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7.1 Formation of Ionic Bonds: Donating and Accepting Electrons 7.2 Energetics of Formation of Ionic Compounds 7.3 Sto

7. Ionic Bonding. 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons 7.2 Energetics of Formation of Ionic Compounds 7.3 Stoichiometry of Ionic Compounds 7.4 Ionic Crystals 7.5 Ionic Radii. Introduction (SB p.186). Sodium.

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7.1 Formation of Ionic Bonds: Donating and Accepting Electrons 7.2 Energetics of Formation of Ionic Compounds 7.3 Sto

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  1. 7 Ionic Bonding 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons 7.2 Energetics of Formation of Ionic Compounds 7.3 Stoichiometry of Ionic Compounds 7.4 Ionic Crystals 7.5 Ionic Radii

  2. Introduction (SB p.186) Sodium When sodium exposed in air, it becomes tarnished rapidly Reacts with oxygen in air  Form a dull oxide layer on the metal surface

  3. Introduction (SB p.186) When sodium is placed in a bottle containing chlorine gas Burns fiercely Gives a white coating of sodium chloride

  4. Introduction (SB p.186) Noble gases • Very stable • Rarely participate in chemical reactions and form bonds with other elements •  Octet configuration

  5. Introduction (SB p.186) Formation of compounds • Transfer or sharing of valence electron(s) takes place • Atoms achieve the electronic configuration of the nearest noble gas in the Periodic Table • Atoms are joined together by chemical bonds

  6. Introduction (SB p.186) Three types of chemical bonds 1. Ionic bond Electrostatic attraction between positively charged particles and negatively charged particles

  7. Introduction (SB p.186) Three types of chemical bonds 2. Covalent bond Electrostatic attraction between nuclei and shared electrons

  8. Introduction (SB p.186) Three types of chemical bonds 3. Metallic bond Electrostatic attraction between metallic cations and delocalized electrons (electrons that have no fixed positions) Let's Think 1

  9. 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons

  10. 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.187) Ionic Bonds • Formed by a transfer of electrons from metallic atoms to non-metallic atoms • e.g. Formation of sodium chloride • Both the sodium ion and chloride ion attain the electronic configurations of noble gases which give rise to stability

  11. 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.187) Formation of ionic bond between sodium atom and chlorine atom Cl Na Sodium atom, Na 1s22s22p63s1 Chlorine atom, Cl 1s22s22p63s23p5

  12. - + linked up together by ionic bond 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.187) Formation of ionic bond between sodium atom and chlorine atom Cl Na Sodium ion, Na+ 1s22s22p6 Chloride ion, Cl- 1s22s22p63s23p6

  13. 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.187) Ionic Bonds: Donating and Accepting Electrons

  14. – + Internuclear distance 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.187) Ionic Bonds: Donating and Accepting Electrons

  15. – + – + 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.187) Ionic Bonds: Donating and Accepting Electrons Cationic radius(r+) Anionic radius(r-) Internuclear distance Internuclear distance = r+ + r-

  16. 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.187) Ionic Bonds: Donating and Accepting Electrons Ionic bonds are the strong non-directional electrostatic attraction between ions of opposite charges.

  17. 7.1 Formation of Ionic Bonds: Donating and Accepting Electrons (SB p.188) Electron transfer from a magnesium atom to two chlorine atoms Electron transfer from two lithium atoms to an oxygen atom

  18. 7.2 Energetics of Formation of Ionic Compounds

  19.  Hf ø Actually passing through many steps at the molecular level 7.2 Energetics of Formation of Ionic Compounds (SB p.189) Energetics of Formation of Ionic Compound macroscopiclevel Na(s) + Cl2(g)  NaCl(s) microscopic level

  20. 7.2 Energetics of Formation of Ionic Compounds (SB p.189) Consider the formation of the ionic compound via a serious of steps: 1. The conversion of the elements to the gaseous atoms (standard enthalpy change of atomization, )

  21. 7.2 Energetics of Formation of Ionic Compounds (SB p.189) Consider the formation of the ionic compound via a serious of steps: 2. The conversion of the gaseous atoms to gaseous ions (ionization enthalpy, and electron affinity, )

  22. 7.2 Energetics of Formation of Ionic Compounds (SB p.189) Consider the formation of the ionic compound via a serious of steps: 3. The combination of the gaseous ions to form an ionic crystal (lattice enthalpy, )

  23. Na(s) + Cl2(g) NaCl(s)Hf = –411 kJ mol-1 ø 7.2 Energetics of Formation of Ionic Compounds (SB p.189) 1. Standard Enthalpy Change of Formation (H f) ø The enthalpy change when one mole of the ionic compound is formed from its constituent elements (in their standard states) under standard conditions.

  24. Na(s) Na(g) H atom [Na(s)] = +109 kJ mol-1 ø Cl2(g) Cl(g) H atom [Cl2(g)] = +121 kJ mol-1 ø 7.2 Energetics of Formation of Ionic Compounds (SB p.190) 2. Standard Enthalpy Change of Atomization (H atom) ø The enthalpy change when one mole of gaseous atoms is formed from an element in the standard state under standard conditions. Questions: Why are the changes endothermic? What type of bond is broken in each case?

  25. Na(g) Na+(g) + e- H I.E [Na(g)] = +494 kJ mol-1 Mg(g) Mg+(g) + e-H I.E [Mg(g)] = +736 kJ mol-1 Mg+(g) Mg2+(g) + e- H I.E [Na(g)] = +1 450 kJ mol-1 7.2 Energetics of Formation of Ionic Compounds (SB p.190 – 191) 3. Ionization Enthalpy (HI.E.) The energy required to remove one mole of electrons from one mole of atoms or ions in the gaseous state. Questions: Why are the changes endothermic?

