Ionic bonds and main group chemistry Towards the noble gas configuration Noble gases are unreactive they have filled shells Shells of reactive elements are unfilled Achieve noble gas configuration by gaining or losing electrons Metals lose electrons form positive ions Metals lose electrons form positive ions Nonmetals gain electrons form negative ions Nonmetals gain electrons form negative ions Lewis dot model The nucleus and all of the core electrons are represented by the element symbol The valence electrons are represented by dots one for each Number of dots in Lewis model is equal to group number (in 1 8 system) The Octet Rule All elements strive to become a noble gas, at least as far as the electrons are concerned. Filling the outer shell 8 electrons Achieve this by adding electrons Or taking them away Predicting ion charges s and p block elements are easy: charge = group number for cations charge = group number for cations charge = -(8 group number) for anions charge = -(8 group number) for anions Less predictable for transition metals Occurrence of variable ionic charge Cr 2+, Cr 3+, Cr 4+, Cr 6+ etc. Cr 2+, Cr 3+, Cr 4+, Cr 6+ etc. 4s electrons are lost first and then the 3d Desirable configurations coincide with empty, half-filled or filled 3d orbitals Fe 2+ ([Ar]3d 6 ) is less stable than Fe 3+ ([Ar]3d 5 ) Fe 2+ ([Ar]3d 6 ) is less stable than Fe 3+ ([Ar]3d 5 ) Ionic size and charge Loss of electrons increases the effective nuclear charge ion shrinks Gain of electrons decreases the effective nuclear charge ion expands Ionization energy Energy required to remove an electron from a neutral gaseous atom Always positive Follows periodic trend Increases across period Increases across period Decreases down group Decreases down group Removal of electrons from filled or half-filled shells is not as favourable [He]2s 2 [He]2s 2 2p 3 [He]2s 2 2p 4 [He]2s 2 2p 1 Higher ionization energies Depend on group number Much harder to remove electrons from a filled shell Stepwise trend below illustrates this Partially filled valence electrons Completely filled core electrons Electron affinity Energy released on adding an electron to a neutral gaseous atom Values are either negative energy released, meaning negative ion formation is favourable negative energy released, meaning negative ion formation is favourable Or zero meaning cant be measured and negative ions are not formed Or zero meaning cant be measured and negative ions are not formed Addition of electrons to filled or half-filled shells is not favoured (e.g. He, N) It is easier to add an electron to Na (3s 1 ) than to Mg (3s 2 ) Ionic bonding Reaction between elements that form positive and negative ions Metals (positive ions) and nonmetals (negative ions) Metals (positive ions) and nonmetals (negative ions) Neutral Na + Cl ionic Na + Cl - [Ne]3s 1 + [Ne]3s 2 3p 5 = [Ne] + + [Ar] - [Ne]3s 1 + [Ne]3s 2 3p 5 = [Ne] + + [Ar] - Stability of the ionic lattice Simply forming ions does not give an energy payout: E i (Na) = 496 kJ/mol E i (Na) = 496 kJ/mol E a (Cl) = -349 kJ/mol E a (Cl) = -349 kJ/mol Net energy investment required Formation of a crystal lattice releases energy The lattice energy is the energy released on bringing ions from the gas phase into the solid lattice Depends on coulombic attraction between ions -U = z 1 z 2 /d ( = 8.99x10 9 JmC -2 Born-Haber cycle for calculating energy The lattice energy can be obtained using other experimentally determined quantities and the energy cycle Lattice energies follow simple trends As ionic charge increases, U increases (U z 1 z 2 ) As ion size decreases, U increases (U 1/d) U(LiF) > U(LiCl) > U(LiBr) U(LiF) > U(LiCl) > U(LiBr) U(NaI) < U(MgI 2 ) < U(AlI 3 ) U(NaI) < U(MgI 2 ) < U(AlI 3 ) The Octet Rule Main-group elements undergo reactions which leave them with eight valence electrons Group 1 (ns 1 ) M + Group 1 (ns 1 ) M + Group 2 (ns 2 ) M 2+ Group 2 (ns 2 ) M 2+ Group 6 (ns 2 np 4 ) X 2- Group 6 (ns 2 np 4 ) X 2- Group 7 (ns 2 np 5 ) X - Group 7 (ns 2 np 5 ) X - Works very well for second row (Li F) Many violations in heavier p-block elements (Pb 2+, Tl +, Sb 3+ )