Download Origin of the Elements and Atomic Structure

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?
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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.
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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)
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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.
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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.
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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
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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.
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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”)
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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.
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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
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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.
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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.
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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
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