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Inorganic Chemistry, Gary L. Miessler, Donald A. Tarr Textbook Chapter 2

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McGill University
CHEM 212
Richard Oakley

The theories of atomic and molecular structure depend on quantum mechanics to de scribe atoms and molecules in mathematical terms Although the details of quantum mechanics require considerable mathematical sophistication it is possible to under stand the principles involved with only a moderate amount of mathematics This chap ter presents the fundamentals needed to explain atomic and molecular structures in qualitative or semiquantitative terms 21 Although the Greek philosophers Democritus 460370 BC and Epicurus 341270 BC presented views of nature that included atoms many hundreds of years passed HlSTORlCAL before experimental studies could establish the quantitative relationships needed for a DEVELOPMENT OF coherent atomic theory In 1808 John Dalton published A New System of Chemical ATOMIC THEORY in which he proposed that hilosohthe ultimate particles of all homogeneous bodies are perfectly alike in weight figure etc In other words every particle of water is like every other particle of water every parti cle of hydrogen is like every other particle of hydrogen etce2 and that atoms combine in simple numerical ratios to form compounds The terminolo gy he used has since been modified but he clearly presented the ideas of atoms and molecules described many observations about heat or caloric as it was called and made quantitative observations of the masses and volumes of substances combining to form new compounds Because of confusion about elemental molecules such as Hz and 02 which he assumed to be monatomic H and 0 he did not find the correct formula for water Dalton said that ohn Dalton A New System qf Chemical Philosophy 1808 reprinted wi der Joseph Peter Owen Limited London 1965 lbid p 113 16 Chapter 2 Atomic Structure When two measures of hydrogen and one of oxygen gas are mixed and fired by the elec tric spark the whole is converted into steam and if the pressure be great this steam be It is most probable then that there is the same number of particles in two comes water measures of hydrogen as in one of oxygen3 In fact he then changed his mind about the number of molecules in equal volumes of different gases At the time I formed the theory of mixed gases I had a confused idea as many have I sup pose at this time that the particles of elastic fluids are all of the same size that a given vol ume of oxygenous gas contains just as many particles as the same volume of hydrogenous or if not that we had no data from which the question could be solvedI later became That every species of pure elastic fluid has its particles globular and all of a convinced size but that no two species agree in the size of their particles the pressure and tempera 4 ture being the same Only a few years later Avogadro used data from GayLussac to argue that equal volumes of gas at equal temperatures and pressures contain the same number of mole cules but uncertainties about the nature of sulfur phosphorus arsenic and mercury va pors delayed acceptance of this idea Widespread confusion about atomic weights and 19 different possible Kekul6 gave molecular formulas contributed to the delay in 1861 formulas for acetic acid In the 1850s Cannizzaro revived the argument of Avogadro and argued that everyone should use the same set of atomic weights rather than the many different sets then being used At a meeting in Karlsruhe in 1860 he distributed a views6 His proposal was eventually accepted and a consistent pamphlet describing his set of atomic weights and formulas gradually evolved In 1869 endeleev and eer independently proposed periodic tables nearly like those used today and from that time the development of atomic theory progressed rapidly 211 THE PERIODIC TABLE The idea of arranging the elements into a periodic table had been considered by many chemists but either the data to support the idea were insufficient or the classification schemes were incomplete Mendeleev and Meyer organized the elements in order of atomic weight and then identified families of elements with similar properties By ar ranging these families in rows or columns and by considering similarities in chemical behavior as well as atomic weight Mendeleev found vacancies in the table and was able to predict the properties of several elements gallium scandium germanium polonium that had not yet been discovered When his predictions proved accurate the concept of a periodic table was quickly established see Figure 110 The discovery of additional elements not known in Mendeleevs time and the synthesis of heavy elements have led the front cover of this text to the more complete modern periodic table shown inside period and In the modern periodic table a horizontal row of elements is called a group or family The traditional designations of groups in the a vertical column is a United States differ from those used in Europe The International Union of Pure and Applied Chemistry IUPAC has recommended that the groups be numbered I through 18 a recommendation that has generated considerable controversy In this text we will 31bid p 133 4bid pp 144145 A Short History of Chemistry 3rd ed Macmillan London 1957 reprinted 1960 5 PartingtonRow New York p 255 Harper 6bid pp 256258 7 I Mendeleev J Russ Phys Chem Soc 1869 i 60 Justus Liebigs Ann Chem 1870 Suppl vii 354 8 Meyer 21 Historical Development of Atomic Theory 1 7 Groups American tradition IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA VIIIA Groups European tradition IA IIA IIIA IVA VA VIA VIIA VIII 1B IIB IIIB IVB VB VIB VIE 0 Groups IUPAC123 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 0 Transition metals I I58 Lanthanides 71 I I FIGURE 21 Names for Parts of90 Actinides 103 the Periodic Table use the IUPAC group numbers with the traditional American numbers in parentheses in Figure 21 Some sections of the periodic table have traditional names as shown 212 DISCOVERY OF SUBATOMIC PARTICLES AND THE BOHR ATOM During the 50 years after the periodic tables of Mendeleev and Meyer were proposed experimental advances came rapidly Some of these discoveries are shown in Table 21 Parallel discoveries in atomic spectra showed that each element emits light of specific energies when excited by an electric discharge or heat In 1885 Balmer showed that the energies of visible light emitted by the hydrogen atom are given by the equation TABLE 21 Discoveries in Atomic Structlrte 1896 A H Becquerel Dscovered radioactivity of uranium 1897 J J Thomson Showed that electrons have a negahve charge wth chargemass1 76 X 1011 Ckg 1909 R A Milhkdn 1 60 X lo C therefore the mass of Medsured the eleclronc charge 1 the electron IS 9 11 X kgof the mass of the H atom 1836 E Rutherford 191 1 Established the nuclear model of the atom very small heavy nucleus surrounded by mostly empty space 1913 H G J Moseley Determined nuclear charges by Xray emission establishing atomic numbers as more fundamental than atomic masses
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