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Geochemistry DM Sherman, University of Bristol 2006/2007 Origin of the Elements and Atomic Structure Geochemistry, DM Sherman University of Bristol The big questions of geochemistry.. • How did the Earth form and differentiate? • What is the composition of the core and mantle? • What were the chemical conditions that allowed the origin of life? • Why do ore-deposits form? • How are the different Earth systems ( lithospherebiosphere-hydrosphere-atmosphere) coupled to each other. • What is the fate of toxic pollutants in the environment? Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 The Big Bang.. • Cosmologists have dated the universe to be 13.7 Ga old. • After the first second (T=1010 K) matter was present as protons, neutrons and electrons. The Big Bang.. • After several minutes (T=109 K), protons and neutrons combined to form light nuclei 2H, 3He and 4He. Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 The Big Bang.. • After 100,000 years (T= 5000 K) neutral atoms of H and He were able to form. 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) Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 Stellar Nucleosynthesis via Fusion 4p → 4He In the sun’s interior, temperatures and pressures are high enough to cause fusion of H atoms (protons) into He nuclei. Stellar Nucleosynthesis via Fusion • If a star is > 8Ms it will eventually enter a brief phase where all He is burned up. • The star will contract and begin other fusion reactions that form elements up to Fe. Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 The Curve of Nuclear Binding Energy Synthesis of elements with Z > 26 (Fe) is not favored by direct fusion. Supernovae and the Heavy Elements • 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. Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 Atomic Structure • Electrons bound to an atom exist in quantized energy states. • This cannot be explained by classical physics. The solar abundance of elements is determined from the absorption lines due to electronic excitations Solar (“Cosmic”) Abundance of Elements This is determined from spectroscopic measurements of the sun’s photosphere Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 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. Electrons in Atoms: Non-Classical Behavior The attraction of electrons (-) to the nucleus(+) obeys Coulomb’s law: F= ! qa q b 4"# 0 r 2 In atoms, however, electrons do not collapse into the nucleus but instead behave like spherical waves with quantized energies and angular momenta. Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 Quantum Mechanics • Instead of Newton’s equations of motion, we solve the Schrodinger Equation ! h2 2m " 2 # + V# = E# which describes the system in terms of a wavefunction Ψ with energy E. •Explains wave/particle duality and quantized states of electrons in atoms. Electronic States and quantum numbers For simplicity, we can represent the electronic wavefunctions in terms of quantum numbers: • Principle quantum number: n =1,2,3… • Angular momentum quantum number: l = 0,1,2,…, n-1. (s, p, d and f for l = 0,1, 2 and 3) • Magnetic quantum number: ml = -l,- l +1,…,+l. • Spin quantum number: ms = +1/2 and -1/2 (“up” and “down”) Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 The Quantum Numbers and the Periodic Table of the Elements Electronic Configurations: Pauli Exclusion Principle We can approximate the electronic structures of multielectronic atoms as electronic configurations over one-electron (hydrogen-like) orbitals. The Pauli Exclusion principle states that no two electrons in an atom can have the same four quantum numbers. Hence, we can only put two electrons (one spin-up and one spin-down) in each nlm orbital. Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 Stable Electronic Configurations Atoms like to adopt closed-shell configurations. To do this, they may gain or lose electrons to become ions 0 : open shell NaNa: open shell : closed Na+Na : closed shellshell + Stable Electronic Configurations (Cont.) The most stable ionization state of Si is the Si4+ ion. The semiclosed shell Si2+ ion does exist in interstellar gas, however. Si0: open shell Si4+: closed shell Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 Stable Electronic Configurations (Cont.) Iron cannot adopt a closed-shell configuration... Fe0: open shell Fe3+: open shell Ions and Electronic Configurations Whether an atom will gain electrons to become an anion or lose electrons to become a cation depends on its electronegativity relative to other atoms. Mg + O → Mg2+ + O2- Atoms with high electronegativity tend to become anions. More precisely, if two neutral atoms come together, the more electronegative atom will receive electrons and become the anion. Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 Electronegativity Stable Ions and Oxidation states The full ionic charge is only realized in completely ionic compounds. Otherwise it is only the formal oxidation state. Page ‹#› Geochemistry DM Sherman, University of Bristol 2006/2007 Chemical bonding: ionic vs. covalent When electrons are completely transferred between atoms to yield cations and anions, the atoms will be held together by ionic bonds. If atoms have similar electronegativities, they adopt closed-shell configurations by sharing electrons with each other; the atoms are held together by covalent bonds. Summary • Origin of the cosmic abundance pattern. – – – – Big Bang Stellar nucleosynthesis via fusion Cosmic dust formed by supernovae. Heavy elements via neutron capture • Electronic Structures of the elements and ions – Quantum numbers and electronic configuration – Stable ions in geochemistry Page ‹#›