CBSE Class 11 Chemistry Classification Of Elements Notes Set B

Class: XI

Chapter 3: Classification of Elements

Chapter Notes

Top Concepts

1. Johann Dobereiner classified elements in group of three elements calledtriads.

2. In Dobereiner’s triad the atomic weight of the middle element is veryclose to the arithmetic mean of the other two elements.

3. Dobereiner’s relationship is referred as Law of triads.

4. Since Dobereiner’s Law of triads worked only for few elements, it wasdismissed.

5. Chancourtois arranged elements in order of increasing atomic weightsand made a cylindrical table of elements.

6. John Newland arranged the elements in the increasing order of atomicweight and noted that the properties of the every eighth element are similar to the first one. This relationship is called as “Law of octaves”

7. Lothar Meyer proposed that on arranging the elements in order ofincreasing atomic weights similarities appear at a regular interval in physical and chemical properties.

8. According to Mendeleev’s periodic law the physical and chemicalproperties of elements are periodic functions of their atomic weights.

9. Merits of Mendeleev’s periodic table:

• Mendeleev’s periodic table was very helpful in remembering andstudying the properties of large number of elements

• Mendeleev’s periodic table helped in correcting the atomic masses ofsome of the elements like gold, beryllium and platinum based on their positions in the periodic table

• Mendeleev could predict the properties of some undiscovered elementslike scandium, gallium and germanium. By this intuition, he had left gaps for the undiscovered elements while arranging elements in his periodic table.

10. Demerits of Merits of Mendeleev’s periodic table:

• Position of hydrogen is not correctly defined in periodic table. It isplaced in group I though it resembles both group 1 and 17.

• In certain pairs of elements increasing order of atomic masses was notobeyed. For example argon (Ar, atomic mass 39.9) is placed before potassium (K, atomic mass 39.1)

• Isotopes were not given separate places in the periodic table althoughMendeleev's classification is based on the atomic masses.

• Some similar elements are separated and dissimilar elements aregrouped together. For example copper and mercury resembled in their properties but had been placed in different groups. On the other hand lithium and copper were placed together although their properties are quite different.

• Mendeleev did not explain the cause of periodicity among theelements.

• Lanthanoids and actinoids were not given a separated position in thetable

11. Moseley performed experiments and studied the frequencies of the X-rays emitted from the elements. With these experiments he concluded that atomic number is more fundamental property of an element than its atomic mass.

12. After Moseley’s experimental results Mendeleev’s periodic law wasmodified to modern periodic law.

13. According to Modern periodic law the physical and chemical properties ofthe elements are periodic functions of their atomic numbers.

14. Modern periodic table is also referred to as long form of periodic table

15. Horizontal rows in the periodic table are called periods.

16. Vertical columns in the periodic table are called groups.

17. In the modern periodic table there are 7 periods and 18 groups.

18. The period number corresponds to highest principal quantum number ofelements.

19. First period contains 2 elements

20. Second and third period contains 8 elements

21. Fourth and fifth period contains 18 elements

22. Sixth period contains 32 elements

23. In the modern periodic table, 14 elements of both sixth and seventhperiods i.e. lanthanoids and actinoids respectively are placed separately at the bottom of the periodic table.

24. Elements with atomic number greater than 92 are called transuranicelements.

25. According to IUPAC, until a new element’s discovery is proved and itsname is officially recognized it is given a temporary name. This nomenclature is based Latin words for their numbers.

26. The interim names of the newly discovered elements are derived bycombining together the roots in order of digits which make up the atomic number and ium is added at the end. 

27. Notation for the IUPAC nomenclature of elements

28. The distribution of electrons into orbitals of an atom is called itselectronic configuration.

29. The electrons in an orbital are filled according to n+l rule.

30. The number of elements in each period is twice the number of atomicorbitals available in the energy level that is being filled.

31. On moving down a group in a periodic table the number of shellincreases from 1 to 7

32. Elements in the same group have same number of valence electrons

33. Value of the principal quantum number for the valence or outermostshell gives the period.

34. The first period has principal quantum number n=1, contains twoelements and corresponds to K-shell.

35. Since K-shell contains only one orbital (1s) it can accommodate twoelectrons. Thus there are two elements in K-shell.

