ionisation of metals- (4) - Important Terms and Concepts 27...

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Important Terms and Concepts 27 condensed liquid and solid states, bonds between molecules are weak secondary ones. Consequently, molecular materials have relatively low melting and boiling temperatures. Most of those that have small molecules composed of a few atoms are gases at ordinary, or ambient, temperatures and pressures. On the other hand, many of the modern polymers, being molecular materials composed of extremely large molecules, exist as solids; some of their properties are strongly dependent on the presence of van der Waals and hydrogen secondary bonds. S U M M A R Y This chapter began with a survey of the fundamentals of atomic structure, presenting the Bohr and wave-mechanical models of electrons in atoms. Whereas the Bohr model assumes electrons to be particles orbiting the nucleus in discrete paths, in wave mechanics we consider them to be wavelike and treat electron position in terms of a probability distribution. Electron energy states are specified in terms of quantum numbers that give rise to electron shells and subshells. The electron configuration of an atom corre- sponds to the manner in which these shells and subshells are filled with electrons in compliance with the Pauli exclusion principle. The periodic table of the elements is generated by arrangement of the various elements according to valence electron configuration. Atomic bonding in solids may be considered in terms of attractive and repulsive forces and energies. The three types of primary bond in solids are ionic, covalent, and metallic. For ionic bonds, electrically charged ions are formed by the transference of valence electrons from one atom type to another; forces are coulombic. There is a sharing of valence electrons between adjacent atoms when bonding is covalent. With metallic bonding, the valence electrons form a ‘‘sea of electrons’’ that is uniformly dispersed around the metal ion cores and acts as a form of glue for them. Both van der Waals and hydrogen bonds are termed secondary, being weak in comparison to the primary ones. They result from attractive forces between electric dipoles, of which there are two types—induced and permanent. For the hydrogen bond, highly polar molecules form when hydrogen covalently bonds to a nonmetallic element such as fluorine. I M P O R T A N T T E R M S A N D C O N C E P T S Atomic mass unit (amu) Atomic number Atomic weight Bohr atomic model Bonding energy Coulombic force Covalent bond Dipole (electric) Electron configuration Electron state Note: In each chapter, most of the terms listed in the ‘‘Important Terms and Concepts’’ section are defined in the Glossary, which follows Appendix E. The others are important enough to warrant treatment in a full section of the text and can be referenced from the table of contents or the index.
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  • Spring '14
  • Colombo
  • Atom, Electron, Materials, Metals, Chemical bond, Van der Waals

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