Transcript Nuclear Chemistry ppt - Warren County Schools

Nuclear Chemistry
Chapter 22
The Nucleus
G Protons and neutrons are collectively
referred to as nucleons.
G In nuclear chemistry, an atom is
referred to as a nuclide, and is
identified by the number of protons
and neutrons.
G Nuclides can be written as 228
88 Ra or as
Radium-228.
An Error in Mass???
Since an atom is made up of protons,
neutrons and electrons, you might assume
that the mass of the atom would be equal to
the sum of the individual subatomic
particles.
Let’s look at 42He:
2 protons: (2 x 1.007276 amu)
2 neutrons: (2 x 1.008665 amu)
2 electrons: (2 x 0.0005486 amu)
An Error in Mass???
When you add the components of the
He atom, you get a mass of 4.032979
amu.
However, the atomic mass of He has
been measured to be 4.00260 amu.
So, as you might have noticed the
calculated mass is 0.03038 amu more
than the actual mass.
How can this be so?
Mass Defect
The difference between the measured
mass and the sum of its protons,
neutrons and electrons is called the
mass defect.
The differences in mass can be
explained by Einstein’s equation :
E= mc2
E=mc2
E stands for energy
m stands for the mass defect
c2 stands for the speed of light squared
In order to use the equation, you must
convert amu to kg, to match the units
for energy (kgm2/s2)
1 amu= 1.6605 x 10-27 kg
Calculating Nuclear Binding
Energy
So using the example of 24He from
before:
0.03038 amu x 1.6605 x 10-27 kg
1 amu
equals 5.0446 x 10-29 kg.
E= (5.0446 x 10-29 kg)(3.00 x 108 m/s)2
= 4.54 x 10-12 kgm2/s2 or J
Nuclear Binding Energy
The nuclear binding energy, that we just
calculated for He, is the energy released
when a nucleus is formed from nucleons.
This binding energy per nucleon is used to
compare the stability of different nuclides.
To find the binding energy per nucleon, you
simply divide the nuclear binding energy by
the total number of nucleons present.
Stability of the Nuclide
The higher the binding energy per
nucleon, the more tightly the nucleons
are held together.
Generally, the elements with
intermediate atomic masses are those
that are most stable.
Band of Stability
When the number of
protons of stable
nuclei are plotted
against the
neutrons, a belt-like
graph is obtained.
Stability
Atoms that have low atomic numbers
are most stable with a 1:1 neutron to
proton ratio. ie: Helium
As the atomic number increases, the
most stable ratio increases to 1.5:1.
ie: Lead-206 has 124 neutrons to 82
protons (1.51:1 ratio).
Strong Nuclear Force
The trend in stability is explained by the
relationship between the nuclear force and
the electrostatic forces between protons.
You know that protons repel each other as
far as the electrostatic force goes. (“like
repels like”).
The strong nuclear force allows protons to
be attracted, but only to protons that are
close to one another.
Strong Nuclear Force
As the atomic number increases, the
electrostatic force between protons
increases more quickly than the
nuclear force.
More neutrons are required to
strengthen the strong nuclear force.
However, beyond the atomic number
83 (Bismuth), no stable nuclides exist.
Magic Numbers
Stable nuclei tend to have even
numbers of nucleons.
This is because the nucleus is most
stable when nucleons (like electrons)
are paired!
The most stable nuclides are those
that have 2, 8, 20, 28, 50, 82, or 126
protons, neutrons or total nucleons.
Nuclear Shell Model
This theory says that nucleons exist in
different energy levels, or shells, in
the nucleus.
2, 8, 20, 28, 50, 82, 126 are
representative of completed nuclear
energy levels and are called magic
numbers.
Nuclear Reactions
A nuclear reaction, or nuclear decay
reaction, is a reaction that affects the
nucleus of an atom.
Unstable nuclei undergo spontaneous
changes that change the number of
protons and neutrons in an atom.
Large amounts of energy are given off
in this process.
How to Solve Nuclear
Reactions
1) Remember that the total of atomic
numbers and mass numbers should be
the same on each side of the equation.
*Notice that when the atomic number
(number of protons) changes, the
identity of the element changes. This
is called transmutation.
Helpful symbols
Neutrons are represented as 10n
Electrons are represented as -10e
Protons are represented as 11p
Examples:
212Po
84
 42He + _____
Mass number: 212 - 4 = 208
Atomic number: 84 - 2 = 82
So the missing nuclide must have a mass of
208 and an atomic number of 82.
Answer: 208
Pb
82
Complete these on your own:
1)
2)
3)
238
93
Np ® b + ______
0
-1
History of Radioactive
Decay
1896- Henri
Becquerel
discovered
radioactivity when a
uranium compound
was left on a
photographic plate.
Even though the
plate was protected
from sun exposure,
it was still exposed
due to the
radioactive x-rays
given off by
uranium.
Radioactive Decay
Defined: the
spontaneous
disintegration of a
nucleus into a slightly
lighter nucleus,
accompanied by the
emission of particles,
electromagnetic
radiation, or both.
This is what
happened to the
photographic plate
in Becquerel’s
experiment.
The Curies
Marie and Pierre found that of the
elements known in 1896, only uranium
and thorium were radioactive.
