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Transcript
IONIC BONDS MAIN GROUP CHEMISTRY Periodicity of Properties Main Group Chemistry IONIC BONDS IONIC SOLIDS • Cation + Anion Ionic Bond • The solid that forms is an ionic solid with a continuous network of cations surrounded by anions and anions surrounded by cations. • The formation of ionic bonds is driven by favorable energy considerations: this is illustrated by the Born-Haber cycle. Born-Haber Cycle (Fig 6.8) • • • • • • • Start with elemental K and F2 (p 215) K(s) K(g) Sublimation energy > 0 K(g) K+(g) + eEi > 0 ½ F2(g) F(g) Dissociation energy > 0 F(g) + e- F-(g) Eea < 0 K+(g) + F-(g) KF(s) - Lattice energy Sum all of these rxns to get energy for K(s) + ½ F2(g) KF(s) -562 kJ/mol Lattice Energy, U • KF(s) K+(g) + F-(g) U>0 • Electrostatic attraction between Cation and Anion. • Table 6.3 ALKALI METALS (1A) • Valence electron config: ns1 • This single s electron is easily lost to form +1 cations. Therefore, these elements have low Ei, are very strong reducing agents (recall Activity Series), metallic, very reactive so they are not found in nature in the elemental form. • Reduction of metal chlorides yield commercial metals. • Table 6.4 RXNS OF 1A METALS • Reactivity of 1A metals increases as you go down group. • + halogen ionic solid metal halide salt • + H2 metal hydride • + N2 metal nitride • + O2 metal oxide, peroxide, superoxide. Note these oxides + water base • + water H2 + metal hydroxide (base) ALKALINE EARTH METALS (2A) • Valence electron config: ns2 • These s electrons are easily lost to form +2 cations. Therefore, these elements have low Ei, are very strong reducing agents (recall Activity Series), metallic, very reactive so they are not found in nature in the elemental form. • Reduction of metal salts yield commercial metals • Table 6.5 RXNS OF 2A METALS • Reactivity of 2A metals increases as you go down group. These reactions are similar to the 1A metals but rxns are less vigorous. • + H2 or N2 metal hydride or nitride • + O2 metal oxide, peroxide, superoxide. Note these oxides + water base • + water H2 + metal hydroxide (base) GROUP 3A ELEMENTS • Valence electron config: ns2 np • These s and p electrons are lost to form +3 cations. Therefore, these elements have low Ei, are reducing agents, metallic except for B. • Table 6.6 • Forms halides, oxides, nitrides. • Reacts with acid and base to form H2 HALOGENS (7A) • Valence electron config: ns2 np5 • These elements readily accept electrons. Therefore, they have high Ei, very negative Eea, are very strong oxidizing agents, nonmetals and exist as diatomics, X2. • Oxidation of anions yield commercial metals • Table 6.7 REACTIONS OF HALOGENS • Reactivity of 7A nonmetals decreases as you go down group. • + metal ionic solid metal halide salt • + H2 hydrogen halide (acid in water) • + Y2 XY • + O2 nonmetal oxides. Note these oxides + water acid • + water acid NOBLE GASES (8A) • Valence electron config: ns2 np6 • These elements are unreactive gases, are neither metal nor nonmetal, have very high Ei and low (@ 0 kJ/mol) Eea. • Table 6.8 • Only Kr and Xe react and they only react with fluorine. OCTET RULE • Main group elements tend to undergo rxns that leave them with 8 valence electrons (octet, Noble gas configuration). • Exceptions occur when there are low energy d orbitals available for additional electrons to fill. (Figure 6.10)