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1. The substance made up of similar atoms is called an element.
2. Elements are to be classified in order to understand them in a better way with respect to their properties.
3. In early days elements were classified into metals and non metals.
4. Robert Boyle (1661) defined an element as any substance that can not be decomposed into a further simple substance by a Physical or Chemical change.
5. Towards the end of the eighteenth century by the time of Lavoisier another eleven element were discovered.
6. By 1865 about sixty three elements were known and by 1940, a total of ninety one elements from natural sources and another seventeen elements synthetically were obtained.
7. By now, including synthetic elements, there are more than 115 elements.
8. As the number of elements increased, it became difficult to keep in memory the chemistries of individual elements and their compounds. It is necessary to classify the elements to understand easily.

9. Joseph Louis Proust stated that Hydrogen atom is the building material and atoms of all other elements are simply due to the combination of number of Hydrogen atoms.
10. Dobereiner proposed the law of triads for the classification of elements.
11. The group of three elements having the similar properties is called triad.
12. In Doberenier triad, the atomic weight of middle element is nearly the arithmetic mean of first and third elements or the atomic weights of all the three elements are approximately the same.
13. Doberenier triads are
i) Li (7), Na (23), K (39)
ii) Ca (40), Sr (87.5), Ba (137)
iii) C(35.5), Br (80), I (127)
iv) S (32), Se (78), Te (125)
v) Mn (55), Cr (52), Fe (56)
14. Limitations of Dobereiner law of triads
i) All the known elements at that time could not be arranged in the form of triads.
ii) The law failed for very low mass or for very high mass elements.
iii) As the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly valid.

15. New land proposed the law of octaves for the classification of elements.
16. When elements are arranged in the increasing order of their atomic weights. Newland noted that the eight element is a kind of repetition of the first, like the eight note of an octave in music. It is called Newlands law of octaves.
17. Newland was the first to assign atomic numbers to the elements.
18. Failures of Newland's table is not with out problems.
i) There are instances of two elements fitted into the same slot.

e.g.: Cobalt and Nickel.
ii) Certain elements totally dissimilar in their properties, were fitted into the same group.
For example he arranged Co, Ni, Pd, Pt and Ir which have different properties compared with halogens in the same row F, Cl, Br, I.
iii) Newlands periodic table was restricted to only 56 elements and did not leave any room for new elements.

19. Mendeleev and Lothar Meyer used atomic weight for the classification of elements.
20. The Physical and Chemical properties of elements are the periodic function of their atomic weights  is called Mendeleev's periodic law.

21. Mendeleev tried to explain the similarities of elements in the same group in terms of their common valency.
22. There are eight vertical columns in mendeleev's periodic table called as groups. They are represented by Roman numerals I to VIII.
23. Elements present in a given vertical column or group have similar properties.
24. Each group is divided into two sub - groups "A" and "B"
25. The elements within any sub group resemble one another to great extent.
26. Group IA elements are called alkali metals (Li, Na, K, Rb, Cs).
27. Group IIA elements are called alkaline earth metals (Be, Mg, Ca, Sr, Ba).
28. The horizontal rows in Mendeleev's periodic table are called periods.
29. There are seven periods in the Mendeleev's periodic table, which are denoted by Arabic numbers 1 to 7.
30. Elements in a period differ in their properties from one another.
31. Mendeleev's periodic table helped in correcting the atomic masses of some elements like, beryllium, lndium and gold.
32. In Mendeleevs periodic table, elements with dissimilar properties were placed in same group as sub group 'A' and sub group 'B'.

33. In Mendeleev's periodic table, three pairs of elements atomic weights are reversed. These are anomalous pairs.
(i) Tellurium and Iodide.
(ii) Cobalt and Nickel
(iii) Argon and Potassium.
34. Mendeleev's periodic table predicted the existence of some of the unknown elements from their properties.

Predicted element

            After discovered element







35. The Physical and chemical properties of elements are periodic functions of their atomic numbers or electronic configurations. It is called modern periodic law.
36. In the long form or extended form of a periodic table the elements are arranged on basis of electronic configuration.
37. In the periodic table the vertical columns are called groups and the horizontal rows are called periods.
38. Long form of the periodic table contains 7 periods and 18 groups.
39. Depending upon to which sub-shell the differentiating electron. i.e, the last coming electron centers in the elements are classified as s, p, d and f block elements.
40. In elements of 1, 2, 13, 14, 15, 16, 17 groups, the outer most orbit is only incomplete. These are called normal elements.
41. In 18 group elements all shells are completely filled these are called Inert gases.
42. In groups 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 two shells are incompletely filled. these elements are called transition elements.
43. First period contains 2 elements, second and third periods contains 8 elements each, fourth and fifth periods contains 18 elements each, sixth period contains 32 elements and seventh period is incomplete.
44. Elements from atomic number 57 (La) to 71 (Lu) are known as Lanthanides.

45. Elements from atomic number 89 (AC) to 103 (Lr) are known as Actinides.
46. The Lanthanides and actinides are placed at the bottom of the periodic table in separate blocks.
47. Based on the electronic configuration, the elements are classified into four classes
(i) Inert gases
(ii) Representative elements
(iii) Transition elements
(iv) Inner Transition elements.

