Transcript Ch. 18: The Nucleus

Ch. 18: The Nucleus
Review
21.1: Nuclear Stability and Radioactive
Decay
21.2 Kinetics of Decay
21.3 Nuclear Transformations
Review
 Nucleus
contains protons and neutrons
 Atomic number (Z) - # p+
 Mass number (A) - # p+ + # n0
 Isotopes- differ in number of neutrons
only
 Same Z but different A
 Nuclide- specific type of atom, member
of a group of isotopes
A
14
 Nuclear symbol notation
Z
X
6
C
Radioactivity and Stability
A nucleus that decomposes forming another
nucleus and one or more particles
 All nuclides are unstable with 84 p+ or more
 Lightweight nuclides are stable with equal
numbers of n0 and p+
 Heavy nuclides should have ratio >1 to be
stable

Radioactivity
and Stability
Types of Decay- A changes
 Alpha

particle production
α particle: helium nucleus
238
4
234
92
2
90
4
2
He
U  He  Th
 Spontaneous

fission
Splitting of a heavy nucleus into 2
lighter nuclides that are about the
same size
Types of Decay- A is Constant
 Beta


Particle Production
β particle is an electron
Can assume the mass is zero
234
0
234
90
1
91
0
1
e
Th e Pa

Net effect : changing a n0 into a p+
Types of Decay- A is Constant
 Gamma


Ray Production
γ ray is collection of high energy
photons
Occurs with other types of decay
238
4
234
0
92
2
90
0
U  He  Th  2 

Helps a nucleus release extra energy
so it can relax to a lower energy state
Types of Decay- A is Constant
0
1
e
Production
Occurs for nuclides below the line of
stability
Positron is a positive particle with
same mass of electron
Also called antiparticle of electron
 Positron



Th e Ne
22
11

0
1
22
10
Net effect: change p+ into n0
Types of Decay- A is Constant
0
1
 Electron


Capture
e
One of the inner electrons in an atom
is captured by nucleus
Gamma rays always produced
Hg  e Au  
201
0
80
1
 Decay Series

201
79
0
0
When several types of decay occur until a
stable nuclide is produced
Writing Equations
 116C
produces a positron
 21483Bi
C  e B
11
6
0
1
11
5
produces a beta particle
214
83
 23793Np
Bi  e Po
0
1
214
84
produces an alpha particle
237
93
Np He  Pa
4
2
233
91
Writing Equations
195
79


Au  ? Pt
Beta particle (electron)
Electron capture
38
19


195
78
K  ? Ar
38
18
Positron
Positron production
Kinetics of Decay
 Rate
of decay is
directly
proportional to
number of
nuclides available
 All are first order
 Constant half-life
Rate  kN
 N 
   kt
ln 
 N0 
0.693
t1/ 2 
k
Example
 If
the half-life of a decay is 67.0 hours,
how much of a 1.000 mg sample will
remain after 335 hours?
 335 / 67 = 5 half-life’s
 1.000 mg  0.500 mg 
0.250 mg  0.125 mg 
0.062 mg  0.031 mg
Nuclear Transformations
 Change
of one element into another
 Scientists have been able to use this to
make the periodic table larger by
creating new elements
 Since 1940, have been able to make
transuranium elements (93-112)
18.4 Detection and Uses of Radioactivity
18.5 Thermodynamic Stability
18.6 Nuclear Fission and Fusion
18.7 Effects of Radiation
Carbon-14 Dating
Used to date items made out of natural fibers
 Created by Willard Libby in 1940s
 Based on the decay of naturally existing carbon14 isotope by β-particle production


It is also created
14
6
14
7
C e
0
1
14
7
N
N n H C
1
0
1
1
14
6
Carbon-14 Dating
These happen at the same
rate as long as the plant is
alive but when it dies, the
decay happens more
rapidly than the creation
 Ratio of 14-C to 12-C
decreases
 Most accurate for pieces
older than 10,000 years

Medical Applications
Radiotracers
 Radioactive nuclides that can be traced in
people by monitoring their radioactivity
 Thallium-201
 For assessing heart damage from heart attacks
 Is taken up by healthy heart tissue only

Medical Applications
 Iodine-131


For diagnosing thyroid problems
Patients drink a solution of 131-I
and the uptake is monitored
Thermodynamic Stability
Can be determined by calculating the change
in potential energy if the nucleus is made
from individual particles
1
1
16
0
1
8
 We can create energy changes by comparing
the sum of the masses: mass defect
 Mass of 168O – mass of (8 10n + 8 11n)
 Convert amu on periodic table to g
(1amu=1.66x10-24 g)

8 n 8 H  O

2.65535x10-23 – [8(1.67493x10-24) + 8(1.67262x10-24)]

-2.269x10-25 g/nucleus = -0.1366 g/mol : lost
when 1 mol of 16-O is formed
Thermodynamic Stability
 Find
energy (J) using E=mc2
 E = (-1.366x10-4kg)(3.00x108m/s)2
= -1.23x1013J/mol
mass must be in kg!
 Binding energy
 Energy required to decompose this
nucleus into its particles
 Often in MeV / nucleon
 1.23 1013 J
1mol
1MeV
1nucleus



23
13
mol
6.02 10 nuclei 1.60 10 J 16nucleons
 7.98MeV / nucleon
Thermodynamic Stability
Nuclear Fission and Fusion
 Fission


Splitting a heavy nucleus into 2 smaller
nuclei with smaller mass numbers
Can use neutrons to create instability
1
0


n U  Ba  Kr 3 n
235
92
141
56
92
36
1
0
Neutrons produced are used to cause
more fission
Produces a huge amount of energy
Nuclear
Fission
and
Fusion
Nuclear Fission and Fusion
 Fusion


Combination of 2 light nuclei to form
a heavier, more stable nucleus
Stars produce their energy using this
1
1


H H H e
1
1
2
1
0
1
Requires very high temperatures
Must be shot at each other to get
close enough
Effects of Radiation
 Any
sort of energy transferred to cells
can break bonds and cause damage
 Radioactive species are sources of
high energy particles so can be very
harmful
 Types
 Somatic: cause illness, cancer, death
 Genetic: produce damage in
offspring
Factors in Effects of
Radiation
 The
more energy, the more damage
 How deep it goes into body
 γ rays > β particles (1 cm) > α particles
(skin)
 How easily they attract electrons from
biomolecules (ionization)
 γ rays cause less than α particles
 How long it stays inside body