Teaching Resources
HST 2008
A brief History of our thinking
The Greek thinker Empedocles first classified the
fundamental elements as fire, air, earth, and water,
although our particular diagram reflects Aristotle's
classification.
Did you know?
The ancient Chinese believed that the five basic
components (in Pinyin, Wu Xing) of the physical universe
were earth, wood, metal, fire, and water. And in India,
the Samkhya-karikas by Ishvarakrsna (c. 3rd century
AD) proclaims the five gross elements to be space, air,
fire, water, and earth.
People have long asked,
 "What is the world made of?" and "What holds it together?"
 As far back as in the days of Aristotle, it was thought that
things were made up of four types of fundamental elements.
The word "fundamental" is key here. By fundamental
building blocks we mean objects that are simple and
structureless -- not made of anything smaller.

Based on scientific observations, our thinking has variously
changed in the recent past. The story begins with John
Dalton ......
Dalton suggested that …
… everything is made up of very tiny particles. He named
these smallest possible piece of an element ‘an atom’ –
which in Greek means unbreakable.
Further he suggested that….
•Atoms are different for different elements.
•Imagine atoms to be solid like billiard balls.
Oxygen Atom
Hydrogen
Atom
Gold Atom



From our scientific observations in chemical reactions, x-ray
diffraction etc, we know today that this view was not entirely
correct. Something was missing..
However, Dalton’s ideas became the basis for our modern
quest for the real constituents of matter (and antimatter!)
Possible web link to a CERN site with pictures, simulations or
videos?
Gold
Atom

Proposed after discovery of “cathode rays” when a gas is
ionized by a high voltage.

Neutral atoms contain smaller particles, called electrons.

Electrons exist in a “sea of positive charge” like plums in a
pudding.

Shot high energy a particles at a thin gold foil

Observed the pattern of scattered particles

Found some scattered by a larger angle than predicted by
the Thomson model

Positive charge is concentrated in the nucleus (only way
to produce large scattering angles observed)

Coulomb force causes electrons to orbit the positive
nucleus

Planetary model
Hydrogen
Atom

Accelerating electrons should radiate energy according to
Maxwell’s Electromagnetic theory

Radiating electrons are losing energy so they will spiral in
towards the nucleus as they lose energy

No atom would be stable -- all would radiate away their
energy
Predicted Hydrogen Emission Spectrum as atoms
radiate energy



Hydrogen electrons may only exist at certain distances
from the nucleus (energy levels)
If they stay in the same energy level, they are stable
(don’t radiate)
If they move from one level to another, they radiate to
produce the discrete spectrum observed

Ionized gas atoms are injected into a mass spectrometer

All atoms have the same charge and the velocity selector
guarantees that they had the same velocity

Different radii are observed in the B-field deflection

Only way for this is to have atoms with same charge and
different mass. (R = mv/qB)

There must be a neutral particle in the nucleus with
significant mass = neutron

Atoms with same charge (protons) and different mass
(neutrons) are called Isotopes
electron
nucleus
Helium Atom

Specific Nucleus = nuclide

Nuclear particles (protons and neutrons) = nucleons

Identically charged nuclei with different mass = isotopes

Generic nucleus symbol =

Number of nucleons = A = mass number

Number of protons = Z = charge (atomic) number
A
Z
X




Clearly, if the nucleus contains a
number of protons, the Coulomb
force would predict that the
protons should repel each other
There must some force that is
stronger than the Coulomb force to
hold them together = Strong
Nuclear Force
Strong Nuclear Force also keeps
neutrons in check in the nucleus
Acts like a spring between
nucleons
FE
FE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
Alpha Particle KE
charge = +Ze
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
charge = +2e
charge = +Ze
E = KE
Alpha Particle KE
Alpha Particle PE

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
a Particle
charge =
+2e
charge = +Ze
R
E = PE =
Alpha Particle KE
Alpha Particle PE
(2e)( Ze) 2Ze2

4 o R
4 o R

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
Initial E = (KE)
Final E = PE =
(2e)( Ze)
2Ze 2

4 o R
4 o R
Conservation of Energy: (KE) =
Solve for R =
2 Ze 2
4 o ( KE )
2 Ze 2
4 o R

We can use the results from the scattering of positively
charged particles to determine the nuclear radius
Solve for R =
2 Ze 2
4 o ( KE )
Adjust initial KE until the a particle is no longer scattered
back. Last scattered KE gives estimate of R.
Empirical Result: R = (1.2 x 10-15)A1/3

Alpha particles emitted by an unstable nucleus are found
to have limited possible amounts of kinetic energy

Gamma rays (pure energy) emitted by an excited nucleus
are found to produce discrete spectra

These results suggest that nuclei have energy levels just
like atoms do

The nature of the strong nuclear force is that it is effective
over very small distances
A-Z
12
0
10
0
80
Segre Plot of Stable Nuclides
= stable
60
40
20
10 20
30
40 50
60
70
80
Z
 Small nuclei are stable when the number of neutrons =
number of protons.

The nature of the strong nuclear force is that it is effective
over very small distances
A-Z
12
0
10
0
80
Segre Plot of Stable Nuclides
= stable
60
40
20
10 20
30
40 50
60
70
80
Z
 As Z increases, the number of neutrons necessary becomes
larger than the number of protons.

The nature of the strong nuclear force is that it is effective
over very small distances
A-Z
12
0
10
0
80
Segre Plot of Stable Nuclides
= stable
60
40
20
10 20
30
40 50
60
70
80
Z
 For large values of Z, the number of neutrons becomes very
large.

The nature of the strong nuclear force is that it is effective
over very small distances
A-Z
12
0
10
0
80
Segre Plot of Stable Nuclides
= stable
60
40
20
10 20
30
40 50
60
70
80
Z
• For large nuclei, the Coulomb force wins out over the strong
nuclear force

The nature of the strong nuclear force is that it is effective
over very small distances
= range of nuclear
force
this proton can repel the proton
on the opposite side of the
nucleus
this neutron is too far away
to exert an attractive nuclear
force on the nucleons on the
opposite side of the nucleus
• For large nuclei, the Coulomb force wins out over the strong
nuclear force

The nature of the strong nuclear force is that it is effective
over very small distances
= range of nuclear
force
this proton can repel the proton
on the opposite side of the
nucleus
this neutron is too far away
to exert an attractive nuclear
force on the proton on the
opposite side of the nucleus
• In addition, as the nucleons become too tightly packed the
nuclear force will cause them to repel each other.

To achieve stability, the nucleus must either decrease its
size by emitting clusters of nucleons (a particles)
a particle emission

To achieve stability, the nucleus must either decrease its
size by emitting clusters of nucleons (a particles)
a particle emission
neutron
decays into
proton and b
particle
b particle emission
• Or, it must rearrange the nucleons to balance the nuclear
force and Coulomb force (b particles)