Chapter_10_Notes

Chapter_10_Notes - Alkali Metals Li Na K Rb Cs What should...

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1 Alkali Metals Shriver, Chapter 10 Li, Na, K, Rb, Cs What should everyone know about the alkali metals? - very reactive in the metallic form - low-melting and soft (can cut them with a spatula) - easy to ionize (stable M + ) - salts are generally very soluble Origins Potassium gets its name from naturally-occurring potash (K 2 CO 3 ) K 2 CO 3 obtained by extraction from wood ash for use in making glass, soap metallic K first isolated by electrolysis of molten potash symbol K comes from its Latin name, kalium (means alkali) Sodium gets its name from soda ash, Na 2 CO 3 obtained from natural deposits of the same mineral, called natron symbol Na comes from its Latin name, natria Lithium was first isolated from minerals (in contrast to wood origin of K) Name comes from Greek lithos (stone) Rubidium and Cesium were first discovered as traces in mineral water by spectroscope Both are named after prominent lines in their spectra rubidus : deepest red caesius , sky blue Francium predicted by Mendeleev discovered by Marguerite Perey, an assistant to Mme Curie named for France radioactive, very rare What are they like, physically? High thermal and electrical conductivity Low melting points: Li (180 °C); Na (97 °C); K (63 °C); Rb (39 °C); Cs (28 °C) Soft: you can cut them with a not-very-sharp knife (or spatula) body-centered cubic structure, not close-packed, so not very dense metallic bonding is weak (only 1 bonding electron per metal: ns 1 ) Each Na atom has 8 neighbors, so if we consider only the unit cell , we could combine the 9 valence atomic orbitals to create a series of bonding, nonbonding and antibonding orbitals:
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2 !" ! 3s 3s nb bcc unit cell In a particular layer, the atoms are arranged in a square. The next layer has the same square structure, displaced so that the atoms lie over the holes in the first layer. Each atom has only 8 nearest neighbors (four above, and four below). There are six next- nearest neighbors in the same layer. top view side view In the solid, each neighbor has 8 neighbors, and so on. The molecular orbitals therefore extend over the entire solid. There are so many of them, and they are so close in energy, that it is no longer reasonable to talk about molecular orbitals. Instead, the orbital combinations generate bands . The bonding orbitals form the valence band, while the antibonding orbitals form the conduction band. The separation between valence and conduction bands is called the band gap. Since the number of orbitals is not evenly distributed over all energy levels in the band, a more informative representation of the bands includes information about the abundance of orbitals – called the density of states . valence band N 3s conduction band band gap density of states Fermi level energy
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3 Valence electrons fill the bonding orbitals first, in order of ascending energy. If the valence band is filled, and the band gap is large, electrons do not have enough thermal energy to populate the conduction band: the material is an insulator . If the band gap is small, a few electrons will reach the conduction band: the material is a
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