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    Add as Friendclassification of elements

    by: suraj

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    2 : Genesis of periodic classification
    3 : Johann Wolfgang Döbereiner (1780-1849) Examples of Döbereiner’ Triads: “…the atomic weight of bromine might be the arithmetical mean of the atomic weights of chlorine and iodine. This mean is (35.470+126.470)/2 = 80.470 This number is not much greater than that found by Berzelius (78.383)” Using modern values: Cl = 35.45 Br = 79.90 I = 126.90 Döbereiner’ Triads: In his paper, An Attempt to Group Elementary Substances according to Their Analogies (Poggendorf's Annalen der Physik und Chemie, 1829), Döbereiner grouped elements to show that atomic weights of a middle element were an average of two similar elements.
    4 : Examples of Döbereiner’ Triads: (continued) “…the specific gravity and atomic weight of strontia are very close to the arithmetic mean of the specific gravities and atomic weights of lime and baryta, since [356.019(=Ca.)+956.880(=Ba.)]/2 = 656.449(=Sr.) and the actual value for strontia is 647.285. “ Using modern values: Ca = 40.08 Sr = 87.62 Ba = 137.33 “In the alkali group, according to this view, soda stands in the middle, since if we take the value for the atomic weight of lithia, determined by Gmelin, = 195.310, and the value for potash = 589.916, then the arithmetic mean of these numbers, (195.310+589.916)/2 = 392.613,which comes very close to the atomic value for soda, which Berzelius determined as = 390.897.” Using modern values: Li = 6.94 Na = 22.99 K = 39.10 “If sulfur, selenium, and tellurium belong to one group, which can well be assumed, since the specific gravity of selenium is exactly the arithmetic mean of the specific gravities of sulfur and tellurium, and all three substances combine with hydrogen to form characteristic hydrogen acids, then selenium forms the middle member, since [32.239(=S) + 129.243(=Te)]/2 = 80.741 and the empirically found atomic value for selenium is 79.263.” Using modern values: S = 32.07 Se = 78.96 Te = 127.60
    5 : John A. R. Newlands (1837-1898)Law of Octaves From 1863 through 1866, Newlands published papers on the relationships between the elements. In 1865 he stated: “If the elements are arranged in the order of their equivalents, with a few slight transpositions, as in the accompanying table, it will be observed that elements belonging to the same group usually appear on the same horizontal line.”
    6 : Newlands continued: “It will also be seen that the numbers of analogous elements generally differ either by 7 or by some multiple of seven; in other words, members of the same group stand to each other in the same relation as the extremities of one or more octaves in music. Thus, in the nitrogen group, between nitrogen and phosphorus there are 7 elements; between phosphorus and arsenic, 14; between arsenic and antimony, 14; and lastly, between antimony and bismuth, 14 also. This peculiar relationship I propose to provisionally term the "Law of Octaves". The complete text of all Newlands’ papers can be found in his book The Periodic Law (1884), at Periodic Law.pdf
    7 : Dmitri Mendeleev (1834-1907) The Periodic Law, 1869 “I began to look about and write down the elements with their atomic weights and typical properties, analogous elements and like atomic weights on separate cards, and this soon convinced me that the properties of elements are in periodic dependence upon their atomic weights.” --Mendeleev, Principles of Chemistry, 1905, Vol. II The problem with previous attempts to organize the elements was that the pattern of repeating properties did not hold after the element calcium. Mendeleev proposed longer columns of elements to allow him to align those elements with similar properties Right: Mendeleev’s first sketch of his periodic table An interesting story relating Mendeleev’s solution to the periodic classification can be found at of New Spheres.pdf
    8 : Mendeleev’s arrangement of the elements were presented to the Russian Physico-chemical Society by Professor Menschutkin because Mendeleev was ill. The table was first published in the German chemistry periodical, Zeitschrift f?r Chemie, in 1869.
    9 : Translation of the German text in Zeitschrift f?r Chemie, 1869: Concerning the relation between the properties and atomic weights of elements. By D. Mendeleev. Arranging the elements in vertical columns with increasing atomic weights, so that the horizontal rows contain similar elements, again in increasing weight order, the following table is obtained from which general predictions can be drawn. Elements show a periodicity of properties if listed in order of size of atomic weights. Elements with similar properties either have atomic weights that are about the same (Pt, Ir, Os) or increase regularly (K, Rb, Cs). The arrangement of the elements corresponds to their valency, and somewhat according to their chemical properties (eg Li, Be, B, C, N, O, F). The commonest elements have small atomic weights. The value of the atomic weight determines the character of the element. There are unknown elements to discover eg., elements similar to Al and Si with atomic weights in range 65-75. The atomic weights of some elements may be changed from knowing the properties of neighbouring elements. Thus the atomic weight of Te must be in range 123-126. It cannot be 128. Some typical properties of an element can be predicted from its atomic weight.
