Transcript The Genesis of the Elements

The Genesis of the Elements
Saliya Ratnayaka
For Chem 510
October 22,2004
Theories…?
How the universe was formed?
 How the various elements were formed?
 Why the different elements and their
isotopes occur in the relative abundance we
observe on earth?

Big Bang Theory
All the matter in the universe was packed as
elementary particles into a “nucleus”
 This exploded!
 Dispersed the matter uniformly as neutrons
 These neutrons then decayed

1
0
n11p  10e  
Big Bang Theory


Initially the temperature was 106-109 K
Number of nuclear reactions occurred
A
z
Z=
M
2
atomic number 1
A = mass number

1
1
H  n H
1
0
2
1
H  H  He
1
1
3
2
3
2
He 01n24He
4
2
He n He
1
0
5
2
88.6% H
11.3% He
-------------------------------------------
99.9% of the atoms in
the universe!
Once the temperature drops all these reactions stop
A Star is Born!
Hertzsprung-Russell (H-R) diagram
The Cosmic Perspective, J. Bennett et al, 2002, Pearson Education, Inc.,Publishing as Addison Wesley
A Star is Born!
Eagle Nebula
Hubble Space Telescope
Pleiades
The Cosmic Perspective, J. Bennett et al, 2002, Pearson Education, Inc.,Publishing as Addison Wesley
Synthesis of heavier elements





Stars are extremely dense (108 g cm-3) and the
temperature is about 107 K
Nuclei can undergo nuclear fusion at these
conditions
The first process in the synthesis is Hydrogen
burning
A small amount of mass is lost and energy is
evolved (E = mc2)
More stable nuclei are formed
Hydrogen Burning
1
1
H  H  D   
1
1
2
1
3
2
Overall
4


2
1
D H  He  
1
1
3
2
 
He He He  2 H
1
1
3
2
4
2

1
1
H  24 He  2    2  2
Concise Inorganic Chemistry, 4th edition, J.D. Lee, Chapman & Hall Ltd
 e

0
1
End of the Hydrogen burning






Helium accumulates in
the core
The core begins to
collapse
H shell heats up and H
fusion begins there at a
higher rate
Gravity cannot balance
this pressure
So the outer layers of the
star expand
The star is now in the sub
giant phase of its life
moving to the red giant
phase
The Cosmic Perspective, J. Bennett et al, 2002, Pearson Education, Inc.,Publishing as Addison Wesley
He Burning
When the core collapsed and the temperature reached 108 K, He began to fuse
4
2
He He Be  
4
2
8
4
The nucleus formed in this way fuse with more He
8
4
Be He C  
4
2
12
6
12
6
C  He O  
16
8
O  He Ne  
4
2
20
10
16
8
4
2
20
10
Ne  He Mg  
4
2
24
12
For a smaller mass star this cycle ends with C and become a white dwarf
Carbon-nitrogen Cycle
In larger stars (1.4 times the mass of the Sun or greater)
C H  N  
12
6
1
1
13
7
13
7
N  C   
Decays
13
6

C H  N  
13
6
1
1
14
7
14
7
N H O 
15
7
N  H  He C
1
1
1
1
15
8
4
2
12
6
O Decays
157 N    
15
8
- Processes
In temperatures as high as 109 K
24
12
Mg  He Si  
28
14
Si He S  
32
16
S  He Ar  
36
18
Ar  He Ca  
4
2
4
2
4
2
28
14
32
16
36
18
4
2
40
20
These fusion reactions happen up to 56Fe and are exothermic
The Iron (Fe) Problem

The supergiant has an inert Fe core which collapses
& heats
 Fe can not fuse
 It has the lowest mass per nuclear particle of
any element
 It can not fuse into another element without
creating mass
The Cosmic Perspective, J. Bennett et al, 2002, Pearson Education, Inc.,Publishing as Addison Wesley
Supernova

Gravity makes
electrons combine
with protons to form
neutrons, releasing
neutrinos in the
process
The Cosmic Perspective, J. Bennett et al, 2002, Pearson Education, Inc.,Publishing as Addison Wesley
Supernova
The amount of energy
released is so great,
that most of the
elements heavier than
Fe are instantly created
Crab Nebula in Taurus
supernova exploded in 1054
In the last millennium,
four supernovae have
been observed in our
part of the Milky Way
Galaxy: in 1006, 1054,
1572, & 1604
The Cosmic Perspective, J. Bennett et al, 2002, Pearson Education, Inc.,Publishing as Addison Wesley
Evidence for the formation of heavier elements
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Young stars have more
heavy elements (2-3%
mass) than old stars
(0.1%)
Even numbered nuclei
are relatively abundant
than nearest odd
numbered nuclei
Elements heavier than
iron are extremely rare
The Cosmic Perspective, J. Bennett et al, 2002, Pearson Education, Inc.,Publishing as Addison Wesley
References

1. Concise Inorganic Chemistry - 4th edition, Chapter 31 - by J.D. Lee
2. Inorganic Chemistry - 3rd edition, chapter 1 - by D.F. Shriever, P.W.
Atkins
3. Chemistry and Chemical reactivity - 4th edition, Chapter 24 - by
Kotz and Treichel
4. The Cosmic perspective - 2nd edition, chapter 15- by J. Bennett, M.
Donahue, N. Schneider, M. Voit
5. Astrophysical Formulae -2nd edition, Pages 418-429, K. R. Lang