i\GRI L. HES. INSTT'I'Urr.: Nl'=W DE LHI. MELLOR'S MOD'ERN INORGANIC CHEMISTRY Revised and Edited by G. D. PAH.KES, M.A., D.Phil., Fellow of Koble College, Oxford in collaboration with J. W. MELLOR, D.Se. With diagrams and illustrations JARJ LONGMANS, GREEN AND CO, LONDON .tJ MODERN CHEMiSTRY' were calleel D6berciller's Triads, but it was soon felt that his list was but a fragment of a more general law. Between lSn:l allli 180G, J. A. R. Newlands published a series of papers in which he arnUlgecl the elements in the ascending oreler of their atomic weights, and noticed that every sllcceeding eighth element was "a kiIlCl of repetition of the first." TIlliS, H Li Be' B C N 0 F N a Mg Al Si P S Cl K Ca Cr Ti Mn Fe " In other words," said N ewlands, "members of tlte same group of elements stand to each other in the same relation as the extremit.ies of one or more octaves in music. This peculiar relationship I propose to provisionally term the law of octaves." Newlancls noticed that elements belonging to the same group "usnally'" appeared in tbe same. column, and !te declared that all the numerical relatiolls which had been observed among the atomic weights "including the wuIl-known triads, are merely arithmetical results {towing frolll the existence of the law of oct a yes." The" law of octaves" did not attract much attention, prolmlJly because fanlty atomic weights seriously with the arrange-ment.* Similar remarks apply to some papers l,y A. E. B. de Chau-courtois in 1802, where also it was proposed i.o classify the elements by .their atomi.c weights. S 2 The Periodic Law-D. I. Mendeleeff and L. Meyer D; 1. Mcndeleeff and L. Meyer, quite independently aud, so far as 'we can tell, quite in ignorance of Newlands's and Challcourtois's work, (1)tained a. far clearcT vision of the "law of octavcs" ahout 18tJU. Mendeleeff said: "When I arranged the elements according tll the magnitude of their atomic weights, beginuing with the smallest, it became evident that there exists a kind of periodicity in their pro-perties." Otherwise expressed, if the elements be arranged ill the order of increasing atomic weights, their propert/es vary from member to member in a definite way, but return more or less nearly to the same value at fixed points in the series. lVI ende16eff continued: "I cksig-nate by the name' periodic law' the mutual relations between the properties of the elements and. their atomic weights, these relations arc '" 'When Mr. Newlands read a paper on " The La.w of Octaves" at a meeting of the London Chemical Society in 1866, Prof. G. C. Fostel' said that any arrange-ment of the clements WOUld. present occasional coincidences, ami inquired if Mr .. Newlaucls had ever exammecl the elements accOJ'ding to their initial letters. Twenty-one years later the Royal Society awarded Ncwlancls the Davy Medal for his discovery. . TIlE CLASSIFICATION OF THE ELEMENTS II7 applicable to all the elements, and have the nature of a periodic* function." Expressed more concisely, we have MendclEieff's periodic law: The properties of the elements are a periodij: function of their atomic weights. Consequelltly, was able to construct a table in which the elements were arranged horizontally in onler of their atomic weights and vertically according to their resemblances in properties. The early tables were very imperfect on account of the unreliability (if many atomic weights, as then assigned, but they were afterwards amended and modified in the light of the more accurate datu which became available. The table on page U8 shows Mendel6efi's scheme modified so as to conform with modern knowledge (Table VIII). Mendcleeff's table was constructed on the basis of the atomic weights of the elements, the most fundamental property of the atom then known. . It is now known, however, that the properties of the elements are in reality a periodic function of an even more fundamental number than the atomic weight, viz., the Atomic Number. This will be discussed fully in Chapter IX-for the present it may be defined as tlte -ordinal 1Htmber of the elemellt 11 the periodic table of MendcUefJ. As we shall see in 3, Mendel6eff reversed the order of certain pairs of elements, as indicated by the valnes of their atomic weights, in order to bring them into their proper places (as determined by their properties),. This procedure has been abundantly justified, and the Periodic System a.s a whole placed on a firm, theoretical basis by recent discoveries concerning the structure of the atom, and the atomic number has been shown to possess a fundamental significance. Table VIII accordingly includes the values of the Atomic NUfl1bers, . as well as of the Atomic Weights. 