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CHAPTER 5: PERIODIC CLASSIFICATION OF ELEMENTS EARLY ATTEMPTS OF CLASSIFICATION *Elements were classified as metals and nonmetals based on their physical and chemical properties. *In 1808, Dalton stated in his atomic theory that atom of one element can be distinguished from the atom of another element based on it’s atomic mass. *So, all earlier classifications were based on atomic mass. DOBERENIER’S LAW OF TRIADS “When the elements are arranged in groups of 3 in increasing order of atomic masses, the middle element of a group has the atomic mass and properties roughly the average of the other 2 elements.” These elements show similarity in their properties. Eg Elements Li Atomic 7 Mass Na 23 K 39 Cl 35.5 Br 80 I 127 Average atomic mass of first and second (7+39)/2 = 23 (35.5+127)/2 = 81.2 LIMITATION All the elements could not be classified into triads discovered at that time. Only a limited number of elements could be arranged in such triads. NEWLAND’S LAW OF OCTAVES Newland classified elements on the basis of “Law of Octaves” stated by him. According to this law, “when the elements are arranged in the order of increasing atomic mass, the properties of the 8th element are repetition of the properties of the first element.” LIMITATIONS All the elements discovered @ that time could not be classified into octaves. MENDELEEV’S CLASSIFICATION Mendeleev’s arranged 63 elements known at that time on the basis of similarity in properties that is on the basis of similarities in the formula of their oxides, hydrides, etc formed by these elements. He observed that most of the elements were placed in the increasing order of their atomic masses. Mendeleev’s periodic law states that “the physical and chemical properties of elements are periodic function of their atomic mass”. In Mendeleev’s Periodic table: 1. There are 8 vertical columns called groups. The groups from one to seven are subdivided into 2 groups, A and B. Thus, on the whole, 15 vertical columns. 2. The horizontal rows in the periodic table are called periods. 3. In a period, the properties of elements gradually changes from metallic to nonmetallic characters. ADVANTAGES OF MENDELEEV’S PERIODIC TABLE 1. He could classify all the 63 elements discovered at that time on the basis of similarities in properties. 2. He left gaps for yet to be discovered elements. 3. He predicted the properties of undiscovered elements and thus helped in the discovery of these elements later on. 4. He named them by prefixing a Sanskrit numeral Eka to the name of the preceding element in the same group. 5. It helped in correction of atomic weight of cirtain elements on the basis of their position. LIMITATIONS OF MENDELEEV’S PERIODIC TABLE 1. Although most of the elements were placed in the order of increasing atomic masses, increasing order could not be maintained in all cases. Eg cobalt (atomic mass 58.9 preseded nickel 58.7) but he could maintain similarity in properties. 2. Mendeleev’s periodic table did not provide place for noble gasses which were discovered later. 3. There were no place for isotopes in Mendeleev’s periodic table although they differ in atomic mass. MODERN PERIODIC TABLE Henry Maseley, an English physicist found that the atomic number (Z) was the fundamental property of an element and not the atomic mass of classification of elements. MODERN PERIODIC LAW Properties of elements are periodic functions of their atomic number. LONG FORM OF PERIODIC TABLE In long form of periodic table, the elements are arranged in groups and periods on the basis of electronic configuration of elements. MAIN FEATURES OF THE LONG FORM OF THE PERIODIC TABLE 1. There are 18 vertical columns known as groups. 2. The group 1 is on the extreme left-hand side and group 18 on the extreme right-hand side of the table. 3. The group 1, 2, and then 13 to 17 contain normal elements. The normal elements are also called representative elements. 4. The elements in group 3 to group 12 are called transition elements. 5. The elements in the group 18 are known as noble gasses or inert gasses. 6. Group 1 elements are called alkate metals. Group 2 elements are called alkaline earth metals. Group 13 elements belong to boron family. Group 14 elements belong to carbon family. Group 15 elements belong to nitrogen family. Group 16 elements belong to oxygen family Group 17 elements are called halogens. 7. The horizontal rows in the periodic table are called periods. 8. There are 7 periods in the long form of periodic table. 9. The first period contains 2 elements hydrogen and Helium. They have only one shell (K). 10. The 2nd period contains 8 elements. Lithium, Baryllium, Borbon, carbon, nitrogen, oxygen, fluorine and neon. The second period has 2 shells. (K and L). 11. The third period contains 8 elements. It has 3 shells (KLM). 12. Fourth period has 18 elements. Fifth period has 18 elements. Sixth period has 32 elements. Seventh period has 32 elements. 13. The number of period to which an element belongs is given by the number of outermost shell. 14. Elements with atomic number 57 to 71 are called lanthanide series and elements with atomic number 89 to 103 are called actinide. 15. Elements having 1 valence electron are placed in group one. Elements having 2 valence electrons are placed in group 2. 16. Elements having 3 valence electrons are placed in group 13. Elements having 4 valence electrons are placed in group 14. Elements having 5 valence electrons are placed in group 15. Elements having 6 valence electrons are placed in group 16. Elements having 7 valence electrons are placed in group 17. Elements having 8 valence electrons are placed in group 18 except helium which has 2 electrons in the valence shell. 17. First period is called very short period. Second and third periods are called short period. Fourth and fifth periods are called long period. Sixth period is called very long period. Seventh period is incomplete. CHARACTERISTICS OF PERIODS 1. Valence electrons: on moving from left to right in a period, the number of valence electrons in elements increases from one to eight. 