Ionic Bonding Chapter 8 1

Ionic Compound Review Formed between cation (positive metal ion) and anion (negative nonmetal ion) Formed by elements on opposite sides of periodic table

Ionic Compound Structure Cations and anions in an ionic crystal are arranged in a systematic, periodic 3-D array that maximizes the attractive forces among cations and anions while minimizing the repulsive forces Determined by forces of Coulomb’s Law Larger charged ions, lead to stronger ionic compounds Smaller ions, lead to stronger forces holding the compound together

Energetics of Ionic Bonding it takes 495 kJ/mol to remove electrons from sodium. We get 349 kJ/mol back by giving electrons to chlorine. 4

Energetics of Ionic Bonding But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic! 495- 349 = 146 (endothermic 5

Energetics of Ionic Bonding There must be a third piece to the puzzle. What is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion. 6

Lattice Energy This third piece of the puzzle is the lattice energy: The energy required to completely separate a mole of a solid ionic compound into its gaseous ions. The energy associated with electrostatic interactions is governed by Coulomb’s law: Eel =  Q1Q2 d 7

Lattice Energy Lattice energy, then, increases with the charge on the ions. Greater force between the ions (Coulomb’s Law) It also increases with decreasing size of ions. Greater forces between ions when their distance decreases (Coulomb’s Law) 8

Energetics of Ionic Bonding By accounting for all three energies (ionization energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process. 9

Energetics of Ionic Bonding These phenomena also helps explain the “octet rule.” Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies. 10

Ionic Compound Properties High melting and Boiling points High temperatures (amounts of energy) are required to overcome the attraction between positive and negative ions Hard Crystals are hard because the positive and negative ions are strongly attracted to each other and difficult to separate Brittle, lack ductility and malleability When pressure is applied the electrostatic repulsion can be enough to split the crystal Soluble in water When ionic compounds dissolve in water, the water can dissociate the ions making the lattice structure fall apart and overcome the positive and negative attractions holding the compound together

Ionic Compound Properties Low volatility (does not want to become a vapor) High temperatures (amounts of energy) are required to overcome the attraction between positive and negative ions Lack conductivity as solid (only conductive in aqueous solution) Ions are bound tightly to each other impeding electrons to flow and conduct heat and electricity The stronger the coulombic forces between the ions in an ionic compound the more these properties will be expressed