3 General Structure of the PeriodiC Table Beginning with hydrogen, the element of lowest atomic weight, and writing down the elements in a horizontal row in the order of their atomic weights, we come to elements showing a recurrence of the properties of elements already \vritten down, at intervals of 2, 8, 8, 18, 18 and 32 elements respectively. If we start a new line in the table at each of these recurrences, that is with the beginning of each new period, we shall find the elements arranged vertically in famiNes or gro1tps of like chemical properties. In the tftble the groups are numbered from I to VIII with the addition of Group 0 as shown. Groups VIn and 0 are unusual in that they arc alternatives. The Periods are numbered 1, 2, etc., up to 7; the first three are known as short periods, the others as long periods. In the long periods the groups are sub-divided into two in the by being placed to the left or nght respectIvely 111 therr appropnate columns. These are designated A and B respectively. * A. periodic function is one whose value repeats itself at regular intervals . The interval is called a" period." 118 ci p, " I': '-' .... :.-' 0 :g' r}, 11 :.--d " "" Ci '" Q) a S " ... o .... ",1 ,-,'" -C{O -,.; l .. ==,"',=, ,-:""=' ,..,.,- ",. '" . 2..:., _ ..... -,.., p;", .::jt-,.!Jr.l .... Ul,... :fs .0' ,0 ______ 2:8 ;;)$ -::> 0' . Jig:J -'I -rr) 0{'" Ul,... ... OJ .... No '" Sb; --":'1 .... ,- rQ", So '0 -'" -" ,.., r3 r", ;::'0 rJit:) >:g; Ul"" fj)':; .,.. ;Ct:"l u::: ggJ OCO i})M "". 'Z!-9 ..... >q" "'" U"'" [jJOO C't. """ -1-"0 _",I.( .,;,.., .-I ;::,,_ ,- 00 """" .- .,y,lmll t1JldCl c()n-sldetatJon 1Ms t\\O deglees or hel'dOlIl \Vlth these lOllU, cIe,l.l 1Il ollr mmd, we C,ll1 now st,tto the Ph.ISO Rule willch Ib tlmt a system will be in equilibrium when j.ts Vd.ri,lnce is equ,]l to the number of components in the system less the number of phases, incl e']sed by two. In symbols. F = C - P -I-- 2 wherr C dCllotl'o tho llUlllbCl of LOlll-pOl1onts, P the 1111111bel of anI1STRY positions where the uriginal aspect of the crystal is the same. Fig. 58, B, C, D, rcspecth'ely, denote tetrad, tri,ld, and dyad ;s, none being 8hsolutely C)':;lCt, so in the application of the corrcspomline: equation to solutions the eqnation expresse;; th" heha\-ionr of an idflll solution to which aetnal solutions conform 1110re or loss. Tbe devia-tions which occur nrc probably to be accounted for in a nmnner similar to that of gases, "Viz., (i) attraction between the molecules of the solute; (ii) attraction between molecules of the solute and solvent; (iii) the volume of molecules themsehcs. As in the case of gases, the diYergcllce is wider at high concentration (Le., high pressure aud small \'O!UlllC), and the simple Van't HotT theory is in reality only applicahle to dilule solnt.iolls and \vitllin a moderate l"iiIlge of temperature. Also as already indic;Ltcd, it requires modification when applietl to sulutions 1") electro-lytC's. (Compare Cll. XII, p. J !H.) 4 Mechanism of Osmotic Pressure and Membrane Action The close analogy between the behaviour of dilute solutions and g,tses demon stratecJ hy Van't Hoff le(l naturally to the view that osmotic pressure and gas pressure arc due to similar causes, i.e., to bombardment by the lllolecnles of thl] solute or gas of the semipermeable membrane or (he walls of the containing vessel At the same time it was natural to assume th[lt the action of the semipormeal,lc mc;mbrane is that of an atomic sieve whose pores, while large enough to permit the passage of solvent l11olecules, are too small to allow of soll1te molecules passing through them. These explanations arc not now felt to be completely satisfactory as they slant!, par1.il'lIlarly as it has been shown that the actual Jlorc-Llhmetcr of a copper-[crrocyanitle 'l11embrane is too large 1.0 blor.:k the passa.ge of even a sng;ar molecule. This has led to various mndiflcatiolls in thn theory, sur,h as the onggestion that the pores arc hydrated (or soh'atcLl) to an extent sufficient to prevent the passage of solute ;nolcculcs. Callendar's Theory Callcndar's vapour pressure hypothesis (HIOfl) is one of the most successful of the purely physical of osmotic pressure. Experiment shows that the maximum vapour pre;;:iure of a solution can be altertxl in three ways: (I) by altering the temperature (p. :11); (2) byvarying * This statement is only true for solutions of non-electrolytes. The corre. spolldillg behaviour of cit'ctrolytes is discnssed in Chapter XII. l\[oDEH.N INnr,CANIC CHEl\lISTHS llie cOlll'(:niration of t.he solution; and (:I) by 1Iliering the pressure under ';vhich tlll! li,plitl itself i:; coniilll:ll. -The relation between vapour pressure- and osrnotic pressure.-It has been provecl experimentally that the maximulU vapour' pressure of r. solution under very great pressures if; rather greater than the llla,ximUln vapour pressure of the same solulion under prcssures. Again, the val)Qur pressure of a solution is tess ilum the vapour pressure of ihe pure solvent, Fig. i,2. COIl-sequently, if the pressure on a soluiion he sufficiently augmentecl, the pi'essme o[ its yapour can 11" made equal to ihe ,'apour pressure of the pure solvent under atmospheric pn'SSlll'C'. ;L'his is the comlitioll nec:csSkrljone mmlificatioll in recont years; principally at the hands of DelJye and l-lii!:keL These l11odific"UOllS have beon suggested in the attempt to meet two diflic111tieei, viz" first the fact that 1ll(l{1ern knowledge of the structure of the atum ,mel the nature of crystllls (eh, IX; eh, X, p, 17l) has shown that a crystal of flU ejr-ctrolyte {'onsists already of ions held together only by electro5iatic forcus; and slwrc rouIll1 eacl1 ion, (1:he arrangemcnt thus