Chapter 09 - Chapter 9: Ionic and Covalent Bonding Ionic...

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Chapter 9 09 1 of 7 Chapter 9: Ionic and Covalent Bonding Ionic Bonds In Chapter 8, we studied electron configurations and their relationship to the periodic table. In Chapter 9 we will study the bonds that elements form. This, too, is related to periodic properties. 1. Describing Ionic Bonds An ionic bond is a bond formed by the electrostatic attraction between cations (positive ions) and anions (negative ions). The ions form when one or more electron is transferred from one atom to another. Atoms lose or gain electrons, in order to form ions that have a noble gas or pseudo-noble gas configuration. a. Lewis Electron-Dot Symbols The Lewis electron-dot symbol is a symbol based on the chemical symbol for an element. The chemical symbol is surrounded by dots that represent the valence electrons of that element. The dots are arranged in an imaginary square around the chemical symbol, one per side before doubling up. This results in elements within a group having the same number of dots. Table 9.1, below show the Lewis electron-dot symbols for the second and third period main-group elements. b. Energy Involved in Ionic Bonding The energy to form an ionic compound (bond) can be thought of as the sum of several processes. Each element begins in its standard state. For metals the standard state is generally as a monatomic solid. For nonmetals, the standard state is often a diatomic molecule and is sometimes liquid or solid. So, the first step is to make each element gaseous: for solids this is the sublimation energy; for liquids this is the vaporization energy. Both requires energy. If the nonmetal is diatomic, that bond must be broken to form monatomic atoms: this is the dissociation energy and also requires energy. The next step is to make each an ion. For metals that involves the ionization energy; this requires energy. For nonmetals that involves the electron affinity; this generally releases energy. The attraction of the two gaseous ions into a solid ionic compound releases energy.
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Chapter 9 09 2 of 7 The cycle of these energies gives us the lattice energy: the energy change when a solid ionic compound is separated into gaseous ions. This process of breaking down the steps in order to calculate the lattice energy is called the Born-Haber cycle. c. Properties of Ionic Substances Ionic compound are almost exclusively solid crystalline compounds. Most have very high melting points. This is due to the strength of the ionic attraction. The strength depends on two factors: the charge on the ions (higher charges result in a stronger force) and the size of the ions (the smaller the ions the stronger the force – inverse square relation). Relatively recently ionic liquids have been discovered.(S EE A C HEMIST L OOKS A T Ionic Liquids and Green Chemistry, page 335.) 2. Electron Configurations of Ions a. Ions of the Main Group Elements Metals will form cations by losing electrons to attain a noble gas configuration. Nonmetals will form anions by gaining electrons to attain a noble gas configuration.
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Chapter 09 - Chapter 9: Ionic and Covalent Bonding Ionic...

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