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Chemistry

Chemistry. Classification of elements-II. Session Objectives. Session Opener. Perspective. Understanding of basic properties like atomic size ionisation energy, electron affinity and electronegativity will help in understanding general trends in s and p block elements. Session Objectives.

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Chemistry

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  1. Chemistry

  2. Classification of elements-II

  3. Session Objectives

  4. Session Opener

  5. Perspective Understanding of basic properties like atomic size ionisation energy, electron affinity and electronegativity will help in understanding general trends in s and p block elements.

  6. Session Objectives • Causes of periodicity • Atomic size,ionic radii,trend in groups and periods • Ionisation energy. • Electron affinity • Electronegativity • Valency and its trend • Anomalous behaviour of first element of group • Diagonal relationship

  7. Element Atomic no. Electronic configuration Li 3 1s2,2s1 Na 11 1s2,2s2,2p6,3s1 K 19 1s2,2s2,2p6,3s2,3p6,4s1 Rb 37 1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s1 Causes of periodicity Repetition of similar valence shell configuration after regular interval.

  8. c a b Atomic size Covalent and van der waal’s radius:

  9. Trends of atomic size

  10. Ask your self Which element has highest covalent radius? Solution: Cs

  11. Size of cation

  12. Fe Fe3+ Fe2+ Protons 26 26 26 23 Electrons 24 26 Cl Cl– Protons 17 17 Electrons 17 18 Size of cation Size of anion

  13. Isoelectronic ions • Note for isoelectronic series Na+, Mg2+, Al3+, N3-, O2-, F-, • N3-> O2-> F-> Na+> Mg2+> Al3+ • Most positive ion the smallest, most negative the largest

  14. + hn IE -e- Ionisation energy • Minimum energy required to remove an electron from a ground-state, gaseous atom • Energy always positive (requires energy) • Measures how tightly the e- is held in atom (think size also) • Energy associated with this reaction Isolated gaseous atom

  15. M – e M+ M2+ – e – e M3+ IE1 IE3 IE2 Successive ionisation energies IE3 > IE2 > IE1

  16. SizeIonisation energy  1 Atomic size Li 1.23 520 Be 0.89 899 Atomic size Ionisation energy KJ/mole Factors affecting values of ionisation energy

  17. Li +3 520 Be +4 899 Effective nuclearcharge Ionisation energy KJ/mole Factors affecting values of ionisation energy 2. Effective nuclear charge Is net nuclear attraction towards the valence shell electrons . Ionization energy  Effective nuclear charge

  18. 3. Screening effect orshielding effect 1 Combined effect of attractive and repulsive forces between electron and proton. Number of inner shells Li 1 520 Na 2 496 No. of inner shells Ionisation energy  Ionisation energy KJ/mole Factors affecting values of ionisation energy

  19. 5. Complete octet Elements having ns2,np6 configuration have extremelyhigh ionisation energy. 4. Penetrating power of orbitals s>p>d>f Factors affecting values of ionisation energy

  20. Factors affecting values of ionisation energy 1  Ionisation energy 6. Stable Configuration Stability of configuration Be 2s2 899 B 2s22p1 801 Configuration Ionisation energy KJ/mole

  21. Trend of ionisation energy in period and groups Exceptions • IE > IE • II A III A • ns2 ns2,np1 (ii) IE > IE V A VI A ns2,np3 ns2,np4 (iii) Ionisation energy of Al > Ga Absence of d electrons in Al

  22. Variation of I1 with Z In a group (column), I1decreases with increasing Z. valence e’s with larger nare further from the nucleus, less tightly held

  23. Variation of I1 with Z Across a period (row), I1 mainly increases with increasing Z. Because of increasing nuclear charge (Z).

  24.  Be has stable (2s2) configuration. IE1Be> IE1 Li IE2 Li > IE2 Be  Li acquires stable configuration when it loses one electron. Illustrative example First ionisation energy of Be is more than Li but the second ionisation energy of Be is less than Li. Why? Solution:

  25. e– EA Electron affinity • Electron affinity is energy change when an e- adds to a gas-phase, ground-state atom • Positive EA means that energy is released, e- addition is favorable and anion is stable! • First EA’s mostly positive, a few negative Successive affinities Isolated gaseous atom

  26. 1  1 atomic size  Stable configuration Li Na At. size 1.23 1.57 EA kJ/mol -57 -21 Li Be Config. 2s1 2s2 EA kJ/mol -57 66 Li Na Inner shells 1 2 EA kJ/mol -57 -21 1 Penetrating power   Effective nuclear charge  Screening effect s>p>d>f Li Be E.N.C 1.23 0.89 EA kJ/mol -57 66 Factors affecting electron affinity Electron affinity

  27. Trends in electron affinities • Decrease down a group and increase across a period in general but there are not clear cut trends as with atomic size and I.E. • Nonmetals are more likely to accept e-s than metals. VIIA’s like to accept e-s the most. Exceptions • EA of Cl > EA of F • Group II A have almost zero electron affinities due to stable ns2 configuration of valence shell. • Group V A have very low values of electron affinities • due to ns2,np3configuration of valence shell.

