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 F 2 (p 215) K(s)  K(g)Sublimation energy > 0 K(g)  K + (g) + e - E i > 0 ½ F 2 (g)  F(g)Dissociation energy > 0 F(g) + e -  F - (g) E ea < 0 K + (g) + F - (g)  KF(s) - Lattice energy Sum all of these rxns to get energy for K(s) + ½ F 2 (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: ns 1 This single s electron is easily lost to form +1 cations. Therefore, these elements have low E i, 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 + H 2  metal hydride + N 2  metal nitride + O 2  metal oxide, peroxide, superoxide. Note these oxides + water  base + water  H 2 + metal hydroxide (base)

ALKALINE EARTH METALS (2A) Valence electron config: ns 2 These s electrons are easily lost to form +2 cations. Therefore, these elements have low E i, 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. + H 2 or N 2  metal hydride or nitride + O 2  metal oxide, peroxide, superoxide. Note these oxides + water  base + water  H 2 + metal hydroxide (base)

GROUP 3A ELEMENTS Valence electron config: ns 2 np These s and p electrons are lost to form +3 cations. Therefore, these elements have low E i, are reducing agents, metallic except for B. Table 6.6 Forms halides, oxides, nitrides. Reacts with acid and base to form H 2

HALOGENS (7A) Valence electron config: ns 2 np 5 These elements readily accept electrons. Therefore, they have high E i, very negative E ea, are very strong oxidizing agents, nonmetals and exist as diatomics, X 2. 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 + H 2  hydrogen halide (acid in water) + Y 2  XY + O 2  nonmetal oxides. Note these oxides + water  acid + water  acid

NOBLE GASES (8A) Valence electron config: ns 2 np 6 These elements are unreactive gases, are neither metal nor nonmetal, have very high E i and low 0 kJ/mol) E ea. 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)