Lecture 32 Formation of an Ionic Bond Ozgur Unal 1

 Na-Cl reaction 2  Mg-O reaction

 In the Na-Cl reaction, Na donates one valence electron to Cl.  Cl accepts one electron from Na.  Both Na + and Cl - have the electron configurations of noble gases after the reaction.  The electrostatic force between these oppositely charged ions holds them together  Ionic bond is formed.  Due to this ionic bond, Na + and Cl - form an ionic compound.  Compounds that contain ionic bonds are ionic compounds. 3

Binary Ionic Compounds:  Thousands of compounds contain ionic bonds.  Many ionic compounds are binary, which means that they contain only two different elements.  Binary ionic compounds contain a metallic cation and a nonmetallic anion.  Example: NaCl and MgO are binary ionic compounds. 4

 The charges of ions are important in the formation of ionic compounds.  The number of electrons accepted and donated are equal.  The total charge of an ionic compound is zero.  Therefore, the ionic compound formed by X +m and Y -n can be written as: X n Y m. 5  Example: What is the ionic compound formed by Ca and F?  Cation: Ca +2  Anion: F -  Ionic compound formed: CaF 2

 Example: What is the ionic compound formed by Al and O? 6  Cation: Al +3  Anion: O -2  Ionic compound formed: Al 2 O 3

Lecture 33 Properties of Ionic Compounds Ozgur Unal 7

 In an ionic compound, large number of positive and negative ions exist together.  These ions are packed into a regular repeating pattern that balances the forces of attraction and repulsion between the ions.  Example: NaCl crystal  Each Na is surrounded by 6 Cl, an each Cl is surrounded by 6 Na.  Figure 7.7 8

9  The strong attraction among the positive ions and negative ions in an ionic compound result in the formation of a crystal lattice.  A crystal lattice is a 3-dimensional geometric arrangement of particles.  Minerals in Figure 7.8 have crystal lattice structure.  There are thousands of minerals and mineralogists classify them according to their color, hardness, magnetic and electric structure etc.  The types of anions minerals contain can also be used to identify minerals.  Example: Silicates (Si and O), Carbonates (C and O), Borates (B) etc.

 Examples of physical properties:  Melting point  Boiling point  Hardness  Electrical conductivity etc. 10  Electrical conductivity depends on the freely moving charged particles.  Ions are charged particles.  In solid state, ions cannot move freely, therefore ionic solids cannot conduct electricity.

 What happens when you put table salt in water? 11  Salty water and liquid salt conduct electricity. Because in both cases there are ions that can move freely.  Similarly, some other ionic compounds dissolved in water, or their liquid state conduct electricity.  An ionic compound whose aqueous solution conducts an electric current is salled an electrolyte.  Does the solution conduct electric current?

 Check out Table 7.5  Ionic bonds are strong, therefore the melting and boiling points of ionic compounds are high. 12  Ionic crystals are also hard, rigid and brittle because of the strong attractive forces that hold the ions in place.  It is possible to break these ionic bonds by applying a force on the crystal structure.  Check out Figure 7.9

 Chemical reactions can be exothermic and endothermic.  Exothermic reactions releases energy.  Example: burning a paper  Endothermic reactions require energy.  Example: chemical reactions when cooking food 13  Formation of ionic bonds release energy, therefore those chemical reactions are exothermic.  Breaking ionic bonds require energy, therefore it is endothermic.

 The energy required to separate 1 mol of the ions of an ionic compound is called the lattice energy.  Mol is a unit for amount of substance.  Check out Table  Lattice energy depends on:  The size of the ions bonded  Charge of the ions  Ionic compounds with smaller ions have greater lattice energy.  Ionic compounds of ions with greater charge have greater lattice energy.