11 12 13 Na Mg Al 2, 8, 1 2, 8, 2 2, 8, 3 1 2 3 (number of valence electrons) 14 Si 2, 8, 4 4 15 P 2, 8, 5 5 16 S 2, 8, 6 6 17 Cl 2, 8, 7 7 18 Ar 2, 8, 8 8 2. Valency: On moving from left to right in each short period, the valency of elements increases from one to 4 and then decreases to 0. Na Mg 2, 8, 1 2, 8, 2 1 2 (valency) Al 2, 8, 3 3 Si 2, 8, 4 4 P 2, 8, 5 3 S 2, 8, 6 2 Cl 2, 8, 7 1 Ar 2, 8, 8 0 3. Size of atoms (or atomic size): The size of an atom is the distance between the center of the neucleous and the outermost shell of an atom. The size of an atom is indicated by writing it’s radius called atomic radius. It is expressed in Picometre whose symbol is “pm”. On moving from left to right, in a period of the periodic table, the size of atoms decreases. That is, as we move from left to right the atomic number increases which means the number of protons and electrons in the atom increases. Due to large positive charge on the neucleous, the electrons are pulled in more close to the nucleous and the size of the atom decreases. Na 186 Mg 160 AL 143 Si 118 P 110 S 104 Cl 99 Ar 4. Metallic character: On moving from left to right in a period, the metallic character of elements decreases but the nonmetallic character increases. Na Metals Mg Al Si P S Metalloids Nonmetals Cl Ar Noble gas (metallic character decrease, nonmetallic character increases). So the greatest metallic character is found in the element on the extreme left side of a period and the greatest nonmetallic character is found in the elements on the right side. So, in this period, sodium is the most metallic element and clorin is the most nonmetallic element. On moving from left to right in a period, the electropositive character of elements decreases, but the electronegative character increases. Metals loose electrons and form positive ion. So metals are electropositive elements. Nonmetals accept electrons and form negative ions, so nonmetals are called electronegative elements. So, on moving from left to right in a period, the tendency of atoms to loose electrons decreases and the tendency of atoms to gain electrons increases. 5. Chemical reactivity: On moving from left to right in a period, the chemical reactivity of elements first decreases then increases. The elements in the last group is chemically inert. Na Very reactive MG Al Si Least reactive P S Cl Very high reactive Ar Noble gas 6. Nature oxides: On moving from left to right in a period, the basic nature of oxides decreases and the acidic nature of oxides increases. Na Highly basic Mg Al Si P Amphoteric Amphoteric S Cl Highly acidic Ar CHARACTERISTICS OF GROUPS 1. Valence electrons: All the elements of a group of the periodic table has the same number of electrons. Group 1 H Li Na K Rb Cs Fr 1 2, 1 2, 8, 1 2, 8, 8 1 2, 8, 18 8 1 2, 8, 18, 18, 8 1 2, 8, 18, 32, 18, 8 1 2. Valency: All the elements in a group have the same valency. Valency of group 1 elements is 1. Valency of group 13 elements is 3. Valency of group 17 elements is 20. 3. Size of atoms: On moving down in a group of the periodic table, the size of atoms increases. Li Na K Rb Cs Fr Atomic Size 152 186 231 244 262 270 Smallest atom Biggest atom The number of electron shell in the atoms increases gradually due to which size of atoms also increases. 4. Metallic character: On going down in a group of the periodic table, the metallic character of elements increases. The greatest metallic character is found in the elements in the lowest part of a group. Li Na K Rb Cs Fr Least metallic character Most metallic character Metallic character or electropositive character increases on going down in a group. On moving down a group, the tendency of an atom to loose electrons (electropositive character) increases. Group 17 F Cl Br I Most electronegative (nonmetallic) Least electronegative On moving down a group, the tendency of an atom to gain electrons (electronagetive character) decreases. 5. Chemical reactivity: The chemical reactivity of metals increases on going down in a group of the periodic table. Due to an increase in size of atom, valence electrons of metal atoms (which take part in chemical reactions) become more and more far away from the neucleous and hence can be removed easily. So, as we go down in a group of metals, the tendency of their atoms to loose electrons increases and hence their chemical reactivity increases. The chemical reactivity of nonmetals decreases on going down in a group. Due to increase in size of atom, the neucleous of atom gos more and more deep and hence it’s attraction for the incoming electrons decreases. So the tendency of their atoms to gain electrons decreases, due to which their reactivity also decreases. Group 1 (metals) Li Na Least reactive K Rb Cs Fr Most reactive Most reactive Chemical reactivity of metals increases on going down in a group. Group 17 (nonmetals) Group 17 (nonmetals) F Cl Br I Most reactive Least reactive Chemical reactivity of nonmetals decreases on going down in a group. 6. Nature of oxides: On going down in a group of the periodic table, there is no change in the nature of oxides of elements. Eg group 1 will be basic, and group 17 will be acidic. MERITS OF MODERN PERIODIC TABLE 1. The modern periodic table is based on atomic number of element which is the most fundamental property of elements. 2. It helps us understand why element shows similar properties but elements in different groups show different properties. 3. It explains the reasons for the periodicity in properties 4. It tells us why elements are repeted after 2, 8, 18 and 32 elements. ADVANTAGES OF THE PERIODIC TABLE 1. The periodic table has made the study of chemistry systematic and easy. 2. It is easier to remember the properties element if it’s position in the periodic table is known. 3. The type of compounds formed by an element can b predicted by knowing the position in the periodic table. 4. A periodic table chart is used as a teaching aid in chemistry.