    10 : Mendeleev published a revised, horizontal table in 1871 Note the structure of this table. It was clear, that there were a number of gaps of “missing” elements in the table. Mendeleev, made several predictions about properties of some missing elements.
    11 : Mendeleev predicted four elements: ekaboron (Eb), ekaaluminium (El), ekamanganese (Em), and ekasilicon (Es) Ekaboron and scandium Scandium was isolated as the oxide in autumn, 1879, by Lars Fredrick Nilson. Per Teodor Cleve recognized the correspondence and notified Mendeleev late in that year. Mendeleev had predicted an atomic mass of 44 for ekaboron in 1871 while scandium has an atomic mass of 44.955910.
    12 : Ekaaluminium and gallium In 1871 Mendeleev predicted the existence of yet undiscovered element he named eka-aluminum (because of its proximity to aluminum in the periodic table). The table below compares the qualities of the element predicted by Mendeleev with actual characteristics of Gallium, discovered by Lecoq de Boisbaudran in 1875).
    13 : Ekasilicon and germanium Germanium was isolated in 1882 by Clemens Alexander Winkler, and provided the best confirmation of the theory up to that time, due to its contrasting more clearly with its neighboring elements than the two previously confirmed predictions of Mendeleev do with theirs.
    14 : Ekamanganese and technetium Technetium was isolated by Carlo Perrier and Emilio Segrè in 1937, well after Mendeleev’s lifetime, from samples of molybdenum that had been bombarded with deuterium nuclei in a cyclotron by Ernest Lawrence. Mendeleev had predicted an atomic mass of 100 for ekamanganese in 1871 and the most stable isotope of technetium is 98Tc.
    15 : Henry Moseley (1887-1915) In 1913, using x-ray diffraction spectra, Moseley showed a systematic relation between wavelength and atomic number Resulted in arrangement of the periodic table by atomic number rather than atomic weight
    16 : 1894. The work of H. G. J. Moseley brought the elements in to a new arrangement (although minimal). He selected to arrange the elements by their atomic number (number of protons) rather than their weight. His work in 1914 led to what we call the Law of Chemical Periodicity.
    17 : Henry G. J. Moseley Periodic Law states that: the physical and chemical properties of elements are a periodic function of atomic number.
    18 : Glenn Seaborg (1912-1999) Starting in 1940, Seaborg was the principal or co-discoverer of ten elements: plutonium (94), americium (95), curium (96), berkelium (97), californium (98), einsteinium (99), fermium (100), mendelevium (101), nobelium (102) and element 106, which was later named seaborgium in his honor. He also developed more than 100 atomic isotopes, and is credited with important contributions to the separation of the isotope of uranium used in the atomic bomb at Hiroshima. Seaborg reconfigured the periodic table by placing the lanthanide/actinide series at the bottom of the table. Seaborg also proposed extending the periodic table to include predicted elements up to atomic number 168. the Periodic Table.pdf
    19 : Finishing out this illustrious group of people is Glenn Seaborg. His work involved the discovery of transuranium elements 94 to 102. This work led to the addition of the lanthanides and actinides (lathanoids and actinoids) [some say actinoins and lathanoins] in the periodic table. He and his colleagues are credited with discovering over 100 isotopes.
    20 : From those works the Periodic Table emerged.
    21 : Period The horizontal rows in the PT are known as PERIOD or SERIES. The first period is the shortest period of all and contains only 2 elements, H and He. The second and third periods are called short periods and contain 8 elements each.
    22 : Fourth and fifth periods are long periods and contain 18 elements each. Sixth and seventh periods are very long periods containing 32 elements* * each.
    23 : The sixth series is called Lanthanide series And the last is called the Actinide series
    24 : Group A group, also known as a family, is a vertical column in the periodic table of the chemical elements. There are 18 groups in the standard periodic table. The letters A and B were designated to main group elements (A) and transition elements (B).
    25 : Group The stairway that starts from Boron separates metals from the non metals. About 20 elements are nonmetals and these are found above and to the right of the stairway. Element found along the stairway are the so called Metalloids
    26 : The modern explanation of the pattern of the periodic table is that the elements in a group have similar configurations of the outermost electron shellsof their atoms: as most chemical properties are dominated by the orbital location of the outermost electron
    27 : Group 1 Group 17 All elements of group 1 have only one valence electron. Li has electrons in two shells, Na in three, K in four while Rb has electrons in five shells. Similarly all the elements of group 17 have seven valence electrons however the number of shells is increasing from two in F to five in I.
    28 : The gradual filing of the third shell can be seen below.
