CBSE Class 11 Chemistry Classification Of Elements Notes Set C

Class XI: Chemistry

Chapter 3: Classification of Elements

Top Concepts

1. Johann Dobereiner classified elements in group of three elements called triads.

2. In Dobereiner’s triad the atomic weight of the middle element is very close 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 was dismissed.

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

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

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

8. According to Mendeleev’s periodic law the physical and chemical properties 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 and studying the properties of large number of elements

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

  • Mendeleev could predict the properties of some undiscovered elements like 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 is placed in group I though it resembles both group 1 and 17.

  • In certain pairs of elements increasing order of atomic masses was not obeyed. For example argon (Ar, atomic mass 39.9) is placed before  otassium (K, atomic mass 39.1).

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

  • Some similar elements are separated and dissimilar elements are grouped 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 the elements.

  • Lanthanoids and actinoids were not given a separated position in the table. 

11. Moseley performed experiments and studied the frequencies of the Xrays 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 was modified to modern periodic law.

13. According to Modern periodic law the physical and chemical properties of the 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 of elements.

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 seventh periods 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 transuranic elements.

25. According to IUPAC, until a new element’s discovery is proved and its name 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 by combining 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 its electronic 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 atomic orbitals available in the energy level that is being filled.

31. On moving down a group in a periodic table the number of shell increases 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 outermost shell gives the period.

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

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

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

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

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

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

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

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 eighteen elements.

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 4d  transition series of elements or second transition series.

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

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 called elements of inner transition series or lanthanoid series.

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

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

53. The modern periodic table is divided into four main blocks – s -block, pblock,

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 outermost energy 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 the general outer electronic configuration.

57. The elements of Group-2 are called alkaline earth metals and have ns2 as the general outer electronic configuration.

58. The elements in which last electron enter the p-orbital of their outermost 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 representative element.

61. The outermost electronic configuration of p-block elements varies from ns2np1 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 dblock elements.

Please refer to attached file for CBSE Class 11 Classification of Elements-Concepts