9.1-9.4 Bonding Models – Ionic Bonding

Differences in Bonding Why do chemical bonds form? Chemical bonds forms because they lower potential energy between charged particles that compose atoms. (To gain stability) If we were to categorize bonds into three main types, we would have: 1.) Ionic (between cation and anion (most of the time metal and non-metal)) Here, electrons are transferred. 2.) Covalent (between two non-metals) Here, electrons are shared. 3.) Metallic (between metals) Here, electrons are pooled.

Representing Valence Electrons with Dots Since valence electrons are held loosely, these are the electrons involved in a chemical bond. Lewis symbol: represents the valence electrons of main group elements as dots surrounding the elements chemical symbol. Example: He: Chemical bond: the sharing or transferring of electrons to attain stable electron configurations of bonding atoms. Since the stable configuration is usually eight electrons in the outermost shell, this is known as the octet rule.

Lewis Symbols (Continued) Lewis symbols can be use to show electron transfer during ionic bonds: .. .. Example: K. + :Cl:  K+ [:Cl:]-  KCl . .. Use Lewis symbols to predict the formula for the compound that forms between magnesium and nitrogen. .. .. Mg: + .N.  3Mg2+ + 2[:N:]3-  Mg3N2 . ..

Lattice Energy When ionic compounds are formed, energy is usually very exothermic, and the resulting structure is a lattice of cations and anions bonded together. Lattice energy – the energy associated with the formation of a crystalline lattice of alternating cations and anions from gaseous ions. According to Coulombs law, as these ions get closer together, potential energy decreases, and when the lattice is formed, energy is emitted as heat. Na+(g) + Cl-(g)  NaCl(s) ΔHo = Lattice energy

Calculating Lattice Energy The Born-Haber Process: A hypothetical series of steps that represent an ionic compound from its constituent elements. The change in enthalpy of each step is known, except the last step (the lattice energy). We can use Hess’s law to determine lattice energy. The formation of ionic compounds is not exothermic due to electron transfer, but rather as a result of the large amount of heat released when the cations and anions coalesce to form a crystalline lattice.

Trends in Lattice Energies Lattice energies become less exothermic (less negative) with increasing ionic radius (down a group). This happens because as the ionic radius increases, ions cannot get as close to each other, and therefore do not release as much energy when the lattice forms. Lattice energies become more exothermic (more negative) with increasing magnitude of ionic charge. If the charge on the ions is great, the ions can get closer together, and as a result, release more energy when they form their lattice. This can all be explained using Coulombs law!

Let’s Try a Practice Problem Arrange the following in order of increasing magnitude of lattice energy: LiBr, KI, and CaO KI<LiBr<CaO

9.1-9.4 pg. 419 #’s 36, 38, 40, 42, 44, & 46 Read 9.5-9.6 pgs. 391-398