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CHAPTER-5, PERIODIC CLASSIFICATION OF ELEMENTS

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MODERN PERIODIC TABLE, Modern periodic law states that, ‘ The properties of elements are periodic functions of their, atomic numbers’., , In the modern periodic table elements are arranged in the increasing order of their atomic, numbers in the form of a table having 7 horizontal rows of elements called periods and 18, vertical rows of elements called groups.

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Periods :, , There are 7 periods of elements as follows :i., , First period has 2 elements H and He called very short period., , ii. Second period has 8 elements Li to Ne called short period., iii. Third period has 8 elements Na to Ar called short period., iv. Fourth period has 18 elements K to Kr called long period., v. Fifth period has 18 elements Rb to Xe called long period., vi. Sixth period has 32 elements Cs to Rn called very long period., vii. Seventh period has 28 elements from Fr to atomic number 114 called incomplete period. 14 elements each of he, , sixth and seventh periods are placed separately at the bottom of the table. The 14 elements of the sixth period, from La to Lu are called Lanthanides and the 14 elements of the seventh period from Ac to Lr are called, Actinides.

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Groups :, , There are 18 groups of elements divided into 9 main groups. They are I, II, III, IV, V, VI, VII, VIII and 0 groups. The groups I to, VII has two sub groups each called A – sub group and B – sub group. Group VIII has 3 rows of elements and 0 group has one row, , of elements., The A sub group elements are called normal elements., The B sub group elements are called transition elements., Lanthanides and Actinides are called inner transition elements., Group 1 (I A ) elements are called alkali metals, Group 2 (II A) elements are called alkaline earth metals., Group 17 (VII A) elements are called halogens., , Group 18 (0 group) are called noble gases., In a group all the elements have the same number of valence electrons. Group I elements have 1 valence electron, Group II, elements have 2 valence electron, Group III elements have 3 valence electrons etc., In a period all the elements contain the same number of shells.

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PROPERTIES OF ELEMENTS, IN PERIODS AND GROUPS, 1. Valence electrons :, In a period the number of valence electrons increases from 1 to 8 from the left to the right and the number of shells is the same., Example : 2nd Period, , In a group the number of valence electrons is the same for all the elements but the number of shells increases from top to bottom., Example : Group – I A

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2. Valency:, In a period the valency of the elements increases from 1 to 4 and then decreases from 4 to 0 from the left to the, right., Example : 2nd Period, , In a group the valency is the same for all elements of the group., Example : Group – I A

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3. Atomic size ( Radius of the atom):, In a period the atomic size of the elements decreases from the left to the right because the nuclear charge (number of, protons) increases and so the electrons are pulled closer to the nucleus., Example: 2nd Period, , In a group the atomic size of the elements increases from top to bottom because the number of shells increases and, the distance between the nucleus and shells also increases., Example: Group – I A

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4. Metallic property (Electropositive nature) :, In a period the metallic property of the elements decreases from the left to the right., Example: 3rd Period, , In a group the metallic property of the elements increases from the top to the bottom., Example: Group VI A

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5. Non metallic property (Electronegative nature):, In a period the non metallic property of the elements increases from the left to the right., Example: 3rd Period, , In a group the non metallic property of the elements decreases from the top to the bottom., Example: Group VI A

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SUMMARY, ● The modern periodic table:, In 1913 Henry Moseley showed that properties of the elements are determined by atomic, numbers instead of the atomic mass. It formed the basis of modern periodic law. The law, is - "The physical and chemical properties of the elements are periodic function of their, atomic numbers". Since atomic mass is a nuclear property where as atomic number, implies for the no. of electrons in neutral atom or no. of protons in nucleus. Nucleus is, deep seated in the atoms and does not take part in chemical reactions. Therefore, the, physical and chemical properties depend upon the no. of electrons and their electronic, configuration which in turn depends upon atomic number (Z). So, when elements are, arranged in the increasing order of atomic numbers, after a regular interval elements have, similar no. of valence electrons therefore chemical properties are repeated i.e. periodicity, in the chemical properties of the elements occurs., It is also called as Bohr, Bury & Rang, Werner periodic table, (1) It is based on the Bohr-Bury electronic configuration concept and atomic number., (2) This model is proposed by Rang and Werner. This table is based on modern periodic, law, the elements are arranged in the increasing order of atomic numbers in such a way, that elements having the same number of valence electrons are placed in the same vertical, column. It consists of 18 vertical columns and seven horizontal rows. Vertical columns of, periodic table are known as groups while horizontal rows are known as periods., Characteristics of the elements present in groups are:, 1. The elements present in a group are separated by definite gaps of atomic numbers, (8, 8, 18, 18, 32)., 2. There are eighteen (18) independent groups in the Long from of Periodic Table., These are numbered from 1 to 18., 3. The elements present in a group have the same number of electrons in the valence, shell of their atoms., 4. The elements present in a group have the same valence., 5. The elements present in group have identical chemical properties., 6. The physical properties of the elements in group such as melting point, boiling, point, density vary gradually., 7. Atomic radii of the elements present in a group increase downwards.

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Characteristics of the element present in period:, 1. In all the elements present in a period the electrons are filled in the valence shell., 2. As the number of electrons in the valence shell change, the chemical properties of the, elements present in a period also change., 3. Atomic radii of the elements in a period decrease from left to the right., 4. Along a period, the metallic character of the elements decreases and the non-metallic, character increases., 5. Along a period, the reducing character of the elements decreases and their oxidizing, character increases., Advantages of Long form over Mendeleev's Periodic Table:, There are several advantages of long form of periodic table over Mendellev's periodic table., Some of these are as follows:, 1. It is based upon atomic number which is a fundamental property instead of atomic, mass., 2. The elements have been grouped as s, p, d and f-block elements. Which helps us to, understand the electronic configuration in a better way., 3. In the long form of periodic table, the elements are arranged in the increasing order of, their atomic numbers therefore, no separate place is required for isotopes., 4. The position of some of the elements which were a misfit on the basis of atomic mass, is not because argon has atomic number 18 which is less than that of potassium which, is 19., 5. Metals, non-metals, metalloids, transition elements, lanthanoids and actinoids and, actinoids are now better classified.