1
n
e
electron
alpha
beta
positron
4
2
α 42 He
0
1
0
1
β
β
0
1
0
1
e
e
15
Copyright © 2009 Charles Hicks
5
Nuclear Equations
• parent nuclide – isotope undergoing nuclear
reaction
• Daughter nuclide – new isotope formed in
the nuclear reaction
Copyright © 2009 Charles Hicks
Balancing Nuclear Equations
• in a nuclear equation mass numbers and
atomic numbers are conserved
• we can use this fact to determine the
identity of a daughter nuclide if we know
the parent nuclide and the mode of decay
Copyright © 2009 Charles Hicks
Write a balanced nuclear equation
for each of the following
• alpha emission from U-238
• beta emission from Ne-24
• positron emission from N-13
• electron capture by Be-7
238
4
234
92 U2 He  90Th
24
0
10 Ne-1 e
13
7N
7
4 Be


24
11Na
01 e 
0
1e
13
6C
 73Li
Copyright © 2009 Charles Hicks
6
Natural Radioactive Decay Series
• radioactive isotopes with Z>83 follow a
series of decays
• three pathways exist
• each is a series of  and b decays
• all three pathways end with different
isotopes of Pb
Copyright © 2009 Charles Hicks
Natural Radioactive Decay Series
one of three pathways shown
like three train lines:
• each isotope is a station
• elements that catch the train at
any stop all go to the last stop- Pb (lead)
stability !! Pb !! Z less than 83
Copyright © 2009 Charles Hicks
Copyright © 2009 Charles Hicks
7
Copyright © 2009 Charles Hicks
Copyright © 2009 Charles Hicks
Write a balanced nuclear equation for the
electron capture decay of palladium-103
Copyright © 2009 Charles Hicks
8
Copyright © 2009 Charles Hicks
Four factors affect nuclear stability
1) size of nucleus - no nucleus larger than Z
= 83 (Bi) is stable
2) ratio of neutrons to protons must be in
certain range called the Stability Band
3) magic numbers of protons or neutrons
increase stability
4) nuclei with even numbers of protons
and/or neutrons more stable
Copyright © 2009 Charles Hicks
Stability Band
Neutron / Proton ratio
for Z = 1  20,
stable N/Z ≈ 1
know
this fact
• For higher Z values N/Z increases
Z = 20  40,
stable N/Z approaches 1.25
for Z = 40  83,
stable N/Z approaches 1.5
yellow = known isotopes
points = stable isotopes
Copyright © 2009 Charles Hicks
for Z > 83,
there are no stable nuclei
Most radioactive isotopes have
N/Z > 1.5
heavier nuclei require
more neutrons to be stable
9
Strong force aka nuclear force
• attractive force between
- protons to protons
Explains the Stability Band
- protons to neutrons
- neutrons to neutrons
• only strong at close distances ~10-15 m
within nucleus
+
+
N
+
+
N N
N
+ N + +
N + N
protons repel each other
like charges
protons neutrons attract
strong force
boron-11
• neutrons acts like a glue to hold
nucleus together
Copyright © 2009 Charles Hicks
Natural Radioactive Decay Series
Revisted
•  decays reduce size nucleus closer to 83
• b decays decrease N/Z ratio so it will land
in stability band eventually
• all three pathways end with isotopes of Pb!
Copyright © 2009 Charles Hicks
Magic numbers
• protons and neutrons each have energy levels like
electrons
• magic numbers N or Z = 2, 8, 20, 28, 50, 82; or N
= 126 correspond to filled shells of protons or
neutrons
- like octets of electrons
• some elements are doubly magic extra stable
4
2
He
2 protons
2 neutrons
16
40
48
8
20
20
O
8 protons
8 neutrons
Ca
20 protons
20 neutrons
Ca
20 protons
28 neutrons
all doubly magic
Copyright © 2009 Charles Hicks
10
Nucleon pairing
• protons and neutrons both are more stable if
they form pairs
• odd numbers P or N1 unpaired  less stable
• even numbers P or N all paired  stable
Copyright © 2009 Charles Hicks
Nucleon pairing and stability
• odd # protons = less stable
• graph appears to alternate
because more stable isotopes
(even # protons)
are more abundant
Copyright © 2009 Charles Hicks
N/Z ratio, decay modes, stability band
decay mode
 emission
b emission
g emission
positron emission
electron capture
effect N/Z
increase
decrease
no effect
increase
increase
• alpha emission from U -238
238
4
234
92 U2 He 90Th
• beta emission from Ne -24
24
0
10 Ne-1 e

