Ionic bonds and main group chemistry

Transcription

1 Ionic bonds and main group chemistry

2 Learning objectives Write Lewis dot structures of atoms and ions Describe physical basis underlying octet rule Predict ionic charges using periodic table Define lattice energy Apply Born-Haber diagrams to calculations of lattice energy

3 Three types of bonding Ionic Metal + nonmetal Electron transfer Covalent Nonmetal + nonmetal Electron sharing Metallic Metal + metal Electron pooling

4 It s all about Coulomb s law Like charges attract E inversely proportional to r E proportional to q x q E 1 4 o q q 1 r 2

5 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 Nonmetals gain electrons form negative ions

6 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)

7 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

8 Predicting ion charges s and p block elements are easy: charge = group number for cations charge = -(8 group number) for anions Group 1A Group 2A Group 3A Group 5A Group 6A Group 7A H + Li + Be 2+ N 3- O 2- F - Na + Mg 2+ Al 3+ P 3- S 2- Cl - K + Ca 2+ Ga 3+ As 3- Se 2- Br - Rb + Sr 2+ In 3+ Te 2- I - Cs + Ba 2+ Tl 3+

9 The Octet Rule Main-group elements undergo reactions which leave them with eight valence electrons Group 1 [NG](ns 1 ) e M + [NG] + Group 2 [NG](ns 2 ) 2e M 2+ [NG] 2+ Group 6 [NG](ns 2 np 4 ) + 2e X 2- [NG] 2- Group 7 [NG](ns 2 np 5 ) + e X - [NG] - Works very well for second row (Li F) Many violations in heavier p-block elements (Pb 2+ not Pb 4+, Tl + not Tl 3+, Sb 3+ not Sb 5+ or Sb 3- )

10 Less predictable for transition metals Occurrence of variable ionic charge Cr 2+, Cr 3+, Cr 4+, Cr 6+ etc. 4s electrons are lost first and then the 3d Maximum oxidation states in first half correspond to loss of all electrons (4s + 3d) Ti 4+, V 5+, Cr 6+, Mn 7+ Doesn t continue beyond half-filled shell 3d electron energy decreases (more tightly held) across period No Fe 8+ etc. Desirable configurations tend to coincide with empty, halffilled or filled 3d orbitals Fe 2+ ([Ar]3d 6 ) is readily oxidized to Fe 3+ ([Ar]3d 5 )

11 Ionization energy Energy required to remove an electron from a neutral gaseous atom Always positive Follows periodic trend Increases across period Decreases down group Removal of electrons from filled or half-filled shells is not as favourable [He]2s 2 2p 3 [He]2s 2 2p 4 [He]2s 2 [He]2s 2 2p 1

12 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

13 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 Or zero meaning can t 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 )

14 Ionic bonding Reaction between elements that form positive and 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] -

15 Stability of the ionic lattice Forming ions does not give energy payout: E i (Na) = 496 kj/mol E a (Cl) = -349 kj/mol Net energy investment (+150 kj/mol) Formation of lattice from gaseous ions releases energy to compensate M + (g) + X - (g) MX(s) + energy Lattice energy is energy released on bringing ions from gas phase into lattice (negative value) Or: lattice energy is energy required to separate lattice into gas phase ions (positive value)

16 Lattice energies follow simple trends Depends on coulombic attraction between ions -U = κz 1 z 2 /d (κ = 8.99x10 9 JmC -2 ) As ionic charge increases, U increases (U z 1 z 2 ) U(NaF) < U(CaO) As ion size decreases, U increases (U 1/d) U(LiCl) > U(NaCl) > U(KCl) Cation F - Cl - Br - I - O 2- Li Na K Be Mg Ca Al ,916 d

17 Born-Haber cycle for calculating Lattice energy difficult to measure directly Can be estimated very well by models Can be obtained using other experimentally determined quantities and conservation of energy energy

18 Drawing the Born-Haber cycle