Nuclear synthesis

1. K.SATHISH KUMAR
M.ED
DEPT OF EDUCATION

2. WHERE DID ALL THE ELEMENTS COME FROM??
1.In the very beginning, both space and time were created in the Big Bang. It happened 13.7 billion
years ago. Afterwards, the universe was a very hot, expanding soup of fundamental particles.
2.The universe expanded rapidly during inflation and expands at a more or less constant rate now. As it
grows, it cools.
3.At first, the universe was dominated by radiation. Soon, quarks combined together to form baryons
(protons and neutrons). When the universe was 3 minutes old, it had cooled enough for these protons
and neutrons to combine into nuclei.
4.This is known as the time of nucleosynthesis. Hydrogen, helium, lithium, and beryllium were
produced.
5.Today, about 90% of the universe is still hydrogen.
6.Remember that only the nuclei of these atoms were created at this time.
7.The universe was still far too hot to allow these nuclei to attract electrons and form atoms.
8.That didn’t happen for another 300,000 years, at the time of recombination.
9.At this time the universe had cooled sufficiently for atoms to exist.
10.Echoes from these first atoms can still be seen in the Cosmic Microwave Background. But this
process only created the lightest four elements. How did the other 88 natural elements come about? To
understand this, we need to understand fusion.

3. 1. All protons have positive charge and therefore repel one other.
2. How then, can they be packed tightly into a nucleus? What holds them there? The answer is
another force, the strong force.
3. When protons collide with enough energy, their electric repulsion can be overwhelmed by this new
force and they will be bound together.
4. Similar processes take place between neutrons and protons and between small nuclei.
5. This building up of heavier nuclei is called nuclear fusion.
6. Three important fusion processes are: the proton-proton chain which details how helium is made in
our sun, the CNO cycle which explains how hydrogen is fused in hotter stars, and the
triple alpha process which accounts for the helium fusing that occurs in mature stars.
7. Sketches of these three processes are shown below.
8. In each case, the sketch is incomplete. These reactions occur in chains with more than one possible
path. Only the most probable paths are shown below.
Proton-proton chain

4. Triple alpha process
1. Another important concept is the behavior of an ideal gas. When a gas is compressed, its temperature increases.
2. When a gas expands, its temperature drops.
3. This is why a bike pump warms up when in use. It is also why air rushing out of a balloon feels cool. Now back
to our story.
4. The atoms left over by the big bang were gravitationally attracted to one another and condensed into huge
clouds.
5. The gravitational pressure on the centers of these clouds heated them to temperatures of millions of degrees.

5. 1. This led to the fusion of hydrogen into helium. Stars were born.
2. As the fuel in their cores is used up, the cores shrink and warm. This causes
hydrogen fusing to occur in the outer parts of the star, its shell. This new source of
energy causes the star to expand and cool, turning it into a red giant.
1. Once all the hydrogen in the core has been used up, helium is fused into carbon,
nitrogen and oxygen. This causes the core to expand and cool once again.
2. The shell, which is still fusing hydrogen, also cools. When all the helium has been
used, the core contracts and warms. This heats up the shell which now starts fusing
helium. The star enters a second red giant phase.

6. 1. During these cycles, convection within the star increases.
2. After hydrogen fusing is stopped, the first dredge-up carries oxygen, nitrogen, and carbon from the CNO
process to the surface of the star.
3. After helium fusing stops, a second dredge-up carries more of these elements from the triple alpha reactions
to the surface.
4. If the star has a mass greater than 2 solar masses, a third dredge-up can occur. This time mostly carbon and
carbon molecules are brought to the surface.
5. These stars are called AGB or carbon stars.
6. They have very active solar winds and are typically surrounded by a sooty planetary nebula.
1. At this point, low-mass stars (below 8 solar masses) eject their outer layers and evolve into white dwarfs.
2. White dwarfs are carbon-oxygen-rich and made of super-dense matter.
3. Most of their hydrogen and helium are lost to the stellar wind.
4. These stars are so dense that they form a new type of “degenerate” or nuclear matter.

7. 1. High-mass stars do not evolve into white-dwarves.
2. The gravitational pressure on these stars does not allow their cores to expand and
cool.
3. Their gravity is so great that not even the degenerate electron pressure in the core
can cool it.
4. After helium fusing ends in these stars, carbon fusing begins creating oxygen, neon,
sodium, and magnesium.

8. 1. If the star has a mass of more than 8 solar masses (called a supergiant) its gravity is strong enough to fuse
neon after it has finished fusing carbon.
2. This forms more oxygen and magnesium.
3. Afterwards, oxygen will be fused into sulfur, silicon, phosphorus, and magnesium.
4. Then silicon will be fused to new elements as heavy as iron.
5. Each stage burns more quickly, at a higher temperature, and at a greater density.
6. As each new stage begins, the earlier stages continue their reactions in onion-like shells about the core.
7. The star heats up and expands as each new phase begins. It constantly loses mass to its stellar wind.

9. 1. Some of the reactions which lead to the creation of these new elements
also produce neutrons.
2. These neutrons are captured by some of the new atoms to form different
isotopes.
3. This process cannot create a nucleus with more than 26 protons (iron)
because at that point the energy it takes to overcome the electric
repulsion is greater than the energy released by the strong force.
4. Creating elements heavier than iron requires energy, it doesn’t release
energy.

