9 - 1 The Octet Rule Except for hydrogen and helium, atoms are most energetically stable if they have a completely filled valence shell. A completely filled valence shell is called an octet because eight electrons are involved. Atoms will form ionic compounds by gaining or losing valence electrons.

9 - 2 Ionic Bonding Ionic bonds result from the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). Na + Cl - Ionic bonds are commonly formed between reactive metals and nonmetals.

9 - 3 When forming cations, the electrons that are removed from an atom or ion are those with a maximum value for n (valence electrons). Within a principal energy level, the easiest electrons to remove are those with a maximum value of l. Higher charges and smaller radii produce stronger bonds.

9 - 4 Ions and the Octet Rule Simple ions are atoms that have lost or gained electrons to satisfy the octet rule. They typically form positive or negative ions based on what requires the smallest gain or loss of electrons to complete an octet.

9 - 5 Formation of Na + and Cl - Na Na + + e - 1s 2 2s 2 2p 6 3s 1 1s 2 2s 2 2p 6 complete octet Cl + e - Cl - 1s 2 2s 2 2p 6 3s 2 3p 5 1s 2 2s 2 2p 6 3s 2 3p 6

9 - 6 Formation of Sodium Chloride Na + Cl Na + + Cl - The opposite charges attract each other forming a formula unit. Molecules are never formed in an ionic compound!

9 - 7 Sodium Chloride Lattice  Ionic compounds consist of an endless repetition of unit cells.  The following unit cell illustrates that the sodium cation and the chlorine anion alternate with each other in three dimensions.  The sodium ion in the center of the cube is actually touched by six chloride ions.

9 - 8 Cl - Na + Each of the Na + ions is surrounded by six Cl - ions, and each Cl - ion is surrounded by six Na + ions.

9 - 9 Properties of Sodium Chloride  The greater the attraction between the cation and anion, the more energy is released upon compound formation.  Ionic compounds have high melting and boiling points.  MgO has the same unit structure as NaCl but has a much higher melting and boiling point.

 The Mg 2+ and O 2- ions attract each other more strongly than the Na + and the Cl -.  If the ions are smaller, they are able to get closer together resulting in greater electrostatic attractions.  In RbI, the Rb + and the I - ions are bigger than Na + and Cl - in NaCl.  RbI has a slightly lower boiling and melting point than NaCl.

 The attraction is smaller between the larger ions and less energy is needed to separate them.

Lattice Energy and Ionic Bonds Lattice energy is the energy required to completely separate a mole of a solid ionic compound into its gaseous ions. Magnitude (size) of lattice energy of a solid depends on the magnitude of the charges (1+, 2+), their sizes, and their arrangement in the solid.

The energy can be calculated by E = k × Q 1 × Q 2 /r where k = 8.99 x 10 9 Jm/C 2, Q 1 and Q 2 are the ionic charges, and r is the distance between the centers of the ions. The equation above indicates that the more stable energies (attraction) are represented by lower or negative values and less stable (repulsive) energies represented by higher or positive values.

For a given arrangement of ions, lattice energy increases as the charges increase and as the radii decreases. Ionic compounds are energetically stable because of the attraction between opposite charges. Arrange the following in order of increasing lattice energy: NaF, CsI, CaO CsI < NaF < CaO

Formation of Sodium Chloride Δ H ° (kJ) Na(s) Na(g) +108 ½Cl 2 (g) Cl(g) +122 Na(g) Na + (g) + e Cl(g) + e - Cl - (g) Na + (g) + ½Cl - (g) NaCl(s) +788

Energetics of NaCl Formation Na(s) + ½Cl 2 (g) Na(g) + ½Cl 2 (g) Na(g) + Cl(g) Na + (s) + Cl(g) Na + (s) + Cl - (g) NaCl(s) +496 kJ +121 kJ +92 kJ -349 kJ -771 kJ -411 kJ

Properties of Ionic Compounds The building blocks of ionic compounds are cations (positive ions) and anions (negative ions). The force holding an ionic compound together is the electrostatic attraction. Properties  have only empirical formulas

 high melting and boiling points  solids at room temperature (25°C, 298K)  conductors in the liquid phase (l) or in an aqueous solution (aq)  some are water soluble, some are not – check solubility chart  exists as a three dimensional network of unit cells