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3 CLASSIFICATION OF ELEMENTS AND, , PERIODICITY IN PROPERTIES, 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., , 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., , 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., , Mendeleev's Periodic Law (1905), , The law states that “the physical and chemical properties of elements are periodic functions of, their atomic weights”., , Mendeleey's Periodic Table, , 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., , NOMENCLATURE OF ELEMENTS WITH ATOMIC NUMBERS > 100, , , , , , , , , , , , , , , , , , , , Atomic number | Name Symbol | TUPAC Name | IUPAC Symbol, 101 Unnilunium =| Unu Mendelevium |Md, 102 Unnilbium Unb Nobelium No, 103 Unniltrium Unt Lawrencium | Lr, 104 Unnilquadium | Ung Rutherfordium | Rf, 105 Unnilpentium |Unp Dubnium Db, 106 Unnilhexium |Unh Seaborgium [Sg, 107 TUnnilseptium [Uns | Bohrium Bh

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Atomic number | Name Symbol | TUPAC Name | TUPAC Symbol, 108 Unniloctium |Uno Hassnium Hs, 109 Unnilennium |Une Meitnerium Mt, 110 Ununnillium |Uun Darmstadtium |Ds, 111 Unununnium |Uuu Rontgenium (Rg, 112 Ununbium Uub Copernicium |Cp, 113 Ununtrium Uut, , 114 Ununquadium | Uug, , 115 Ununpentium |Uup, , 116 Ununhexium |Uuh, , 117 Ununseptium |Uus, , 118 Ununoctium |Uuo, , , , , , , , , , , , , , MODERN PERIODIC LAW (1913) (Henry Mosely), , The law states that “the physical and chemical properties of elements are periodic functions of, their atomic numbers”., , Classification of periodic table, , (a) Group and Period, , Vertical columns are called groups and Horizontal rows are called periods. There are 7 periods and 18, , groups in modern periodic table, , (b) s, p, d and f Blocks, , s-Block elements: The elements of Group | (alkali metals) and Group 2 (alkaline earth metals) which, have ns1 and ns? outermost electronic configuration belong to the s-Block Elements., , p-Block Elements: Elements belonging to Group 13 to 18 are called p-block elements. They have a, general electronic configuration ns? np'*, , d-Block Elements: The elements of Group 3 to 12 are called d-block elements. They have a general, electronic configuration ns'* (n-1) d'!°, , f-Block Elements: The elements of Lanthanoids and Actinoids are called f-block elements. They have, , a general electronic configuration ns? (n-1) do: (n-2) f, , (c) Representative, Transition, Inner transition elements and Noble gases, , Representative elements: Elements of group 1,2,13 to 17 are called Representative elements., Transition elements: Elements of group 3 to 12 are called transition elements., 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 ns?, , np® configuration., , (d) 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., , Trends in Physical Properties, @ Atomic Radius, Half of the distance between nuclei in covalently bonded diatomic molecule., Variation of Atomic radius in a periodic table, 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, increases., (a) Tonic Radius, The ionic radii can be estimated by measuring the distances between cations and anions in ionic, crystals,, Cations are smaller than their parent atoms. Anions are larger than their parent atoms., Iso electronic species, Some atoms and ions which contain the same number of electrons, we call them iso electronic, species., For example, O® , F, Na*, Mg?" These have the same number of electrons (10)., (ii) Ionization Enthalpy (TE), 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”. Ionization enthalpies are always, positive., , Xq@) —~X*mre TE;, , X*@ 7X* wre IE>, 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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(iii) Electron Gain Enthalpy, 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 KJ mol '., , Xwr+e —> X~ + EA:, Noble gases have large positive electron gain enthalpies because the electron has to enter the, next higher principal quantum level leading to a very unstable electronic configuration., 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., (iv) _Electronegativity, It is defined as the tendency of atom to attract a shared pair of electrons., Variation of Electronegativity, Across a period, electronegativity increases, because effective nuclear charge increases and, atomic size decreases. Down the group, electronegativity decreases, because effective nuclear, charge decreases and atomic size increases., Questions and Answers, 1. Second ionisation enthalpy is greater than first why?, Ans: 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., 2. The most electronegative element is, Ans: Fluorine, 5. Give the TUPAC name of the element with atomic number 117., Ans; Ununseptium, 4. Consider the following species N*, O?, F, Na", Mg*", AP’, What is common in them?, Ans: They are isoelectronic species. (10 electrons), , 3. Match the following, , , , A_| B__, Sodium f-block, Oxygen s-block, Uranium | d-block, , Silver p-block, , , , , , , , , , , , Ans: Sodium - s-block, Oxygen - p-block, Uranium- f-block, Silver - d-block,, , , , Generated Via PDF Scanner

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6. Which one has greater size: Sodium or potassium? Justify your answer?, , Ans: Potassium. Because down the group no of shells increases so atomic size increases., , te The first ionisation enthalpy of sodium is lower than that of Magnesium, but its second, ionisation enthalpy is higher than that of Magnesium. Why?, , Ans: Na - 1s? 2s? 2p® 3s!, , Mg - 18? 2s? 2p? 3s?, , By removing one electron, Na gets stable noble gas configuration(octet).But, since the s orbital, of Mg is completely filled , removal of first electron will be difficult. So the first ionisation, enthalpy of Na is lower than that of Mg. Removal of 2nd electron from Na is difficult due to its, octet configuration but by removing 2nd electron from Mg, it gets stable noble gas, configuration. So the second ionisation enthalpy of Na is higher than that of Mg., , 8. Electron gain enthalpy of Fluorine is lower than that of Chlorine why?, , Ans: Due to small size and greater electronic repulsion in fluorine., , 9. What is meant by electron gain enthalpy? What are the factors affecting it?, , Ans; It is the heat change (enthalpy change) when an electron is added to the outer most shell, of an isolated gaseous atom. Size of the atom and nuclear charge., , 10. What is meant by electronegativity? How does electronegativity vary in the periodic, table? Justify, , Ans: It is defined as the tendency of atom to attract a shared pair of electrons., Electronegativity decreases down the group because atomic size increases and nuclear, attraction decreases, Electronegativity increases along the period due to decrease in size and, increase in nuclear charge., , 11, Account for the following, , (i) Ionization enthalpy of Nitrogen is greater than that of Oxygen, , (ii) Atomic radius decreases from left to right along a period, , Ans: (i)) Due to the stable half-filled electronic configuration of Nitrogen., , (iii) Along a period, the no. of shells remains the same and the nuclear charge increases, , one by one. So, the atomic radius decreases.