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Atomic structure Protons ,Neutron ,Electorn, Bohr model ,Heisenburg Uncertainty Principle
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Electrons in Atoms: Non-Classical Behavior The attraction of electrons (-) to the nucleus(+) obeys Coulomb’s law: ! F = qaqb 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. Quantum Mechanics
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which describes the system in terms of a wavefunction Ψ with energy E.
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.
Stable Electronic Configurations Na: open shell Na+: closed shell Atoms like to adopt closed-shell configurations. To do this, they may gain or lose electrons to become ions Na^0 : open shell Na+: closed shell Stable Electronic Configurations (Cont.) Si^0 : open shell Si4+: closed shell The most stable ionization state of Si is the Si4+^ ion. The semi- closed shell Si2+^ ion does exist in interstellar gas, however.
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.
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. Ionic Radii
Pauling’s Rules for Ionic Structures (cont.) Rule 2 : For an anion to be stable, the sum of the strengths* of the electrostatic bonds that reach an anion from its coordination of cations will equal the charge on the anion. This is the electrostatic valency principle****. (*The Pauling bond strength that a cation gives to an anion is the cation charge/coordination number). Pauling’s Rules for Ionic Structures (cont.) Rule 3: The sharing of edges and faces by coordination polyhedra decreases the stability of a structure. There are two polymorphs of TiO 2. Rutile is more stable (by 6 kJ/mole) than anatase because there is less edge-sharing in the structure. Anatase Rutile
Breakdown of Pauling’s Rules Pauling’s rules will fail when bonds are not ionic. Sulfide minerals tend to have covalent bonds with S. Metallic bonding is also found is metals and many sulfides. Mackinawite (FeS) Triolite (FeS) Breakdown of Pauling’s Rules At high pressure, Pauling’s rules appear to breakdown as cations adopt high coordination numbers. (We could view this as a pressure-induced decrease in the ionic radius of O-2^ relative to metal ion radii.) MgSiO 3 Perovskite Stishovite (SiO 2 )
