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Chapter 41. The s -Block Elements. 41.1 Introduction 41.2 Characteristic Properties of the s -Block Elements 41.3 Variation in Properties of the s -Block Elements 41.4 Variation in Properties of the Compounds of the s -Block Elements 41.5 Uses of the Compounds of the s -Block Elements.

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The s -Block Elements


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    1. Chapter 41 The s-Block Elements 41.1Introduction 41.2Characteristic Properties of the s-Block Elements 41.3Variation in Properties of the s-Block Elements 41.4Variation in Properties of the Compounds of the s-Block Elements 41.5Uses of the Compounds of the s-Block Elements

    2. 41.1 Introduction (SB p.40) • s-Block elements: • Consists of Group IA and Group IIA elements • Outermost shell electrons in s orbitals • Highly reactive metals • Good reducing agents • Fixed oxidation states +1 for Group I elements+2 for Group II elements

    3. Lithium Sodium Potassium Caesium Rubidium 41.1 Introduction (SB p.41) Group I elements:

    4. Calcium Beryllium Magnesium Radium Strontium Barium 41.1 Introduction (SB p.41) Group II elements:

    5. 41.2 Characteristic Properties of the s-Block Elements (SB p.42) Some characteristic properties of Group I metals

    6. 41.2 Characteristic Properties of the s-Block Elements (SB p.42) Some characteristic properties of Group II elements

    7. 41.2 Characteristic Properties of the s-Block Elements (SB p.42) Metallic Character • Group I elements: • Silvery in colour, tarnish rapidly in air • ∴ keep immersed under paraffin oil or in vacuum sealed ampoules • Soft, low boiling and melting points • ∵ weak metallic bond due to only 1 e– is contributed to form bonds • Low density • ∵ body-centred cubic structure have more spaces

    8. 41.2 Characteristic Properties of the s-Block Elements (SB p.43) Some information about Group I elements “b” denotes body-centred cubic structure

    9. 41.2 Characteristic Properties of the s-Block Elements (SB p.43) • Group II elements: • greyish in colour • harder and higher boiling and melting points (compared to Group I counterparts) • ∵ stronger metallic bond due to 2e– are contributed to form bond and smaller atomic sizes • show different crystal structures

    10. 41.2 Characteristic Properties of the s-Block Elements (SB p.44) Some information about Group II elements “h”, “f” and “b” denote hexagonal close-packed, face-centred cubic and body-centred cubic structures respectively

    11. 41.2 Characteristic Properties of the s-Block Elements (SB p.44) Low Electronegativity • All have low electronegativity values • ∵ the outermost s electrons are effectively shielded by inner electron shells. The tendency of losing electrons is relatively high. • Electronegativity values decrease when going down the group • ∵ the outermost shell electrons are further and further away from nucleus

    12. 41.2 Characteristic Properties of the s-Block Elements (SB p.44) Group II elements are more electronegative than Group I counterparts ∵ higher nuclear charge, stronger attraction to outermost shell electrons i.e. more difficult to remove the electrons

    13. Relationship between three oxides: O2– O22– 2O2– monoxide peroxide superoxide 41.2 Characteristic Properties of the s-Block Elements (SB p.45) Formation of Basic Oxides Group I elements • Produce more than one type of oxides (except Li) • All are ionic • Three types of oxides: normal oxides (monoxides), peroxides, superoxides

    14. Li forms the monoxide only • 4Li(s) + O2(g)  2Li2O(s) 180C • Na forms the monoxide and peroxide when O2 isabundant • 4Na(s) + O2(g)  2Na2O(s) • 2Na2O(s) + O2(g)  2Na2O2(s) 180C 300C • K forms the monoxide, peroxide and superoxide • 4K(s) + O2(g)  2K2O(s) • 2K2O(s) + O2(g)  2K2O2(s) • K2O2(s) + O2(g)  2KO2(s) 180C 300C 3000C 41.2 Characteristic Properties of the s-Block Elements (SB p.45)

