Download Topic 7_1_Ext D__Nuclear structure and force

Topic 7.1 Extended
D –Nuclear Structure and Nuclear Force
Almost everything we observe in the world around us is
sensed by the electromagnetic and the gravitational
forces.
Your seeing an object is because of the interaction
between photons at the electrons orbiting atoms.
Your feeling an object is because of the interaction
between the electrons in your atoms responding to the
electrons in the object you are feeling.
Of course, weight is the other factor you can sense in
an object.
There are four fundamental forces in the universe.
You have studied two of them in detail - namely the
gravitational force (last year) and the electromagnetic
force (this year).
The next slide shows the relative magnitudes of the
four forces of nature:
FYI: In this chapter and the next, we will be studying the strong and the
Topic
7.1 ofExtended
weak forces, completing
our overview
all the fundamental forces of
nature.
D –Nuclear Structure and Nuclear Force
ELECTRO-WEAK
STRONG
ELECTROMAGNETIC
WEAK
GRAVITY
+
+
nuclear
force
STRONGEST
Range:
Extremely Short
Force Carrier:
Gluon
light,
heat and
charge
radioactivity
freefall
WEAKEST
Range:

Range:
Short
Force Carrier:
Photon
Range:

Force Carrier:
Graviton
FYI: An alpha particle () is a double-positively charged particle emitted
Topic
Extended
by radioactive materials
such as 7.1
uranium.
D –Nuclear Structure and Nuclear Force
observing the spectra. But how do we study the
nucleus?
In 1897 British physicist J.J. Thomson
 discovered the electron, and went on to
 propose a "plum pudding" model of the
atom in which all of the electrons were
 embedded in a spherical positive charge
the size of the atom:
In 1911 British physicist Ernest
Rutherford conducted experiments on
the structure of the atom by sending
alpha particles through gold leaf.
If the atomic structure was as
Thomson said, a beam of  particles
should barely be deflected as it
passed through the atom.
Rutherford's experimental results were
quite different, as the next slide
will show:
scintillation screen
We studied the electrons surrounding the nucleus by
Topic 7.1 Extended
D –Nuclear Structure and Nuclear Force
Here we see that the
deflections are much more
scattered...
The nucleus
Rutherford proposed that the
positive charge of the atom was
located in the center, and he
The atom
coined the term nucleus.
"On consideration, I realized that
this scattering backward must be the
result of a single collision, and when
I made the calculations I saw that it
was impossible to get anything of that
order of magnitude unless you took a
system in which the greater part of
the mass of the atom was concentrated
in a minute nucleus."
FYI: By using known values for both mass and speed of the  particle,
Topic
7.1
and the atomic number
of the atom
usedExtended
in the experiment, it was found
thatDthe–Nuclear
radius of the nucleus
was of the order
10-12 m!
Structure
andof Nuclear
Force
FYI: This, by the way, is an upper limit on the size. Why?
In fact, if we look at a head-on collision between
an alpha particle and a nucleus, we can obtain a
rough value for the diameter of a nucleus.
The charge of the alpha particle is 2e, and the charge
of the nucleus is Ze, where Z is the atomic number of
the element.
The alpha particle feels a coulomb potential caused by
the nucleus given by
kQ
kZe
=
V =
r
r
If the  particle approaches from infinity, the work
required to stop it so that it reverses is given by
2
W = qV = 2eV = 2ekZe = 2kZe
rmin
rmin
where rmin is the closest approach of the  particle to
0
the nucleus.
From the work-kinetic energy theorem, W = K = K - K0
2
2
mv
2kZe
so that
=
or
4kZe2
Minimum Radius
rmin =
2
rmin
of a Nucleus
mv2
Topic 7.1 Extended
D –Nuclear Structure and Nuclear Force
All right.
Suppose we have two protons in a
nucleus. Then we have a huge problem. What is it?
Estimate the force between two protons in a helium
atom.
The two positive charges feel the coulomb force:
9109(1.6 10-19)2
kQq
= 0.00023 N
F = 2 =
(110-12)2
r
The two positive charges will want to accelerate apart
with an acceleration of
F
0.00023
= 1.381023 m s-2
=
a =
m
1.6710-27
Since most atoms do not spontaneously disintegrate we
can postulate that there must be a very strong
attractive nuclear force which is able to counter the
repulsive electric force.
Since even in crystals (in which nuclei are very close
together) the nuclei of the nearby atoms are not
attracted to each other, the nuclear force must be very
short ranged.
FYI: Thus there are isotopes of hydrogen, all having the same (+1)
Topic 7.1 Extended
nucleus, but different numbers of neutrons. A neutron is slightly more
D –Nuclear
and Nuclear
massive
then a proton.Structure
In fact, we can compare
the masses ofForce
the
electron, proton, and neutron:
So what, exactly,
a = 1840 electron masses
Mass ofis
proton
nucleus comprised
Mass ofof?
neutron = 1841 electron masses
Perhaps you recall the
FYI: Hydrogen has three common isotopes:
instrument called the
Hydrogen has one proton and no neutrons in the nucleus.
mass spectrometer.
Deuterium has one proton and one neutron.
An element is ionized,
Tritium has one proton and two neutrons.
and accelerated by an
All forms have
a single
applied
voltage
in electron.
the
chamber S.
It is then projected
into a known perpenThrough use of such an
dicular magnetic field.
instrument, scientists
The radius is
discovered different
detected, and the mass
masses for hydrogen
calculated:
nuclei, and postulated
r =
mv
qB
rqB
m = v
the existence of the
neutrally-charged
neutron.
FYI: A = Z + N, so that nuclear reactions do not generally include the N
Topic 7.1 Extended
values (since it can be calculated).
D – Nuclear Structure and Nuclear Force
FYI: Since periodic tables are readily available, we can even get away
with just showing the A number.
An element's characteristics are determined by the
FYI: Tritium
is unstable. But
is stable enough to form heavy
number
of electrons
it deuterium
has (chemistry).
D2O. Note
use of the D
the special isotope
symbol.
water
The number
of the
electrons
isfordetermined
by the
number
of
(opposites
attract).
FYI:protons
A particular
isotope of an
element is called a SPECIES or a
NUCLIDE.
ThereforeThus
it we
follows
that
isotopes
of anof element
have three
species,
or nuclides,
hydrogen.
behave chemically the same.
There are 6 nuclides
of carbon:
FYI:
In chemistry,
you need
only a chemical symbol, such
11
12
13C
14C
15C
16C
C
Chydrogen.
as H, to represent
For
In the
nuclear
physics,weyou
distinguish
the
higher elements,
don'tneed
have to
special
names like we
do for
isotopes.
hydrogen. We name these isotopes like this: carbon-12, carbon-14, etc.
Only carbon-12
and carbon-13 are stable.
NUCLEAR PHYSICS
CHEMISTRY
H
Mass Number = A
H
H
Protons = Z
1
1
H
0
hydrogen
2
1
N = Neutrons
1
deuterium
3
1
H
2
tritium