Origin of Elements, Lecture Notes - Chemistry, Study notes of Chemistry

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DM Sherman, University of Bristol

Origin of the Elements

and their Isotopes

Geochemistry, DM Sherman University of Bristol

The Big Bang..

NASA/WMAP

DM Sherman, University of Bristol

The Big Bang..

• Cosmologists have dated the universe to be 13.7 Ga old.
• After the first second (T=10^10 K) matter was present as protons, neutrons and electrons.

The Big Bang..

• After several minutes (T=10^9 K), protons and neutrons combined to form light nuclei 2 H, 3 He and 4 He and 7 Li.

DM Sherman, University of Bristol

Atomic Structure

• Atoms consist of a nucleus ( protons + neutrons ) surrounded* by electrons.
• Number of protons determines the atomic number (element).
• Number of neutrons + protons determines atomic mass ( isotope ).
• Protons are positively charged, neutrons are neutral and electrons are negatively charged. *It’s not really like this..

Star Formation by Condensation

• Stars form by the gravitational condensation of hydrogen clouds.
• High temperatures and pressures allow nuclear fusion of light element nuclei. Star formation in Orion Nebula ( NASA, Hubble )

DM Sherman, University of Bristol

Nuclear Fusion

Nuclear fusion occurs between light nuclei when subjected to extremely high pressures and temperatures: Mass deficit (Δm) releases energy (E= mc^2 )

Stellar Nucleosynthesis via Fusion

4p → 4 He In a medium star’s interior, temperatures and pressures are high enough to cause fusion of H atoms (protons) into 2 H, 3 He, 4 He nuclei. Hydrogen burning: (^1) H + 1 H → 2 H + γ (^2) H + 1 H → 3 He + γ (^3) He + 3 He → 4 He + 2 (^1) H + γ Plus trace Li, Be and B

DM Sherman, University of Bristol

The Curve of Nuclear Binding Energy

Synthesis of elements with Z > 26 (Fe) is not favored by direct fusion. Fusion Fission

α-decay

Alpha decay is a type of nuclear fission that occurs by spontaneous emission of an alpha particle (^4 He)

DM Sherman, University of Bristol

Magic Numbers and Stable Nuclei

Nuclei with even numbers of protons and neutrons are more stable than those with odd numbers. In particular, nuclei with 2, 8, 20, 28, 50, 82 and 126 nucleons (protons or neutrons) are especially stable. (^4) He 16 O 40 Ca 48 Ca 208 Pb 2 8 20 20 82 This motivates the shell model of the nucleus in analogy with electronic configurations.

Chart of the Nuclides

Neutrons Protons β- Z N Z N Z- N- Z- N+ Z+ N- β+

DM Sherman, University of Bristol

Example: S-process from Ag to Sb

The R-Process (Rapid Neutron Capture)

• After all the material in the star’s core is converted to Fe, the star can no longer produce energy.
• The star collapses; the heat released from gravitation causes a massive explosion known as a supernova.
• The explosion yields a large flux of neutrons.
• Heavy elements (Z > 26) are synthesised by neutron capture. Crab Nebula: a remnant of a supernova explosion.

DM Sherman, University of Bristol

Formation of Li, Be and B

Li, Be and B have low binding energies Li, Be and B are destroyed in steller interiors. Although some 7 Li formed in the big bang; other Li isotopes along with Be and B formed by spallation processes.

Element Origin Summary

DM Sherman, University of Bristol Summary

• Big Bang nucleosynthesis predicts correct abundance of light nuclei.
• Steller nucleosynthesis:
  • Fusion by H, C, O, Si burning
  • S-process (neutron capture + beta decay)
• Spallation
• Explosive nucleosynthesis (supernovae):
  • R-process (neutron capture + beta decay) Reading : White, Chapter 10 (http://www.geo.cornell.edu/geology/classes/Chapters/Chapter10.pdf)