  26. First electron affinity of O(g): O(g) + e- O-(g) H E.A [O(g)] = –142 kJ mol-1 Second electron affinity of O(g): O-(g) + e- O2-(g) H E.A [O(g)] = –844 kJ mol-1 7.2 Energetics of Formation of Ionic Compounds (SB p.191) 4. Electron affinity (ΔHE.A.) The enthalpy change when one mole of electrons is added to one mole of atoms or ions in the gaseous state. Questions: Why may E.A. have -ve or +ve values?

  27. 7.2 Energetics of Formation of Ionic Compounds (SB p.192) Electron affinities (in kJ mol–1) of some elements and ions

  28. ø Na+ (g) + Cl-(g) NaCl(s) H lattice[Na+Cl-(s)] – – + + 7.2 Energetics of Formation of Ionic Compounds (SB p.192) 5. Lattice enthalpy ( ΔHlattice) ø The enthalpy change when one mole of an ionic crystal is formed from its constituent ions in the gaseous state under standard conditions.

  29. ø Na+ (g) + Cl-(g) NaCl(s) H lattice[Na+Cl-(s)] +ve or -ve? – – – – – – + + + + + + Why can’t L.E. be determined directly from experiments? Questions: 7.2 Energetics of Formation of Ionic Compounds (SB p.192) L.E. can be calculated from the values of other experimentally determined enthalpy changes by constructing a Born-Haber cycle and applying Hess’s law

  30. 7.2 Energetics of Formation of Ionic Compounds (SB p.193) Born-Haber Cycle A simplified enthalpy level diagram used to calculate the lattice enthalpy of an ionic compound. • Two different routes to form an ionic compound • Route 1: Direct single-step reaction of the elements to form the ionic compound • Route 2: Consists of a number of steps. The enthalpy change of each step can be found from experiments, except the lattice enthalpy

  31. 7.2 Energetics of Formation of Ionic Compounds (SB p.193) Born-Haber Cycle for the formation of sodium chloride

  32. 7.2 Energetics of Formation of Ionic Compounds (SB p.194) • Or draw enthalpy level diagram to represent the enthalpy changes in the Born-Haber cycle Example 7-2

  33. 7.2 Energetics of Formation of Ionic Compounds (SB p.196) Lattice enthalpy A measure of ionic bond strength which in turn represents the strength of the ionic lattice. • The higher (more negative) the lattice enthalpy of an ionic latticeThe higher is the ionic bond strengthThe more stable is the ionic lattice

  34. 7.2 Energetics of Formation of Ionic Compounds (SB p.196) Factors affect lattice enthalpy Let's Think 2 • Effect of ionic size: • The greater the ionic sizeThe lower (or less negative) is the lattice enthalpy • Effect of ionic charge: • The greater the ionic chargeThe higher (or more negative) is the lattice enthalpy Check Point 7-2

  35. 7.3 Stoichiometry of Ionic Compounds

  36. 7.3 Stoichiometry of Ionic Compounds (SB p.197) Stoichiometry Stoichiometry of a compound is the simplest ratio of the atoms bonded to form the compound. How can the stoichiometry of an ionic compound be determined?

  37. 1 2 7.3 Stoichiometry of Ionic Compounds (SB p.197 – 198) A. In Terms of Electronic Configuration magnesium chloride Example Elements involved Mg(Group II)Cl(Group VII) Ions formed Mg2+Cl- Ratio of ions Chemical formula Mg2+(Cl-)2 or MgCl2

  38. 7.3 Stoichiometry of Ionic Compounds (SB p.198) B. In Terms of Enthalpy Change of Formation • The more negative the enthalpy change of formation of an ionic compound • The greater is the driving force for its formationThe more stable the compound Check Point 7-3

  39. 7.4 Ionic Crystals

  40. 7.4 Ionic Crystals (SB p.201) Structure of Sodium Chloride Unit cell of NaCl Co-ordination number of Na+ = 6 6 : 6 co-ordination Co-ordination number of Cl- = 6

  41. 7.4 Ionic Crystals (SB p.202) Face-centred cubic lattice

  42. 7.4 Ionic Crystals (SB p.202) A unit cell is the smallest basic portion of the crystal lattice that, when repeatedly stacked together at various directions, can reproduce the entire crystal structure.

  43. 7.4 Ionic Crystals (SB p.202) Structure of Caesium Chloride Simple cubic lattice Co-ordination number of Cs+ = 8 8 : 8 co-ordination Co-ordination number of Cl- = 8

  44. 7.4 Ionic Crystals (SB p.203) Structure of Calcium Fluoride Face-centred cubic lattice Co-ordination number of Ca+ = 8 8 : 4 co-ordination Co-ordination number of F- = 4

  45. 7.4 Ionic Crystals (SB p.203) Some simple ionic structures Example 7-4 Check Point 7-4

  46. 7.5 Ionic Radii

  47. X-ray 7.5 Ionic Radii (SB p.205) X-ray and electron diffraction technique Photographic plate

  48. 7.5 Ionic Radii (SB p.205) Electron density plot for sodium chloride crystal

  49. 7.5 Ionic Radii (SB p.206) A. Cations • Smaller radius than the corresponding atom • Reasons: • 1. The number of electron shells decreases • 2. No. of protons > No. of electrons (p/e ratio increases) The nuclear attraction is more effective to cause a contraction in the electron cloud

  50. 7.5 Ionic Radii (SB p.206) Size of ion vs size of atom Comparing relative atomic radii of some elements with the ionic radii of the corresponding ions

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