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Topic 16 Chunk 1

Topic 16 Chunk 1. By Paul and Elliott. Electron Figurations. The Inert Pair Effect (Group 4). All of the elements in the group have the outer electronic structure ns²np² The oxidation state of +4 is where all these outer electrons are directly involved in the bonding

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Topic 16 Chunk 1

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  1. Topic 16 Chunk 1 • By Paul and Elliott

  2. Electron Figurations

  3. The Inert Pair Effect (Group 4) • All of the elements in the group have the outer electronic structure ns²np² • The oxidation state of +4 is where all these outer electrons are directly involved in the bonding • Towards the bottom of the group there is an increasing tendency for the s² pair to not be used in bonding, making it inert • This inert pair effect is particularly dominant in the chemistry of lead

  4. The Inert Pair Effect continued • In the chemistry of group 4 the +2 oxidation state is more stable for lead and tin than for germanium • Carbon and silicon are different as they display oxidation states of +4 with carbon monoxide being the exception • The reasons for increasing stability of the lower oxidation state are complex and we thankfully don’t need to know them.

  5. More Inert Pair Effect • Remember that the lower valency states have a higher percentage of ionic bonding • Although germanium does exist in GeII state, the GeIV state is more stable • SnII and SnIV have similar stabilities but that PbII is more stable than PbIV • Sn²+ and Pb²+ occur predominantly in ionic compounds • In group 3 In+ and Tl+ exist • In group 5 only Bi³+ exists

  6. Maximum Covalency Rule • In period 1 the maximum colvalency of any element is 4 with a maximum of 4 lone pairs of electrons • In period 2 compounds such as PCl5 and SF6 are formed where the maximum covalency is greater than 4, with up 6 lone pairs of electrons possible • The reason for this difference is that period 2 elements have the third quantum shell, with d-electron levels which can be used in electron bonding

  7. Maximum Covalency Rule in Group 5 Halides • Nitrogen only has 3 pairs of electrons available for bonding e.g. NCl3 • The three unpaired 2p electrons can each form a bond but the 2s electrons cannot as there are no ‘2d’ orbitals into which one of the 2s electrons can be promoted

  8. Maximum Covalency Rule in Group 5 Halides • Phosphorus can show valencies of 3 and 5 e.g. in PCl3 and PCl5 • By promoting an electron from the 3s orbital to the 3d orbital, phosphorus now has 5 unpaired electrons available for bonding.

  9. Amphoteric Behaviour • A compound is said to be amphoteric if it reacts with both acids and alkalis • Al³ ion is small and highly charged so it will extensively hydrate in solution and may be considered to be [Al(H2O)]³+ • If aqueous sodium hydroxide is added to a solution containing aqueous Al³+ ions, the OH- ions will remove protons from the hydrated aluminium ion.

  10. Amphoteric Behaviour of Al³+ • [Al(H2O)3(OH)3] +3OH-→ [Al(OH)6]³- + 3H2O • The aluminium hydroxide forms a gelatinous white precipitate • The series of reactions may be reversed by addition of acid to the sodium aluminate solution • Aluminium hydroxide is an amphoteric hydroxide as it reacts with dilute acids to form salt solutions and with aqueous sodium or potassium hydroxide to form the aluminate ion

  11. Amphoteric Behaviour of Pb²+ • Lead(II) nitrate being the most common soluble salt it is usually used to illustrate the reactions of the lead(II) ion • When aqueous sodium hydroxide is added to aqueous lead(II) nitrate, a white precipitate of lead(II) hydroxide is formed • Pb²+(aq) + 2OH-(aq) → Pb(OH)2(s)

  12. Amphoteric Behaviour of Pb²+ • On addition of excess aqueous sodium hydroxide, the white precipitate dissolves to give a colourless solution containing the plumbate(II) ion • Pb(OH)2(s) + 2OH-(aq)→ [Pb(OH)4]²-(aq) • Addition of acid to the solution containing the plumbate (II) ion will re-precipitate the lead (II) hydroxide which in turn will dissolve in more acid to form aqueous Pb²+ ions

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