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Teaching Resources. HST 2008. Are we really made up of Stars?. 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 ? - PowerPoint PPT Presentation
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Teaching Resources
HST 2008
Are we really made up of Stars?
A brief History of our thinkingThe 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.
So what is the MATTER?
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.
What about more recently? 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.
Dalton Atomic Model
Oxygen Atom Hydrogen Atom
Gold Atom
This makes a lot of sense. But!
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?
Thomson Atomic Model
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.
Geiger-Marsden Experiment
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
Rutherford Atomic 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
Problems with Rutherford Model
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
Hydrogen Atom
Problems with Rutherford’s ModelPredicted Hydrogen Emission Spectrum as atoms radiate energy
Bohr’s Solution
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
Neutrons 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
Refined Atomic Model (Bohr)
Helium Atom
electron
nucleus
Vocabulary 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
AZ X
Nuclear Forces 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
Estimating Nuclear Size
We can use the results from the scattering of positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size
We can use the results from the scattering of positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size
We can use the results from the scattering of positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size
We can use the results from the scattering of positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle
Estimating Nuclear Size
We can use the results from the scattering of positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle charge = +2e
E = KE
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
charge = +Ze
Alpha Particle KE Alpha Particle PE
a Particle charge = +2e
E = PE =
R
2(2 )( ) 24 4o o
e Ze ZeR R
Estimating Nuclear Size We can use the results from the scattering of
positively charged particles to determine the nuclear radius
Final E = PE =2(2 )( ) 2
4 4o o
e Ze ZeR R
Initial E = (KE)
Conservation of Energy: (KE) = 22
4 o
ZeR
Solve for R = 22
4 ( )o
ZeKE
Estimating Nuclear Size
We can use the results from the scattering of positively charged particles to determine the nuclear radius
Solve for R = 22
4 ( )o
ZeKE
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
Nuclear Energy Levels 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
Nuclear Stability The nature of the strong nuclear force is that it
is effective over very small distances
Small nuclei are stable when the number of neutrons = number of protons.
Z
A - Z
10 20 30 40 50 60 70 80
20
40
60
80
100
120
= stable
Segre Plot of Stable Nuclides
Nuclear Stability The nature of the strong nuclear force is that it
is effective over very small distances
As Z increases, the number of neutrons necessary becomes larger than the number of protons.
Z
A - Z
10 20 30 40 50 60 70 80
20
40
60
80
100
120
= stable
Segre Plot of Stable Nuclides
Nuclear Stability The nature of the strong nuclear force is that it
is effective over very small distances
For large values of Z, the number of neutrons becomes very large.
Z
A - Z
10 20 30 40 50 60 70 80
20
40
60
80
100
120
= stable
Segre Plot of Stable Nuclides
Nuclear Stability The nature of the strong nuclear force is that it
is effective over very small distances
• For large nuclei, the Coulomb force wins out over the strong nuclear force
Z
A - Z
10 20 30 40 50 60 70 80
20
40
60
80
100
120
= stable
Segre Plot of Stable Nuclides
Nuclear Stability The nature of the strong nuclear force is that it
is effective over very small distances
• For large nuclei, the Coulomb force wins out over the strong nuclear force
= range of nuclear force
this proton can repel the proton on the opposite side of the nucleusthis neutron is too far away to exert an attractive nuclear force on the nucleons on the opposite side of the nucleus
Nuclear Stability The nature of the strong nuclear force is that it
is effective over very small distances
• In addition, as the nucleons become too tightly packed the nuclear force will cause them to repel each other.
= range of nuclear force
this proton can repel the proton on the opposite side of the nucleusthis neutron is too far away to exert an attractive nuclear force on the proton on the opposite side of the nucleus
Nuclear Stability To achieve stability, the nucleus must either
decrease its size by emitting clusters of nucleons (a particles)
a particle emission
Nuclear Stability To achieve stability, the nucleus must either
decrease its size by emitting clusters of nucleons (a particles)
a particle emission
• Or, it must rearrange the nucleons to balance the nuclear force and Coulomb force (b particles)
b particle emissionneutron decays into proton and b particle