    29 : PERIODIC PROPERTIES We have also learned that in a period the number of valence electrons and the nuclear charge increases from left to right. It increases the force of attraction between them. In a group the number of filled shells increases and valence electrons are present in higher shells. This decreases the force of attraction between them and the nucleus of the atom. These changes affect various properties of elements and they show gradual variation in a group and in a period and they repeat themselves after a certain interval of atomic number. Such properties are called periodic properties.
    30 : Periodic Properties and their Variation in the Periodic Table Valency in a period : the number of valence electrons increases in a period. In normal elements it increases from 1 to 8 in a period from left to right. It reaches 8 in group 18 elements (noble gases) which show practically no chemical activity under ordinary conditions and their valency is taken as zero.
    31 : Atomic radii A number of physical properties like density and melting and boiling points are related to the sizes of atoms. Atomic size is difficult to define. Atomic radius determines the size of an atom. For an isolated atom it may be taken as the distance between the centre of atom and the outermost shell.
    32 : Practically, measurement of size of an isolated atom is difficult; therefore, it is measured when an atom is in company of another atom of same element. Atomic radii is defined as one-half the distance between the nuclei of two atoms when they are linked to each other by a single covalent bond.
    33 : Variation of atomic radii in a period Atomic radii of 2nd and 3rd period elements are given in the table below. What do you observe? In a period, atomic radius generally decreases right. 2nd Period Li Be B C N O F 155 112 98 91 92 73 72 3rd Period Na Mg Al Si P S Cl 190 160 143 132 128 127 99 Since valence electrons are added in the same shell, they are more and more strongly attracted towards nucleus. This gradually decreases atomic radii.
    34 : Variation of atomic radii in a group Atomic radii increase in a group from top to bottom.
    35 : Ionic radii Ionic radius is the radius of an ion. On converting into an ion the size of a neutral atom changes. Anion is bigger than the neutral atom. This is because addition of one or more electrons increases repulsions among electrons and they move away from each other.
    36 : On the other hand a cation is smaller than the neutral atom. When one or more electrons are removed, the repulsive force between the remaining electrons decreases and they come a little closer.
    37 : Variation of ionic radii in periods and groups Ionic radii show variations similar to those of atomic radii. Thus, ionic radii increase in a group. Ionic radii decrease in a period .
    38 : Ionization energy Negatively charged electrons in an atom are attracted by the positively charged nucleus. For removing an electron this attractive force must be overcome by spending some energy. The minimum amount of energy required to remove an electron from a gaseous atom in its round state to form a gaseous ion is called ionization energy.
    39 : Variation of ionization energy in a group and period We have already seen earlier, that the force of attraction between valence electrons and nucleus decreases in a group from top to bottom. What should happen to their ionization energy values? Ionization energy decreases in a group from top to bottom. The ionization energy increases in a period from left to right.
    40 : Electron affinity Another important property that determines the chemical properties of an element is the tendency to gain an additional electron. This ability is measured by electron affinity. It is the energy change when an electron is accepted by an atom in the gaseous state. By convention, electron affinity is assigned a positive value when energy is released during the process. The greater value of electron affinity, means more energy is released during the process and greater is the tendency of the atom to gain electron.
    41 : Variation of electron affinity in a group & period In a group, the electron affinity decreases on moving from top to bottom, that is, less and less amount of energy is released. In a period, the electron affinity increases from left to right, that is, more and more amount of energy is released.
    42 : Electronegativity Electronegativity is relative tendency of a bonded atom to attract the bond-electrons towards itself. Electronegativity is a dimensionless quantity and does not have any units. It just compares the tendency of various elements to attract the bond-electrons towards themselves.
    43 : Variation of electronegativity in a group & period Electronegativity decreases in a group from top to bottom. Electronegativity increases in a period from left to right.
    44 : Metallic and non-metallic character You know what are characteristic properties of a metal? They are its electropositive character (the tendency to lose electrons), metallic luster, ductility, malleability and electrical conductance. Metallic character of an element largely depends upon its ionization energy. Smaller the value of ionization energy, more electropositive and hence more metallic the element would be.
    45 : Variation of metallic character in a group & period Metallic character of elements increases in a group from top to bottom. Metallic character of elements decreases in a period from left to right
    46 : Atomic radii Defined as one-half the distance between the nuclei of two atoms when they are linked to each other by a single covalent bond. Ionic size The radius of an ion. On converting into an ion the size of a neutral atom changes. Metallic Property They are its electropositive character (the tendency to lose electrons), metallic luster, ductility, malleability and electrical conductance. SUMMARY
    47 : Ionization Energy The minimum amount of energy required to remove an electron from a gaseous atom in its round state to form a gaseous ion Electron Affinity The tendency of an atom to gain an additional electron Ionization Energy The relative tendency of a bonded atom to attract the bond-electrons towards itself.
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