    15. Rb, Cs also forms superoxides • Rb2O2(s) + O2(g)  2RbO2(s) • Cs2O2(s) + O2(g)  2CsO2(s) 3000C 3000C 41.2 Characteristic Properties of the s-Block Elements (SB p.45)

    16. 41.2 Characteristic Properties of the s-Block Elements (SB p.46) • Li does not form peroxides or superoxides • Reason: •  Li+ is small •  high polarizing power •  serious distortion on electron cloud of peroxide or superoxide •  more distortion , more unstable •  Li2O2 and LiO2 do not exist • K+, Rb+ and Cs+ ions are large •  Low polarizing power  peroxides and superoxides are stable

    17. 2Be(s) + O2(g)  2BeO(s) 2Mg(s) + O2(g)  2MgO(s) 2Ca(s) + O2(g)  2CaO(s) 2Ba(s) + O2(g)  2BaO(s) 2BaO(s) + O2(g) 2BaO2(s) 41.2 Characteristic Properties of the s-Block Elements (SB p.46) Group II Elements • Form normal oxides only, except Sr, Ba which can form peroxides • All are basic (except BeOwhich is amphoteric)

    18. 41.2 Characteristic Properties of the s-Block Elements (SB p.46) Beryllium peroxide (BeO2) does not exist Reason: Be2+ is small high polarizing power  serious distortion on electron cloud of the peroxide ion  polarizing power of Be2+ > Li+, due to smaller size, higher charge  BeO2 does not exist

    19. 41.2 Characteristic Properties of the s-Block Elements (SB p.47) Ba cannot form superoxide while K can Reason: polarizing power of Ba2+ > K+ high polarizing power  more serious distortion on electron cloud of the superoxide ion   Ba(O2)2 does not exist

    20. 41.2 Characteristic Properties of the s-Block Elements (SB p.47) Formation of Hydroxides • All Group I metals (except Li) react with H2O to form metal hydroxides and H2 gas • e.g. 2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g) • 2K(s) + 2H2O(l)  2KOH(aq) + H2(g) • All Group I oxides react with H2O to form metal hydroxides • General equation: • M2O(s) + H2O(l)  2MOH(aq) • M2O2(s) + 2H2O(l)  2MOH(aq) + H2O2(aq) • 2MO2(s) + 2H2O(l)  2MOH(aq) + H2O2(aq) + O2(g)

    21. Ca reacts with H2O readily at room temperature 41.2 Characteristic Properties of the s-Block Elements (SB p.47) • All Group II metals (except Be & Mg) react with H2O to form metal hydroxides and H2 gas • e.g. Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g) • Sr(s) + 2H2O(l)  Sr(OH)2(aq) + H2(g) • Be does not react with H2O(l or g) • Mg does not react with H2O(l) but with H2O(g) • Mg(s) + H2O(g)  MgO(s) + H2(g)

    22. 41.2 Characteristic Properties of the s-Block Elements (SB p.48) • The reactivity increases down the group. • The oxides of Ca, Sr, Ba react with H2O(l) to give 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) • MgO dissolves in acids to form salts but is slightly soluble in water • BeO is insoluble in both acids and water

    23. 41.2 Characteristic Properties of the s-Block Elements (SB p.48) Bonding and Oxidation State in Compounds • s-Block elements form compounds that are predominantly ionic in nature • Oxidation state of Group I metals must be +1 • Reason: • only 1 electron in the outermost s orbitals  once this e- is removed, a stable electronic configuration is obtained  1st I.E. is low • The 2nd e- is removed from the stable octet  2nd I.E. is very high∴ Group I metals predominantly form ions with a fixed oxidation number +1

    24. 41.2 Characteristic Properties of the s-Block Elements (SB p.48) Chemical formulae of some Group I compounds