10. 1. The heaviest elements are created in supernovae, the fantastic death of supergiant stars.
2. As the core of the supergiant becomes saturated with iron, its pressure and temperature increase.
3. Eventually, the blackbody radiation from the core produces gamma rays powerful enough to break apart the iron
atoms in the core.
4. This further increases the pressure to a point where electrons and protons are fused into neutrons.
5. This releases lots of energy in the form of neutrinos.
6. The core cools and contracts; the inner shells rush to fill the void.
7. As the core reaches nuclear density it become rigid and even bounces back a little.
8. When the onrushing material feels this bounce, it creates a wave.
9. As the wave spreads to outer, less-dense regions, it speeds up.
10. Soon it is a shock wave and combines with the wave of neutrinos.
11. The star is doomed. This process blows the star apart releasing 1046
joules of energy. This shock wave is the only
place hot and dense enough to fuse elements heavier than iron, elements up to and including uranium.

11. 1. The supernova just described is termed a type II supernova because it contains the emission lines of metals.
2. Type I supernovae have no such lines. They are caused by a binary star system with a red giant and a white dwarf
(remember that 70% of stars are binary).
3. Matter that flows off the red giant collects on the white dwarf and increases the pressure on its core.
4. Eventually carbon fusing begins in the white dwarf, but this time the star is made of “degenerate” matter and
cannot expand to cool off. The result is a runaway reaction which ends in a supernova.
SO THAT’S WHERE THE ELEMENTS COME FROM!!
1.The first four elements were present after the big bang.
2.Elements up through magnesium were created in red giants. Elements up through iron were created in supergiants
and the elements from iron to uranium were created in supernovae.
3.These atoms can meet one another in nebulae, on dust particles, and even in planets.
4. When they do, chemical reactions take place. These reactions eventually led to life and then . . . us.

12. Models of the AtomModels of the Atom
a Historical Perspectivea Historical Perspective

13. Aristotle
Early Greek Theories
• 400 B.C. - Democritus thought matter could not
be divided indefinitely.
• 350 B.C - Aristotle modified an earlier theory
that matter was made of four “elements”: earth,
fire, water, air.
Democritus
• Aristotle was wrong. However, his theory
persisted for 2000 years.
fire
air
water
earth
• This led to the idea of atoms in a void.

14. John Dalton
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
• All matter is made of atoms.
• Atoms of an element are identical.
• Each element has different atoms.
• Atoms of different elements combine in
constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed) and
the law of constant composition (elements combine
in fixed ratios).

15. Adding Electrons to the Model
1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thompson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
Materials, when rubbed, can develop a charge difference. This
electricity is called “cathode rays” when passed through an
evacuated tube (demos).
These rays have a small mass and are negative.
Thompson noted that these negative subatomic particles were
a fundamental part of all atoms.

16. Ernest Rutherford (movie: 10 min.)
Most particles passed through. So,
atoms are mostly empty.
Some positive α-particles deflected
or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
Radioactive
substance path of invisible
α-particles
• Rutherford shot alpha (α) particles at gold foil.
Lead block
Zinc sulfide screen Thin gold foil

17. Bohr’s model
There are 2 types of spectra: continuous spectra & line
spectra. It’s when electrons fall back down that they release
a photon. These jumps down from “shell” to “shell” account
for the line spectra seen in gas discharge tubes (through
spectroscopes).
• Electrons orbit the nucleus in “shells”
•Electrons can be bumped up to a higher shell if hit
by an electron or a photon of light.

18. Atomic numbers, Mass numbers
• There are 3 types of subatomic particles. We already know about
electrons (e–
) & protons (p+
). Neutrons (n0
) were also shown to exist
(1930s).
• They have: no charge, a mass similar to protons
• Elements are often symbolized with their mass number and atomic
number
E.g. Oxygen: O
16
8
• These values are given on the periodic table.
• For now, round the mass # to a whole number.
• These numbers tell you a lot about atoms.
# of protons = # of electrons = atomic number
# of neutrons = mass number – atomic number
• Calculate # of e–
, n0
, p+
for Ca, Ar, and Br.

19. 3545358035Br
1822184018Ar
2020204020Ca
e–
n0
p+
MassAtomic

20. 3 p+
4 n0
2e–
1e–
Li shorthand
Bohr - Rutherford diagrams
• Putting all this together, we get B-R diagrams
• To draw them you must know the # of protons, neutrons, and electrons
(2,8,8,2 filling order)
• Draw protons (p+
), (n0
) in circle (i.e. “nucleus”)
• Draw electrons around in shells
2 p+
2 n0
He
3 p+
4 n0
Li
Draw Be, B, Al and shorthand diagrams for O, Na

21. 11 p+
12 n°
2e–
8e–
1e–
Na
8 p+
8 n°
2e–
6e–
O
4 p+
5 n°
Be
5 p+
6 n°
B
13 p+
14 n°
Al

22. Isotopes and Radioisotopes
• Atoms of the same element that have different numbers of
neutrons are called isotopes.
• Due to isotopes, mass #s are not round #s.
• Li (6.9) is made up of both 6
Li and 7
Li.
• Often, at least one isotope is unstable.
• It breaks down, releasing radioactivity.
• These types of isotopes are called radioisotopes
Q- Sometimes an isotope is written without its atomic number - e.g.
35
S (or S-35). Why?
Q- Draw B-R diagrams for the two Li isotopes.
A- The atomic # of an element doesn’t change Although the number
of neutrons can vary, atoms have definite numbers of protons.

23. 3 p+
3 n0
2e–
1e–
6
Li 7
Li
3 p+
4 n0
2e–
1e–
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