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 0 1
n
1 1
p
0 1
e
Big Bang Theory
Initially the temperature was 10 6 -10 9 K Number of nuclear reactions occurred
z A M
Z = atomic number A = mass number 1 1
H
0 1
n
1 2
H
1 2
H
1 1
H
2 3
He
2 3
He
0 1
n
2 4
He
2 4
He
0 1
n
2 5
He
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 (10 8 g cm -3 ) and the temperature is about 10 7 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 = mc 2 ) More stable nuclei are formed
Hydrogen Burning
Overall
1 1
H
1 1
H
2 1
D
1 2 3 2
D He
1 1
H
2 3
He
4 2 3 2
He He
2 1 1 1 0
e
4 1
H
2 4
He
2 2 2 Concise Inorganic Chemistry, 4 th edition, J.D. Lee, Chapman & Hall Ltd
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 10 8 K, He began to fuse 2 4
He
2 4
He
4 8
Be
The nucleus formed in this way fuse with more He 4 8
Be
2 4
He
12
C
6 12
C
6 2 4
He
16
O
8 16
O
8 2 4
He
20 10
Ne
20
Ne
10 2 4
He
24 12
Mg
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) 12
C
6 1 1
H
13
N
7 13
C
6 1 1
H
14
N
7 14
N
7 1 1
H
15
O
8 15
N
7 1 1
H
2 4
He
12
C
6 13
N
7 13
C
6 15
O
8 15 7
N
- Processes
In temperatures as high as 10 9 K 24
Mg
12 2 4
He
28 14
Si
28
Si
14 2 4
He
32 16
S
32
S
16 2 4
He
36 18
Ar
36 18
Ar
2 4
He
40
Ca
20 These fusion reactions happen up to 56 Fe 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 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