    25. 41.2 Characteristic Properties of the s-Block Elements (SB p.49) • Oxidation state of Group II metals must be +2 • Reason: • only 2 electrons in the outermost s orbitals  once these 2e- are removed, a stable electronic configuration is obtained  sum of1st I.E. and 2nd I.E. is low • the 3rd e- is removed from the stable octet  3rd I.E. is very high∴ Group II metals predominantly form ions with a fixed oxidation number +2

    26. 41.2 Characteristic Properties of the s-Block Elements (SB p.49) Chemical formulae of some Group II compounds

    27. e.g. 41.2 Characteristic Properties of the s-Block Elements (SB p.49) Weak Tendency to Form Complexes A complex is a polyatomic ion or neutral molecule formed when molecular or ionic groups (called ligands) form dative covalent bonds with a central metal atom or cation.

    28. 41.2 Characteristic Properties of the s-Block Elements (SB p.50) • Complex formation is common in d-block elements • ∵ d-block metal ions utilize their low-lying vacant d-orbitals to accept the lone pair electrons from the surrounding ligands •  a complex is formed • Since s-block metal ions do notpossess low-lying vacant d-orbitals, they do not form complexes • When s-block metal ions are surrounded by polar molecules, there is only electrostatic attraction between the positive ion and the negative ends of the dipoles

    29. 41.2 Characteristic Properties of the s-Block Elements (SB p.50) Characteristic Flame Colours of Salts • Many s-block elements can give characteristic flame colours in the flame test • The outermost shell electrons of Group I & II elements are weakly held •  The electrons can be excited to higher energy levels on heating •  When electrons return to ground state, radiations are emitted •  The radiations fall into the visible light region •  As the energy is quantized, the flame colour is a characteristic property of the element

    30. (a) (b) (c) (d) (a) lithium (b) sodium (c) potassium (d) calcium 41.2 Characteristic Properties of the s-Block Elements (SB p.50)

    31. (a) K+ ion (0.133 nm) has a greater ionic radius than Ca2+ ion (0.099 nm). In fact, K+ and Ca2+ ions are isoelectronic and have the same number of electron shells. However, because Ca2+ ion has one more proton than K+ ion, the electrons of Ca2+ ion will experience a greater attractive force from the nucleus. This leads to a smaller ionic radius of Ca2+ ion. 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (a) Which ion has a greater ionic radius, potassium ion or calcium ion? Give reasons for your choice. Answer

    32. (b) All s-block elements are highly electropositive. This means that they lose their electrons easily. The reason is that the outermost shell electrons of the s-block elements are effectively shielded from the nucleus by the fully-filled inner electron shells. So the electrons are less firmly held by the nucleus, and hence easily to be removed. 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (b) Explain why s-block elements are highly electropositive. Answer

    33. (c) Alkali metals have the [ ] ns1 electronic configuration. The last s electron enters a new electron shell which is much further away from the nucleus. Hence, the attractive force from the nucleus holding this electron is relatively weak. Also, this s electron is shielded from the attraction of the nucleus by the fully-filled inner electron shells. Moreover, once this electron is removed, a stable electronic configuration (octet) is attained. Consequently, the first ionization enthalpies of alkali metals are relatively low. Besides, as their second ionization enthalpies involve the removal of an electron from a fully-filled electron shell, their second ionization enthalpies are relatively large. As a result, alkali metals show a fixed oxidation state of +1 in all their compounds. 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (c) Explain why alkali metals show a fixed oxidation state of +1 in their compounds in terms of ionization enthalpies. Answer

    34. (d) By conducting the flame test, sodium compounds will give a golden yellow flame colour, whereas potassium compounds will give a lilac flame colour. 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (d) Give one test which would enable you to distinguish a sodium compound from a potassium compound. Answer

    35. (e) Since ions of alkali metals and alkaline earth metals do not have low-lying vacant orbitals for forming dative covalent bonds with the lone pair electrons of surrounding ligands, they rarely form complexes. 41.2 Characteristic Properties of the s-Block Elements (SB p.51) Check Point 41-1 (cont’d) (e) Ions of alkali metals and alkaline earth metals have very low tendency to form complexes. Give one reason to account for this. Answer