Marie is credited with the discovery of
Radium and Polonium, both of which
are also radioactive.
Types of Radioactive Decay
Type
Alpha
particle
Beta
particle
Positron
Gamma
ray
Symbol
4
2
He
+2
Mass
(amu)
4.00260
b
-1
0.0005486
b
+1
0.0005486
0
0
0
-1
0
+1

Charge
Alpha Emission
An alpha particle has 2 protons and 2
neutrons bound together and is
emitted during some types of nuclear
decay.
They can be stopped by clothing or
paper because they are so large.
Example reaction:
Beta Emission
A beta particle is an electron emitted
from the nucleus during some kinds of
radioactive decay.
They can be stopped by aluminum foil
or thin metals.
Example reaction:
Positron Emission
A positron is a particle that has the
same mass as an electron but has a
positive charge.
They can be stopped by aluminum foil
and thin metals as well.
Example reaction:
Electron Capture
In electron capture, an inner orbital
electron is captured by the nucleus of
its own atom.
Example reaction:
Gamma Emission
Gamma rays are high-energy
electromagnetic waves emitted from a
nucleus as it changes from an excited
state to a ground state.
Gamma Rays
Unlike the other forms
of radiation we have
discussed, it takes a lot
to stop gamma rays
from penetrating your
skin.
Dense materials, like
lead or concrete, are
used to stop gamma
rays.
Decay Series
Most of the decay
reactions we have
studied thus far
only involve one
transformation.
However, it is often
necessary for a
series of these
reactions to occur
before a stable
nuclide is reached.
The heaviest
nuclide of each
series is called the
parent nuclide.
All the nuclides
produced by the
parent are called
daughter nuclides.
Uranium-238 Decay Series
Artificial Transmutations
Some radioactive
nuclide cannot be
found naturally on
the earth, so they
are called artificial
radioactive
nuclides.
Rutherford’s
apparatus
They are made by
bombardment of
stable nuclei with
charged and
uncharged particles.
Artificial Transmutations
Neutrons are often
used because of
their neutral
charge.
When alpha
particles or protons
are used, they must
be accelerated to
obtain the energy
needed to approach
the positively
charged nucleus.
Getting Radioactive elements
Some radioactive
substances are
found in nature.
However, some can
be created through
artificial
transmutation.
Examples:
Radiation Exposure
Roentgen- unit used to measure nuclear
radiation, equal to the amount of radiation
that produces 2x 109 ion pairs when it passes
through 1 cm3 of dry air.
rems- (roentgen equivalent, man) measures
radiation damage to human tissue
1 rem is the quantity of ionizing radiation
that does as much damage to human tissue
as 1 roentgen of high voltage X-rays.
Exposure to Radiation
The long term effects of continued radiation
damage include cancer and DNA mutations,
causing genetic abnormalities.
Everyone is exposed to background
radiation. The amount of exposure depends
on certain activities. The average exposure
in one year is 0.1 rem. The maximum
permissible dose is 0.5 rem per year.
http://www.epa.gov/rpdweb00/understand
/calculate.html
Radiation Sickness
The larger the dose received at once the
greater the effect on the whole body.
Generally- 25 rem and under cannot be
detected
100 rem reduces white blood cell count
temporarily
>100 rem person experiences nausea,
vomiting and a reduction in white blood cell
count
>300 rem white cell count at zero and
diarrhea, hair loss and infection occur
Lethal Dose
The lethal dose of a substance is often
referred to as LD50. This means that it
is expected to cause death in 50% of
the people with exposure.
The LD50 for humans of radiation is 500
rems.
Dosages of 600 rem would be fatal to
all humans within a few weeks.
LD50 for life forms other than
humans
Insects- 100,000 rems
Bacterium- 50,000 rems
Rat- 800 rems
Humans- 500 rems
Dog- 300 rems
Detecting radiation in your
surroundings
Geiger counterdetect radiation by
counting electric
pulses, kind of
sounds like a metal
detector
Film Badges- used
to measure
exposure of people
working with
radiation
Cloud Chambers
Used to detect
alpha and beta
particles
Chamber is filled
with ethanol or
water vapor. When
the particles collide
with air, ions are
formed.
The vapor
condenses around
the path of ions and
makes it visible.
Applications of nuclear
Chemistry
1) Radioactive
dating- age is
measured by
accumulation of
daughter nuclides or
disappearance of
parent nuclides
2) Radioactive
tracers- radioactive
atoms included in
substances so that
movement can be
followed by
radiation detectors
ie: technetium-99
can be used to
detect bone cancer.
Applications of Nuclear
Chemistry
3) Irradiation of
food
Using radiation to
kill bacteria on
food, like producetomatoes,
blueberries,
mushrooms, and
even spinach
This is in response
to the food borne
illnesses that have
been plaguing the
US.
E. coli
Listeria
Salmonella
Radiation Doses for
Therapeutic Procedures
Lymphoma- 4500
rem
Skin cancer- 50006000 rem
Lung cancer- 6000
rem
Brain tumor- 60007000 rem
Remember that
these are not whole
body dosages.
Many external beam
cancer treatments
are targeted to
specific areas of the
body.