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WHY DO WE NEED TO CLASSIFY ELEMENTS ?, In 1800, 31 elements were known, In 1865, 63 elements were known, , In 2015, 118 elements were known, With a large number of elements it is very difficult to, study individually the chemistry of all these elements, and their innumerable compounds individually. To, ease out this problem, scientists organise their, knowledge by classifying the elements.
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Triads By Johann Dobereiner in 1800, The physical and chemical properties of several, groups of three elements (Triads) are similar. In, each case, he noticed that the middle element of, each of the Triads had an atomic weight about half, way between the atomic weights of the other two.
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A.E.B. de Chancourtois (1862) cylindrical table, , He arranged the then known elements in order of, increasing atomic weights and made a cylindrical, table of elements to display the periodic, recurrence of properties.
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John Alexander Newlands (1865), The Law of Octaves, He arranged the elements in increasing order of their, atomic weights and noted that every eighth element had, properties similar to the first element. Newlands’s Law of, Octaves seemed to be true only for elements up to, calcium.
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Dmitri Mendeleev (1834-1907), Mendeleev's Periodic Law, The properties of the elements are a periodic function of their, atomic weights.
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Mendeleev's Periodic Table (1905), Mendeleev arranged elements in horizontal rows and vertical, columns of a table in order of their increasing atomic weights, in such a way that the elements with similar properties, occupied the same vertical column or group., He realized that some of the elements did not fit in with his, scheme of classification if the order of atomic weight was, strictly followed., , He left the gap under Aluminium and a gap under Silicon, and, called these elements Eka- Aluminium and Eka-Silicon.
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MODERN PERIODIC LAW AND THE PRESENT FORM OF, THE PERIODIC TABLE (1913) Henry Moseley, MODERN PERIODIC LAW, The physical and chemical properties of the elements are, periodic functions of their atomic numbers.
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NOMENCLATURE OF ELEMENTS WITH ATOMIC NUMBERS > 100
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ELECTRONIC CONFIGURATIONS OF ELEMENTS
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Classification of periodic table, (a) Group and Period, Vertical columns are called groups and Horizontal raws are, called periods. There are 7 periods and 18 groups in modern, periodic table
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(b) s, p, d and f Blocks, On the basis of electronic configuration periodic table is, divided in to four blocks
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The s-Block elements, The elements of Group 1 (alkali metals) and Group 2 (alkaline, earth metals) which have ns1 and ns2 outermost electronic, configuration belong to the s-Block Elements.
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The p-Block Elements, Elements belonging to Group 13 to 18 are called p-block, elements. They have a general electronic configuration, , ns2 np1-6
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The d-Block Elements, The elements of Group 3 to 12 are called d-block elements., They have a general electronic configuration ns1-2 (n-1)d1-10
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The f-Block Elements, The elements of Lanthanoids and Actinoids are called f-block, elements. They have a general electronic configuration, ns2 (n-1)d0-1 (n-2)f 1-14
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(c) Representative, Transition, Inner transition elements, and Noble gases, On the basis of elements periodic table is divided in to four
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Representative elements, Elements of group 1,2,13 to 17 are called Representative, elements
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Transition elements, , Elements of group 3 to 12 are called transition elements
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Inner transition elements, , The elements of Lanthanoids and Actinoids are called Inner, transition elements.
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Noble gases, , Elements of group 18 are called noble gases. They are stable, due to the presence of ns2 np6 configuration
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Metals, Non-metals and Metalloids, Metals appear on the left side of the Periodic Table.Non-metals, are located at the top right hand side of the Periodic Table. The, elements (e.g., silicon,germanium, arsenic, antimony and, tellurium) diagonally across the Periodic Table show properties, that are characteristic of both metals and non- metals. These, elements are called Semi-metals or Metalloids.
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Trends in Physical Properties, (i) Atomic and ionic radii, , (ii) Ionization enthalpy, (iii) Electron gain enthalpy, (iv) Electronegativity.
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Atomic Radius, Half of the distance between nuclei in covalently, bonded diatomic molecule., , Across the period atomic radius decreases because, nuclear charge increases and atomic size decreases, Down the group atomic radius increases because, nuclear charge decreases and atomic size inecreases
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Variation of atomic radius with atomic number
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(b) Ionic Radius, The ionic radii can be estimated by measuring the distances, , between cations and anions in ionic crystals.
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Variation of Atomic radius and ionic radius in a periodic table
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Iso electronic species, Some atoms and ions which contain the same number of, electrons, we call them iso electronic species., For example,, , O2- ,, , F , Na+ , Mg2+, , These have the same number of electrons (10).
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(c) Ionization Enthalpy (IE), It is the amount of energy required to remove an electron from, an isolated gaseous atom (X) in its ground state. The unit of, ionization enthalpy is kJ mol–1. Ionization enthalpies are always, positive., X(g) → X, +, , X, , +, , (g) → X, , –, +, e, (g), 2+, , –, +, e, (g), , IE1, , IE2, , Variation of first ionization enthalpies with, atomic number for elements with Z = 1 to 60
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Variation of Ionisation Energy, , First ionization enthalpies of elements of, the second period, , First ionization enthalpies of elements of, first group (alkali metals)
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Variation of Ionisation Energy, Across the period ionisation energy increases because atomic size decreases and, nuclear charge increases., Down the group ionisation energy decreases because atomic size increases and, nuclear charge decreases.
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Second ionisation energy is greater than first, why ?, The second ionization enthalpy will be higher than the first, , ionization enthalpy because it is more difficult to remove an, electron from a positively charged ion than from a neutral, atom.
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Electron Gain Enthalpy or Electron Affinity (EA), It is the amount of energy liberated when an electron is added, to an isolated gaseous atom (X) in its ground state. The unit of, electron gain enthalpy is is kJ mol–1., X(g) + e –→ X, , –, , (g), , +, , EA1, , It may be noted that electron gain enthalpies have large, negative values towards the noble gases.
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Variation of Electron gain enthalpy, Across a period electron gain enthalpy increases, because, effective nuclear charge increases and atomic size decreases., Down the group, electron gain enthalpy decreases, because, effective nuclear charge decreases and atomic size increases.
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Electron gain enthalpy of chlorine is higher than that of, fluorine why ?
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Variation of electronegativity in a periodic table
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The periodic trends of elements in the periodic table
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Anomalous Properties of Second Period Elements
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