    36. 41.3 Variation in Properties of the s-Block Elements (SB p.51) Variation in Physical Properties Atomic Radius and Ionic Radius

    37. 41.3 Variation in Properties of the s-Block Elements (SB p.52) Variations in atomic radius and ionic radius of Groups I and II elements

    38. 41.3 Variation in Properties of the s-Block Elements (SB p.52) Observations: • ionic radius of any Group I or II element is smaller than the atomic radius ∵ after losing the outermost shell electron(s), there is one electron shell less in the cation than in the atom, electrons are hold more strongly by nucleus  electron cloud contracts

    39. 41.3 Variation in Properties of the s-Block Elements (SB p.52) • the atomic and ionic radii increase down the Groups ∵ more and more electron shells occupied, outermost shell electrons become further away, and more inner shells shielding the outermost shell electrons attraction between the nucleus and the outermost shell electrons decreases  atomic and ionic radii increase

    40. 41.3 Variation in Properties of the s-Block Elements (SB p.52) • atomic and ionic radii decrease when going from Group I to II in each period ∵ Group II elements have 1 more proton and electron than Group I elements, increase in the number of protons leads to greater attractive force to hold electrons more strongly, but increase in the number of electrons does not lead to the increase in repulsive force between electrons and shielding effect atomic and ionic radii decrease

    41. 41.3 Variation in Properties of the s-Block Elements (SB p.53) Ionization Enthalpy

    42. 41.3 Variation in Properties of the s-Block Elements (SB p.54) Variations in the 1st and 2nd ionization enthalpies of Group I elements

    43. 41.3 Variation in Properties of the s-Block Elements (SB p.54) Variations in the 1st, 2nd and 3rd ionization enthalpies of Group II elements

    44. 41.3 Variation in Properties of the s-Block Elements (SB p.55) • Observations: • • 1st I.E. of Group I elements are low and the 2nd I.E. are extremely high • Reason: • The outermost s electron enters a new electron shell which is further away from nucleus, and is shielded by inner electron shells •  the attractive force is weak, easy to be removed • The second electron to be removed is from the inner electron shell •  removal of 2nd electron will disrupt the stable electronic configuration •  2nd I.E. are extremely high

    45. 41.3 Variation in Properties of the s-Block Elements (SB p.55) • • 1st and 2nd I.E. of Group II elements are low but the 3rd I.E. are much higher • Reason: • The outermost s electrons are further away from the nucleus and effectively shielded by inner electron shells •  attractive force is weak, easy to be removed • The 3rd electron to be removed is from inner electron shells  removal of 3rd electron will disrupt the stable electronic configuration •  3rd I.E. are much higher

    46. 41.3 Variation in Properties of the s-Block Elements (SB p.55) • • the ionization enthalpies decrease down the Groups • Reason: • atomic sizes increase down the group •  the outermost shell electron(s) will be better shielded by inner electron shells •  less attractive force experienced •  less energy is required to remove the electrons

    47. 41.3 Variation in Properties of the s-Block Elements (SB p.55) Melting Point • Melting points of Groups I and II elements depend on strength of metallic bonds • The stronger the bond, the higher is the melting point • Metallic bond strength depends on: (1) ionic radius, (2) no. of valence electrons

    48. 41.3 Variation in Properties of the s-Block Elements (SB p.55) Melting points of Groups I and II elements

    49. 41.3 Variation in Properties of the s-Block Elements (SB p.55) Variations in melting points of Groups I and II elements

    50. 41.3 Variation in Properties of the s-Block Elements (SB p.56) • Observations: • • melting point decreases as going down Groups I and II • Reason: • the ionic size of the elements increases • the no. of electrons in the delocalized electron sea remains unchanged •  charge density decreases • attraction between ions and electrons becomes weaker • metallic bond is weaker