  28. Do you know? More the value of electron affinitygreater is the oxidising power.

  29. 1 1. Electronegativity  Atomic size Electronegativity It is the relative tendency to attract shared pair of electrons towards itself. Factors effecting electronegativity 2. Electronegativity is higher for nearly filled configuration e.g. O(3.5) and F(4.0).

  30. Li Be B C N O F Valence shellconfiguration 2s1 2s2 2s2,2p1 2s2,2p2 2s2,2P3 2s2 ,2P4 2s2,2P5 Electronegativity 1.0 1.52.0 2.5 3.0 3.5 4.0 Periodic variation (i) In period (ii) In groups-decreases down the group

  31. Pauling scale of electronegativity

  32. Mulliken’s scale of electronegativity Electronegativity represents an average of the binding energy of the outermost electrons over a range of valence-state ionizations (A+ A A- in A-B) In other words, the average of the ionization energy and the electron affinity.

  33. Do you know 1. Smaller atoms have more electronegativities 2. F is most electronegative element. 3. Decreasing order of electro negativity F > O > Cl  N > Br > C > I > H

  34. Valency Li 3 1s2,2s1 Na 11 1s2,2s2,2p6,3s1 K 19 1s2,2s2,2p6,3s2,3p6,4s1 Rb 37 1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s1 • The valency of an element is decided by number of electrons present in outermost shell. • All the elements of a group have same valency.E.g.- All the group I elements show 1 valency.

  35. Valency • Valency of s block elements issame as their group number. • Examples:Ca is member of group 2  its valency is 2 • K is member of group 1 its valency is 1

  36. Valency • Valency of p block elements isequal to number of electrons invalence shell. • e.g.- Al has 3 electrons in valence shell.Therefore, its valency is 3. Or8-number of electrons in valence shell.e.g- valency of oxygen is 8 – 6 =2

  37. Valency • Valency of d and f block elements variable. Iron shows the valence 2 and 3

  38. Valency

  39. Element Li Be B C N O F Ne Number of electrons in valence shell 1 2 3 4 5 6 7 8 Valency 1 2 3 4 3 2 1 0 Valency in period

  40. Anomalous behaviour of first element of group Causes: • Smallest size in group. • Highest value of ionisation energy in the group. • Absence of vacant d orbitals.

  41. Examples of anomalous behaviour of first element of group • Carbon forms multiple bonds but rest of the members form only single bonds. • Nitrogen does not form NCl5 but phosphorous forms PCl5.

  42. 2nd period Li Be B C 3rd period Na Mg Al Si Diagonal relationship

  43. Causes of diagonal relationship • Similarity in size. • Similarity in ionisation energy. • Similarity in electron affinity.

  44. Class Test

  45. Class Exercise - 1 Which has the smallest size?(a) Na+ (b) Mg2+(c) Al3+ (d) P5+ Solution: Size of isoelectronic species decreases with increase innuclear charge. Hence, answer is (d).

  46. Class Exercise - 2 If the first ionization energy of helium is 2.37 kJ/mole, the first ionization energy of neon in kJ/mole is:(a) 0.11 (b) 2.37 (c) 2.68 (d) 2.08 Solution: Ionization energy decreases down the group. Hence, answer is (d).

  47. Class Exercise - 3 The relative electronegativities of F, O, N, C and H are(a) F > C > H > N > O (b) F > O > N > C > H(c) F > N > C > H > O (d) F > N > H > C > O Solution: The correct order of electronegativities is Hence, answer is (b).

  48. Class Exercise - 4 Which of the following ions hassmallest ionic radius?(a) Li+ (b) Be2+(c) H– (d) All have equal radii Solution: More the nuclear charge on cation smallerwill be the size. Hence, answer is (b).

  49. Class Exercise - 5 Which one of the following is correct order of ionic size?(a) Ca2+ > K1+ > Cl- > S2- (b) S2- > Cl- > K+ > Ca2+(c) Ca2+ > Cl- > K1+ > S2-(d) S2- > Ca2+ > Cl- > K+ Solution: Size of iso electronic species decreases with increasein nuclear charge, more interelectronic repulsion inS and Cl is the reason of their increased size. Hence, answer is (b).

  50. Class Exercise - 6 The electron affinities of N,O, S and Cl are(a) N < O < S < Cl (b) O < N < Cl < S(c) O = Cl < N = S(d) O < S < Cl < N Solution: The correct order of electron affirmities isN < O < S < Cl Hence, answer is (a).

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