Transcript Nuclear chem PPT NUC LECTURE

The Atomic Nucleus
Compacted nucleus:
4 protons
5 neutrons
Since atom is electrically neutral, there
must be 4 electrons.
4 electrons
Beryllium Atom
Definitions
A nucleon is a general term to denote a nuclear
particle - that is, either a proton or a neutron.
The atomic number Z of an element is equal to
the number of protons in the nucleus of that
element.
The mass number A of an element is equal to
the total number of nucleons (protons +
neutrons).
The mass number A of any element is equal to
the sum of the atomic number Z and the number
of neutrons N :
A=N+Z
Symbol Notation
A convenient way of describing an element is by
giving its mass number and its atomic number,
along with the chemical symbol for that element.
A
Z
X
Mass number
Atomic number
 Symbol 
9
For example, consider beryllium (Be): 4
Be
Describe the nucleus of a lithium atom which has a mass
number of 7 and an atomic number of 3.
A = 7; Z = 3; N = ?
N=A–Z= 7-3
neutrons: N = 4
Protons:
Z=3
Electrons: Same as Z
7
3
Li
Lithium Atom
Isotopes of Elements
Isotopes are atoms that have the same number
of protons (Z1= Z2), but a different number of
neutrons (N). (A1  A2)
3
2
He
Helium - 3
Isotopes
of helium
4
2
He
Helium - 4
Nuclides
Because of the existence of so many isotopes, the
term element is sometimes confusing. The term
nuclide is better.
A nuclide is an atom that has a definite
mass number A and Z-number. A list of
nuclides will include isotopes.
The following are best described as nuclides:
3
2
He
4
2
He
12
6
C
13
6
C
Radioactivity
As the heavier atoms become
more unstable, particles and
photons are emitted from the
nucleus and it is said to be
radioactive. All elements with
A > 82 are radioactive.
a
bg
Examples are:
Alpha particles a
b- particles (electrons)
Gamma rays g
b+ particles (positrons)
Nuclear Decay and Reactions
When the ratio of N/z gets very large, the nucleus
becomes unstable and often emits particles or photons
Radioactive elements often go through a series of
successive decays, until they form a stable nucleus.
For example Uranium undergoes 14 separate
decays before the stable isotope
Lead is produced.
The Alpha Particle
An alpha particle a is the nucleus of a helium
atom consisting of two protons and two
neutrons tightly bound.
Charge = +2e- = 3.2 x 10-19 C
Mass = 4.001506 u
Relatively low speeds ( 0.1c )
Not very penetrating
The Beta Particle
A beta-minus particle b- is simply an electron
that has been expelled from the nucleus.
-
Charge = e- = -1.6 x 10-19 C
Mass = 0.00055 u
-
High speeds (near c)
-
Very penetrating
The Gamma Photon
A gamma ray g has very high electromagnetic
radiation carrying energy away from the
nucleus.
g
g
Charge = Zero (0)
Mass = zero (0)
g
Speed = c (3 x 108 m/s)
g
Most penetrating radiation
FISSION REACTIONS
Balancing Nuclear Reactions
Rules for Balancing
1. Balance Mass
2. Balance Charge
A
Z
X
Y + a + energy
A- 4
Z -2
4
2
Find the missing product from the neutron
alpha bombardment of Aluminum - 27
27
13
Al+ a 
4
2
27
13
+ n
1
0
Al+ a  P+ n
4
2
30
15
1
0
Alpha Decay
Alpha decay: In alpha decay, an unstable nucleus
produces a daughter nucleus and releases an a particle.
Alpha decay 2 a results in the loss of two
protons and two neutrons from the nucleus.
4
A
Z
X
Y + a + energy
A- 4
Z -2
4
2
X is parent atom and Y is daughter atom
The energy is carried away primarily
by the K.E. of the alpha particle.
Find the missing product from the neutron
bombardment of Lithium-6
A
Z
X
6
3
Y + a + energy
A- 4
Z -2
4
2
+ a
Li+ n 
6
3
1
0
4
2
Li+ n  H + a
1
0
3
1
4
2
U-238 decays to produce (what) and an alpha particle?
A
Z
U
238
92
X  ZA--42Y + 24a + energy
Th+ He
234
90
4
2
Beta Decay
Beta-minus b- decay results when a neutron
decays into a proton and an electron. Thus, the
Z-number increases by one.
A
Z
X
Y + b + energy
A
Z +1
0
-1
X is parent atom and Y is daughter atom
The energy is carried away primarily
by the K.E. of the electron.
-
Find the missing product from the Beta Decay
carbon of 14
C
+ e
14
6
0
-1
C  N+ e
14
6
14
7
0
-1
Gamma Decay
A redistribution of the energy within the nucleus
results in gamma decay.
The γ ray is a high-energy photon.
Neither the mass number nor the atomic number
is changed in gamma decay.
Gamma radiation often accompanies alpha and
beta decay.
Write the reaction that occurs when
Thorium 234 decays by beta emission and
gamma radiation is emitted.
Th
234
90
Pa + e + g
234
91
0
-1
0
0
Nuclear Decay and Reactions
The three types of radiation are summarized in the
table below.
Nuclear Decay and Reactions
Nuclear Reactors
To create a controlled chain reaction and make use of the energy
produced, the neutrons need to interact with the fissionable uranium at
the right rate.
Most of the neutrons released by the fission of 235 U atoms are
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moving at high speeds.
These are called fast neutrons.
In addition, naturally occurring uranium consists of less than one
percent 235 U and more than 99 percent 238 U.
92
92
Nuclear Decay and Reactions
Nuclear Reactors
When a 238U nucleus absorbs a fast neutron, it does not
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undergo fission, but becomes a new isotope, 239
U.
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The absorption of neutrons by 238 U keeps most of the
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neutrons from reaching the fissionable 235U atoms.
92
Thus, most neutrons released by the fission of
to cause the fission of another 235U atom.
92
235 U
92
are unable
Nuclear Decay and Reactions
Nuclear Reactors
To control the reaction, the uranium is broken up into small pieces and
placed in a moderator, a material that can slow down, or moderate, the
fast neutrons.
When a neutron collides with a light atom it transfers momentum and
energy to the atom. In this way, the neutron loses energy.
In this way, the neutron loses energy.
The moderator thus slows many fast neutrons to speeds at
which they can be absorbed more easily by 235U than by
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238U.
92
Nuclear Decay and Reactions
Nuclear Reactors
The type of nuclear reactor
used in the United States, the
pressurized water reactor,
contains about 200 metric tons
of uranium sealed in hundreds
of metal rods.
The rods are immersed in
water, as shown in the figure.
Atomic Mass Unit, u
One atomic mass unit (1 u) is equal to onetwelfth of the mass of the most abundant form of
the carbon atom--carbon-12.
Atomic mass unit: 1 u = 1.6606 x 10-27 kg
Common atomic masses:
Proton: 1.007276 u
Neutron: 1.008665 u
Electron: 0.00055 u
Hydrogen: 1.007825 u
The average atomic mass of Boron-11 is 11.009305 u. What
is the mass of the nucleus of one boron atom in kg?
11
5
B = 11.009305
Electron: 0.00055 u
The mass of the nucleus is the atomic mass
less the mass of Z = 5 electrons:
Mass = 11.009305 u – 5(0.00055 u)
1 boron nucleus = 11.00656 u
 1.6606 x 10-27 kg 
m  11.00656 u 