48. He, Ne, Ar, Kr, Xe and Rn elements are not involve in chemical reactions. So these are known as inert gases the electronic configuration of Inert gases is ns2np6. Electronic configuration of Helium is 1s2
49. Elements having incompletely filled outermost shell are known as representative elements. The general electronic configuration of representative elements is nsto ns2np5
50. Elements having ns1 and ns2 outer electronic configuration are known as s-block elements. These are present in group IA and IIA

51. Elements having ns2np1 to ns2npouter electronic configuration are known as p-block elements. These are present in group IIIA, IVA, VA, VIA and VIIA.
52. Elements having ns2 (n - 1)d1 - 10 outer electronic configuration are known as transition elements. These are also called d-block elements.
53. Elements having ns2(n - 1)d0 or 1 (n - 2)f1-14 outer electronic configuration are known as inner transition elements. These are also called f-block elements.
54. The elements with three or less electrons in the other shell are considered to be metals and those with five or more electrons in the outer shell considered to be non metals.
55. Metalloids or semi metals are elements which have properties that are intermediate between the properties of metals and non-metals.
56. B, Si, As, Ge etc are metallodis.
57. s-block elements are metals, where as P-block elements are metals, metalloids and non-metals.
58. Valance (or) valency of an element was defined as the combining power of an element with respect to hydrogen, oxygen.

59. In general, the valence of an element with respect to hydrogen is its traditional group number. If the element is in the group V or above, its valence is 8-group number.
60. In general, each period starts with valency 1 and then decreases to zero.
61. The distance between the center of the nucleus and the outer most orbit is called atomic radius.
62. Atomic radius is expressed in picometer units (pm).
                                        1 pm = 10-12 m

63. In a group the atomic radius increases from top to bottom. This is due to the addition of one extra shell from one element to another.
64. In a period, the atomic radius decreases from left to right. This is due to increase of nuclear attraction on outermost electrons.
65. The minimum energy required to remove an electron from the outer most orbit of an atom in the gaseous state is called Ionisation energy or Ionisation potential.
66. The energy required to remove the first electron from the outer most orbit or shell of a neutral gaseous atom of the element is called its first ionization energy.
M(g) + IE1 → M+(g) + e(IE= first ionization energy)

67. The energy required to remove an electron from uni - positive ion of the element is called the 2nd ionization energy of that element and so on.
M+(g) + IE2 ⇒M+2(g) + e(IE= second ionization energy)

68. Ionization energy is expressed in K.J./moles.
69. Ionization energy of an element depends on
i) nuclear charge.
ii) screening effect or shielding effect.
iii) penetration power of the orbitals.
iv) stable configuration
v) Atomic radius

70. More the nuclear charge more is the ionization energy.
e.g.: Between 11Na and 17Cl. chlorine atom has more ionization energy
71. More the shells with electrons between the nucleus and the valence shell, they act as screens and decrease nuclear attraction over valence electron. This is called the screening effect (shielding effect)
72. If shielding effect increases, ionization energy decreases.

73. The extent of penetration of orbitals is in the order, s > p > d > f.
74. If the penetration of orbital increases, ionisation energy increases.
75. Half filled or completely filled electronic configurations are stable so that configuration atoms having higher ionisation potentials.
76. Nitrogen contain half filled p orbitals so it contains more stable. Hence the ionisation potential of nitrogen is greater than oxygen.
77. More the atomic radius, less is the ionization energy.
78. In a group the ionisation energy decreases from top to bottom. This is due to increase of atomic size and decrease of nuclear attraction on the outer most electrons.
79. In a period the ionisation energy increases from left to right. This is due to decrease of atomic size and increase of nuclear attraction on the outer most electrons.
80. The energy liberated when and electron is added to its neutral gaseous atom is called electron affinity.
M(g) + e-  M- (g) + E1 (M = Element, E1 = First electron affinity)

81. The energy obsorbed when an electron is added to a uninegative ion of the element is called second electron affinity of that element.
M-(g) + e- → M-2(g) + E2 (E2 = second electron affinity)

82. All the factors which influence the ionisation energy would also influence the electron gain enthalpy.
83. In a group, electron affinity decreases from top to bottom due to increase of atomic size and decrease of nuclear attraction.
84. In a period, electron affinity increases from left to right due to decrease of atomic size and increase of nuclear attraction.
85. The highest electron affinity element is chlorine.

86. The tendency of an atom to attract a shared pair of electrons with other atom in a molecule is called electronegativity.
87. Millikan proposed that the electronegativity of an element is the average value of its ionisation energy and electron affinity.

88. Pauling assigned the electronegativity values for elements on the basis of bond energies. He assumed that the electronegativity of hydrogen is 2.20 and calculated the values of other elements with respect to hydrogen.

89. Electronegativity values of elements decrease as we go down in a group and increase along a period from left to right.
90. The most electronegative element is 'F' and the least electronegative stable element is 'Cs'.
91. The tendency of an atom to lose electrons and become positively charged ion is called electro positive character.
92. Metals are electropositive elements.
93. Metals show less electronegative character.
94. Non metals are more electronegative due to their smaller atomic radii.
95. Metallic character increases and non metallic Character decreases from top to  bottom in a group.
96. Metallic character decreases and non metallic character increases from left to right in a period.
97. Al, Si and Ge are metalloids.
98. Periodic properties of elements and their trends in groups and in periods.

Periodic property

Trend in


  (From top to bottom)


       (From left to right)


  From top to bottom
Same for all elements

From left to right Increases from 1 to 4 and decreases to 0.

Atomic radius



Ionisation energy



Electron affinity



Electron positivity



Electron negativity



Metallic nature



Non-Metallic Nature



Conceptual flow chart


Posted Date : 17-11-2020

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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