Presentation on theme: "Metals: Bonding, Conductivity, and Magnetism (Ch. 6)"— Presentation transcript:
1 Metals: Bonding, Conductivity, and Magnetism (Ch. 6) Big-picture perspective:Metals and alloys are essential to modern technologies, especially as electrical conductors, structural materials, and magnets. We will find that these unique properties arise from the atomic and electronic structures of metals. Most metalsand alloys have relatively simple crystal structures based on sphere packings, although others can be very complex.Learning goals:Identify and assign unit cells, coordination numbers, asymmetric units, numbers of atoms contained within a unit cell, and the fraction of space filled in a given structure.Relate molecular orbital theory to the delocalization of valence electrons in metals.Understand the concepts of electron wavelength and density of states.Understand the consequences of the nearly free electron model for the band structure of metals and their conductivity.Explain why some metals are magnetic and others are diamagnetic, and how these phenomena relate to bonding and orbital overlap.Use the Curie-Weiss law to explain the temperature dependence of magnetic ordering.Acquire a physical picture of different kinds of magnetic ordering and the magnetic hysteresis loops of ferro- and ferrimagnets.
18 Bonding and Conductivity What are the periodic trends?
19 Molecular Orbitals in Metals 1-D Chain of Na atomsWhat is the wavelength of an electron in these MO's?
20 Molecular Orbitals in Metals Infinite chain of Na atomsWhat are the energies of the MO's in metals?
21 Molecular Orbitals in Metals What are the energies of the MO's in metals?Nearly free electron model:KE = ½ mv2 = p2/2m = h2/2mλ2Energyk (= 2π/λ)k = π/aE =h2k28π2m1D D DDensity of StatesEF…(Number of orbitals per unit energy)
22 Metals vs. Insulators (or Semiconductors) Band DiagramsMetals vs. Insulators (or Semiconductors)
23 Conduction in Metals Metals vs. Insulators (or Semiconductors) "Nearly free" electrons conduct electricity and heat
24 Conduction in MetalsElectrons in metals are accelerated by an electric field, but they scatter by interacting with the lattice (lattice vibrations, defects, impurities)The mean free path is long (~40 nm) compared to the atomic spacing (0.2 nm) (football field vs. football). Thus we have an electron "gas"Scattering gives rise to resistance (Ohm's law, V = iR)
25 Bonding, Energetics, and Magnetism Why is Mg (=[Ne]3s2) a metal?The promotion energy (3s s13p1) is less than the bonding energy
26 Bonding, Energetics, and Magnetism How many bonding electrons does each atom have?Why does W have such strong bonding?Why are the 3d elements different from 4d & 5d?
27 Bonding, Energetics, and Magnetism Which transition metals are magnetic?
28 Four kinds of magnetic behavior }No unpaired spins}}Other kinds of spin ordering:AntiferromagneticFerrimagnetic
29 Spin alignment in a magnetic field Paramagnets follow Curie Law behaviorlow T high TSpins align in a magnetic field at low TNo ordering in the absence of an applied field
30 Ferro-, ferri-, and antiferromagnets Above Tc, ferro/ferrimagnets follow the Curie-Weiss LawSpins spontaneously order below TCParamagnetic behavior above TCAntiferromagnets resist parallel alignment of spins=> negative TCAlso paramagnetic above TC
31 Ferro-/ferrimagnets below TC What competing energies cause magnets to have micron-size domains?How do domain walls move in an applied magnetic field?
32 Magnetic hysteresis loops What are hard vs. soft magnets?When would you want a soft magnet?