1
u


m = 1.83 x 10-26 kg
Mass and Energy
Recall Einstein’s equivalency formula for m and E:
E  mc 2 ;
c  3 x 108 m/s
E  mc ; c  3 x 10 m/s
2
8
The energy of a mass of 1 u can be found:
E = (1 u)c2 = (1.66 x 10-27 kg)(3 x 108 m/s)2
E = 1.49 x 10-10 J
When converting
amu to energy:
Or
E = 931.5 MeV
c  931.5
2
MeV
u
What is the rest mass energy of a proton (1.007276 u)?
E = mc2 = (1.00726 u)(931.5 MeV/u)
Proton: E = 938.3 MeV
Similar conversions show other rest mass energies:
Neutron: E = 939.6 MeV
Electron: E = 0.511 MeV
The Mass Defect
The mass defect is the difference between the rest
mass of a nucleus and the sum of the rest masses
of its constituent nucleons.
The whole is less than the sum of the parts!
Consider the carbon-12 atom (12.00000 u):
Nuclear mass = Mass of atom – Electron masses
= 12.00000 u – 6(0.00055 u)
= 11.996706 u
The nucleus of the carbon-12 atom has this mass.
(Continued . . .)
The Binding Energy
The binding energy EB of a nucleus is the energy
required to separate a nucleus into its constituent
parts.
EB = mDc2 where c2 = 931.5 MeV/u
The binding energy for the carbon-12 example is:
EB = (0.098940 u)(931.5 MeV/u)
Binding EB for C-12:
EB = 92.2 MeV
Binding Energy per Nucleon
An important way of comparing the nuclei of
atoms is finding their binding energy per nucleon:
Binding energy EB =  MeV 