24
11Na
• positron emission from N-13
13
7N
 01 e 
7
4 Be

13
6C
• electron capture by Be-7
0
1e
 73 Li
Copyright © 2009 Charles Hicks
11
N/Z ratio, decay modes, stability band
nuclei need to
dec Z & dec N/Z
 + b emission
 decay decreases N and Z
as nuclei gets lighter N/Z
nees to dec
nuclei need
to dec N/Z
b emission
heavy elements decay by series
of  and b decays to achieve this
beta emission decreases N/Z ratio
positron emission and
electron capture increase
N/Z ratio
nuclei need
to inc N/Z
PE and EC
Copyright © 2009 Charles Hicks
Half-Life and Stability
• Radioactive isotopes undergo decay by first order
process  the half life (t½) is a constant
• radioactive nuclei that have factors making their nucleus
more stable will have longer half lives (t½)
Chapter 19 Supplemental Examples
Example#1: Europium has two stable isotopes Eu-151, and Eu-153. Three of
the unstable isotopes Eu-144, Eu,-145, and Eu-146 have half lives of 10.2 sec,
5.93 days, 4.61 days respectively. Eu-146 has an N/P ratio closer to that of the
stable isotopes. Why does why does Eu-145 have a longer half life?
Hint : Consider the four factors that affect nuclear stability:
1) N/Z ratio
2) magic numbers
3) Z < 83
4) odd or even number of nucleons
Copyright © 2009 Charles Hicks
N/Z Ratio and Stability
• the decay mode of a radioactive isotope can be predicted
from its position relative to the stability band.
Example#2: Zirconium-96 is an unstable radioactive isotope. By what mode or
modes of decay would you predict it would decay?
Consider the four factors that affect nuclear stability:
1) N/Z ratio – look at the graph of the stability band
2) magic numbers – has none
3) Z < 83 – yes so there are stable isotopes of Zr
4) odd or even number of nucleons – both N and P are even which is favorable
Zr-96 is above the stability band in the region below z=83 so its “problem”
is its N/Z ratio is too large. Decay by beta emission will decrease N/Z and
creating a more stable nucleus.
Copyright © 2009 Charles Hicks
12
Odd/Even # Protons and Stability
• isotopes with an odd number of neutrons are
less stable
Example#3:Fluorine-18 is a radioactive isotope. Suggest a reason for its instability.
Consider the four factors that affect nuclear stability:
1) N/Z ratio
2) magic numbers
3) Z < 83
4) odd or even number of nucleons
F-18 has two factors in its favor
N/Z=1.0
Z<83
It does not have any extra stability due to magic numbers
It does have an odd number of protons and an odd number of neutrons
These are both unfavorable and must be the reason for the instability of
the nucleus of F-18
Copyright © 2009 Charles Hicks
Z>83 and Stability
• isotopes with Z>83 are all radioactive
Example#3:Radon-222 has a half life of 3.8 days.
Suggest a reason this isotope is radioactive.
There are 4 factors that affect nuclear stability:
N/Z ratio
magic numbers
Z < 83
odd or even number of nucleons
Rn-222 has Z>83
There are no stable isotopes with Z>83 because the repulsion of protons
for each other is too large to be overcome by the addition of any number
of neutrons or having any of the other factors favorable.
Copyright © 2009 Charles Hicks
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Copyright © 2009 Charles Hicks
Copyright © 2009 Charles Hicks
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15
Br-81 is a stable isotope. Br-82 has a half
life of 35.5 hours and Br-83 has a half life
of 2.3 hours. Suggest a reason that Br-83
has a shorter half life.
Copyright © 2009 Charles Hicks
Copyright © 2009 Charles Hicks
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Copyright © 2009 Charles Hicks