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The s -Block Elements

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  1. The s-Block Elements

  2. The s-Block Elements • Elements of Groups IA* (the alkali metals) and IIA* (the alkaline earth metals) •  constitute the s-block elements •  their outermost shell electrons are in the s orbital • *Note: In the following, Groups IA and IIA are abbreviated as Groups I and II respectively.

  3. The s-block elements

  4. The s-Block Elements • Similarities • 1. highly reactive metals • 2. strong reducing agents • 3. form ionic compounds with fixed oxidation states of +1 for Group I elements and +2 for Group II elements

  5. [Ne] 3s1 [Ar] 4s1 [Kr] 5s1 [Xe] 6s1 [Rn] 7s1 [Ne] 3s2 [Ar] 4s2 [Kr] 5s2 [Xe] 6s2 [Rn] 7s2 Q.1 Group I Li Lithium Na Sodium K Potassium Rb Rubidium Cs Caesium *Fr Francium Electronic configuration [He] 2s1 Group II Be Beryllium Mg Magnesium Ca Calcium Sr Strontium Ba Barium *Ra Radium Electronic configuration [He] 2s2

  6. Group I elements • Lithium

  7. Group I elements • Sodium

  8. Group I elements • Potassium

  9. Group I elements • Rubidium

  10. Group I elements • Francium - radioactive

  11. Group I elements • Beryllium

  12. Group I elements • Magnesium

  13. Group I elements • Calcium

  14. Group I elements • Strontium

  15. Group I elements • Barium

  16. Group I elements • Radium - radioactive

  17. Characteristic Properties of thes-Block Elements

  18. All have low electronegativity. electropositive

  19. EN  down the group EN : Group II > Group I (∵ greater ENC)

  20. Bonding Strength of metallic bond : Group II > Group I m.p./b.p. : Group II > Group I

  21. Hardness : - Group I < Group II Na/K…can be easily cut with a knife

  22. Structure Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba Density : Group II > Group I

  23. Structure Group I : b.c.c. Group II : f.c.c. or h.c.p. except Ba Density also depends on size and mass of the atoms

  24. Metallic charater (Reactivity) : - High tendency to lose electrons as shown by –ve E Mn+(aq) + ne  M(s)

  25. Metallic charater (Reactivity) : -  down the groups Group I > Group II

  26. sodium Sodium is stored under paraffin oil

  27. caesium rubidium Caesium and rubidium are stored in vacuum-sealed ampoules

  28. Formation of Basic Oxides 1. Group I Elements • All alkali metals form more than one type of oxide on burning in air (except lithium)

  29. Abundant supply 1. Group I Elements • Three types of oxides: • normal oxides • peroxides • superoxides O2–oxide ion O22–peroxide ion 2O2–superoxide ion

  30. 1. Group I Elements • Type of oxide formed depends on • supply of oxygen • reaction temperature • charge density of M+

  31. 4Li(s) + O2(g) 2Li2O(s) lithium oxide 1. Group I Elements • Lithium •  when it is burnt in air, it forms normaloxide only

  32. 4Na(s) + O2(g) 2Na2O(s) sodium oxide 2Na2O(s) + O2(g) 2Na2O2(s) sodium peroxide excess 1. Group I Elements • Sodium •  when it is burnt in an abundantsupply of oxygen •  forms both the normal oxide and the peroxide

  33. 1. Group I Elements • Potassium, rubidium and caesium •  form All three types of oxides when burnt in sufficient supply of oxygen

  34. 1. Group I Elements • Potassium: • 4K(s) + O2(g)  2K2O(s) potassium oxide • 2K2O(s) + O2(g)  2K2O2(s) potassium peroxide • K2O2(s) + O2(g)  2KO2(s) potassium superoxide

  35. 1. Group I Elements • Rubidium: • 4Rb(s) + O2(g)  2Rb2O(s) • 2Rb2O(s) + O2(g)  2Rb2O2(s) • Rb2O2(s) + O2(g)  2RbO2(s)

  36. 1. Group I Elements • Caesium: • 4Cs(s) + O2(g)  2Cs2O(s) • 2Cs2O(s) + O2(g)  2Cs2O2(s) • Cs2O2(s) + O2(g)  2CsO2(s)

  37. Oxides formed by Group I elements Cations with high charge densities (Li+ or Na+) tend to polarize the large electron clouds of peroxide ions and/or superoxide ions  Making them decompose to give oxide ions

  38. 1. Group I Elements The electron cloud of the superoxide ion is greatly distorted by the small lithium ion

  39. Oxides formed by Group I elements White solids Slightly coloured solids Highly coloured solids

  40. KO2 used as oxygen generators and CO2 scrubbers in spacecrafts 4KO2 + 2H2O  4KOH + 3O2 2KOH + CO2  K2CO3 + H2O

  41. 2. Group II Elements • Beryllium, magnesium and calcium •  form normal oxides only on burning in air • 2Be(s) + O2(g)  2BeO(s) • 2Mg(s) + O2(g)  2MgO(s) • 2Ca(s) + O2(g)  2CaO(s)

  42. Q.2(a) Be2+, Mg2+ and Ba2+ have higher charge densities  more polarizing  distort the electron cloud of O22  O22 decomposes to give O2

  43. Sr(s) + O2(g) SrO2(s) 2SrO(s) + O2(g) 2SrO2(s) strontium peroxide Q.2(b) 2Sr(s) + O2(g)  2SrO(s) strontium oxide

  44. 500C 700C Ba(s) + O2(g) BaO2(s) 2BaO(s) + O2(g) 2BaO2(s) barium peroxide Q.2(b) 2Ba(s) + O2(g)  2BaO(s) barium oxide

  45. Oxides formed by Group II elements KO2 superoxide

  46. Oxides formed by Group II elements All these oxides are basic in nature (except beryllium oxide which is amphoteric)

  47. Formation of hydroxides 1. Group I hydroxides 2Li(s) + 2H2O(l)  2LiOH(aq) + H2(g) 2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g) 2K(s) + 2H2O(l)  2KOH(aq) + H2(g) 2Rb(s) + 2H2O(l)  2RbOH(aq) + H2(g) 2Cs(s) + 2H2O(l)  2CsOH(aq) + H2(g)

  48. Formation of hydroxides 1. Group I hydroxides For normal oxides, M2O(s) + H2O(l)  2MOH(aq) For peroxides, M2O2(s) + 2H2O(l)  2MOH(aq) + H2O2(aq) For superoxides, 2MO2(s) + 2H2O(l)  2MOH(aq) + H2O2(aq) + O2(g)

  49. Formation of hydroxides 2. Group II hydroxides Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g) Sr(s) + 2H2O(l)  Sr(OH)2(aq) + H2(g) Ba(s) + 2H2O(l)  Ba(OH)2(aq) + H2(g) Mg reacts with steam but not water. Mg(s) + H2O(g)  MgO(s) + H2(g) Be does not react with water and steam.

  50. MgO(s) + H2O(l) Mg(OH)2(aq) slightly soluble Formation of hydroxides 2. Group II hydroxides CaO(s) + H2O(l)  Ca(OH)2(aq) SrO(s) + H2O(l)  Sr(OH)2(aq) BaO(s) + H2O(l)  Ba(OH)2(aq) BeO(s) + H2O(l)  No reaction