per nucleon
A
 nucleon 
For our C-12 example A = 12 and:
EB 92.2 MeV
MeV

 7.68 nucleon
A
12
Mass Defect (Continued)
Mass of carbon-12 nucleus: 11.996706
Proton: 1.007276 u
Neutron: 1.008665 u
The nucleus contains 6 protons and 6 neutrons:
6 p = 6(1.007276 u) = 6.043656 u
6 n = 6(1.008665 u) = 6.051990 u
Total mass of parts: = 12.095646 u
Mass defect mD = 12.095646 u – 11.996706 u
mD = 0.098940 u
Where Elements On the Periodic Table
Came From
(and what the sun tells us)
Origin Of Universe
Our Sun is Our Best Indicator
Our Sun is small – largest sun plotted is 100 times larger
Conditions at the Sun’s core are extreme
temperature is 15.6 million Kelvin
pressure is 250 billion atmospheres
The Sun’s energy produced by nuclear fusion
reactions.
Sun’s Basic Forces
1. Gravity
Significant only when masses are large
Attractive only
2. Electro-magnetic force
Like charges repel; opposites attract
Much stronger than gravity for electrons and protons
3. Strong force
Operates only at very short distances
“felt” only by atomic nuclei
Conditions for Fusion
1. Particles must be hot enough (Temperature)
2. Particles must be in sufficient number (High Density)
3. Particles must be well contained (Confinement Time)
Spin 3:20 – 8:20
Why Doesn’t Sun Explode?
Gravity – Keeps the nuclear reaction under control.
Savage Sun 7:30-12:00
Nuclear Fusion
1. The process that unites small mass nuclei into
larger mass nuclei
2. Extremely large amounts of energy released
3. More efficient at producing energy than fission
(BREAK APART)
Nuclear reactions do create new elements
because they are reactions that involve the
nucleus of an atom, called transmutation.
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
Proton-proton chain
Fusion Video 7:26-10:30
In the Normal Life of a Star
1. Early: nuclear fusion turns
Hydrogen into Helium
2. Late stages: massive
star…
Helium converted into a. heavier
elements (carbon, oxygen, …,
iron)
When a small star has consumed its fuel, Iron is the final product.
Other Elements
1. Big Bang produced most of the hydrogen &
helium today.
2. But other elements (naturally occurring elements
as heavy as Uranium)
It is believe that these elements were formed in the
cores of stars (long after the big bang happened).
Can We Harvest the Sun?
If fusion is a long way off…..
https://www.youtube.com/watch?v=sdHo5fPAnc&list=PL0C99D9DFBBEEB6D6&index=12
MYSTERY OF THE HEAVENS
•
•
•
•
•
•
•
•
90% of the atoms in the universe are hydrogen
atoms.
5% of all atoms are helium
All of the other elements taken together make up 1%
of the universe
Li, Be, and B are mysteriously rare.
Elements of even atomic number are more abundant
than those with odd atomic numbers.
There is a general decline in abundance from oxygen
to lead.
However, there is a very pronounced maximum in
relative abundance around iron.
There are no stable elements with mass numbers
greater than about 210 amu.
ANSWERS FROM CHEMISTRY
• Hydrogen fusion continues for 99% of the lifetime of a star
•When the Hydrogen is essentially depleted, the star collapses,
and core temp is > 100 million Celsius.
•In this helium-burning’ cycle, it is the carbon nucleus first formed
•The direct formation of carbon from helium means that the
process has circumvented lithium, beryllium, and boron.
Note that the slope of the curve
is much steeper in the fusion
region than the fission region
(fusion returns much more
energy per nucleon than fission
Advantages of Fusion
1.
2.
3.
Produces large amounts of energy
4.
No atmospheric pollution leading to acid rain or greenhouse
effect
5.
No long-term storage of radioactive waste needed
Fuels are plentiful
Inherently safe since any malfunction results in a shutdown of
the fusion reaction