36. The second period has principal quantum number n=2, contains eightelements and corresponds to L-shell.

37. The 4 orbitals filled in second period are one 2s (with 2 electrons) andthree 2p (6 electrons).

38. The third period has principal quantum number n=3, contains eightelements and corresponds to M-shell.

39. The four orbitals filled in third period are one 3s ( 2 electrons) and three3p ( 6 electrons).

40. The fourth period has principal quantum number n=4, contains eighteenelements.

41. The 9 orbitals filled in fourth period are one 4s (2 electrons), five 3d(with 10 electrons) and three 4p (with 6 electrons).

42. Elements from Scandium (Z=21) to Zinc (Z=30) are called 3d transitionseries of elements or first transition series.

43. The fifth period has principal quantum number n=5, contains eighteenelements.

44. The nine orbitals filled in fifth period are one 5s (2 electrons), five 4d(10 electrons) and three 5p (6 electrons).

45. Elements from Yttrium (Z=39) to Cadmium (Z=48) are called 4dtransition series of elements or second transition series.

46. The sixth period has principal quantum number n=6, contains 32elements.

47. The 16 orbitals filled in sixth period are one 6s (2 electrons), seven 4f(14 electrons), five 5d (10 electrons) and three 6p (6 electrons).

48. The orbitals filled in seventh period are 7s,5f,6d and 7p

49. Elements from lanthanum (Z=57), Hafnium (Z=72) to mercury (Z=80)are called 5d transition series of elements or third transition series.

50. Fourteen elements from Cerium (Z=58) to Lutetium (Z=71) are calledelements of inner transition series or lanthanoid series.

51. Fourteen elements from Thorium (Z=90) to Lawrencium (Z=103) arecalled elements of 5f inner transition series or actinoid series.

52. The 4f and 5f series of elements are placed separately in periodic tableto provide a theoretical justification for periodicity occurring at regular intervals.

53. The modern periodic table is divided into four main blocks – s -block, p-block, d-block and f-block depending on the type of orbital that are being filled with exception of hydrogen and helium.

54. The elements in which last electron enter the s-orbital of their outermostenergy level are called s-block elements.

55. The s-block consists of two groups, Group-1 and Group-2.

56. The elements of Group-1 are called alkali metals and have ns1 as thegeneral outer electronic configuration.

57. The elements of Group-2 are called alkaline earth metals and have ns2as the general outer electronic configuration.

58. The elements in which last electron enter the p-orbital of theiroutermost energy level are called p-block elements.

59. The p-block elements constitute elements belonging to group 13 to 18.

60. Elements of s-block and p-block are collectively called representativeelement

61. The outermost electronic configuration of p-block elements varies fromns2np1 to ns2np6

62. Elements of group 18 having ns2np6 configuration are called noble gases.

63. Elements of group 17 are called halogens

64. Elements of group 16 are called chalcogens

65. Number of valence electrons in group =Group number -10 for elements belonging to group 13 to 18

66. Elements in which the last electron enters d-orbitals of penultimate energy level constitute d-block elements.

67. Elements of group 3 to 12 in the centre of periodic table constitute the d-block elements

68. General outer electronic configuration of d-block elements is (n-1)d1-10 ns1-2

69. d-block elements constitute transition series elements. The name “transition series” is derived from the fact the d-block elements represent transition in character from reactive metals (belonging to group1 and 2 constituting s-block) on one side of the periodic table to non-metals (belonging to group 13 to 18 constituting p-block) on other side of the periodic table .

70. Elements in which last electron enters f-orbitals are called f-block elements

71. Elements of Lanthanoid series have general outer electronic configuration of 4f1-14 5d0-1 6s2

72. Elements of Actinoid series have general outer electronic configuration of 5f1-14 6d0-1 7s2

73. Elements in lanthanoid and actinoid series are called inner transition series.

74. Metals comprise more than 78 % of all known elements and appear on left hand side of periodic table

75. Non-metals are placed on right hand side of periodic table

76. Metal are characterized by having a tendency to loose electron

77. Non-metals are characterised by having tendency to gain electron

78. In general metallic character increases down the group and decreases along period

Please click the link below to download pdf file for CBSE Class 11 Chemistry Notes - Classification of Elements.