snggeste,l wonld be similar to that obtaining in a sodium chloride crystal-p, \VhL'll an ion to move uncler the inllu()llCe of an applied potential differeU(l" this ionic atmosphere has to be renewed in front of th" moving inll, while that hphinr.l it dies away, It is that the formation of the new lags behind the decav of the old, Lhe timo interval being known as the re/axatioll lillli" There will thus always be an excess of ions of opposite sign to the moving ion behind it, which will calise its movement to be retarded. In addition, the applied potential difference will tend to move the ionic atmosphere itself in a direction opposite to that of tho moving ion" which will cause further of the latter. These effects will be larger the greater the concentration, and so with increasing dilution, the speed of a given ion, under a given potenti,LI grarlicllt, will increase and with it the equivalent conductivity, reaching a maximum at infinite dilution, H* clllnlilST[{\' As a n'sult of their mathematical analysis uf effects, Duuye allli I-liickel showed that the effect of the retorLI8.t-ions mentioned be proportional to the square root of the concentration, i.e_, . A", - a"';; where,\ = cOllducti\-ity at COllcelltmtion C alldll is a constant. This theory has met with a certain measure of success whon appliocl to dilute mlutions, ]Jut its range of applicability is \'o1'y limite(1 iLlHI it is cl'ident that the theory i" far from cOlllplete. It is llOW recognized that there is an element o[ truth in the original cledrolylic theory, and that as our ImolVlerige grows, attempts to apply Htat theory to the facts will approach closer and to a more complete 8xjll:lllatioll of facts. The Hydration of the Hydi'ogen Ion Another modification of the details of the Electrolytic Theory, as put forward by Arrhenius, in consequence of the results of later work, is dne to the fact that ions are usually hydratecl, i.e., they carry with them a certain amount of water. The existence of this hydration has been shown by measuring the change in concentration of a non-electrolyte, present in the sulution, which oCcurs as a result of electro-lysis. Evidence has accumulated to show that, for instance, In a solutioll Of'i!l1l1 acid it is not hydrogen ions' themselves which are present, but ions of the formula [H30]', known as hydroxonillm ions. Olle of the important consequences of this fact is referred to in Chapter XV (p, 230) in connection with the strengths of acids. , "\ . \.' 9 Voltaic Cells .. It has long been known that if plates of two dissimilar "metals are connected by a wire and immersed in a solution of an electrolyte, a current will flow along the wire. If plates of zinc and copper are taken and immersed in dilute sulphuric acid the arrangement is known as a simple cell. If the cell is allowed to work for a little time it will be found that zinc is dissolving in the acid and that bubbles of hydrogen appear on the copper plate, which is 110t otherwise affected. If the strength of the current passing through the wire be investigated, it will be fonnd that by the time the copper plate has become covered with hydrogen the current has droppeu almost to zero. On brushing away the bubbles f:!;om the copper plate the current will rise again to . its former value. '. In normal electrical terms, it is found that a current is flowing along tlle wire from the copper to the zinc; although, in fact, it is now known that a stream of electrons is passing along the wire from the zinc to the copper. Zinc is going into solution, forming a solution of zinc sulphate; or, in terms of the ionic theory, zinc ions Zn", that is, atoms of zinc carrying double positive charges. The formation of a zinc ion, which has thus two electrons fewer than the atom from which it is derived, will thus .leave the zinc plate with these two electrons, which are conduded through the wire to the copper plate. In the solution there arc hydrogen ions, caused by the dissociation of ELECTROLYSIS & THE ELECTROLYTIC DISSOCIATIIT\l THEORY :!ll:1 the acid (see ell. XV). When the copper plate receives tlw two electrons, by way of the connecting wire, it becomes negati \"Cly charged and hence attracts hydrogen ions (i.e., hydrogen atoms (';'teh having lost one electron) which are positively charged, and these take up electrons from the copper plate, so becoming hydrogen atoms again and being liberated as hydrogen gas. The production of the current is thus seen to depend upon the tendency of the zinc. atoms to become zinc ions, concerning which more is said in the next section. The simple cell is not a useful cell on account of the tendency for its activity to be stopped by the accumulation of hydrogen 011 the copper plate, a phenomenon which is known as polarization. In conse-quence, various other forms of cell have been devised in urder to avoid this difficulty. The usual method for avoiding it is to replace the copper by carbon and to surround this with an oxidizing agent which ,oxidizes the hydrogen to water as soon as it is formed. Some cells, however, employ two liquids, as ,for example, the Da.niell cell, which consists of a plate of zinc immersed in dilute snlphuric acid contained in a porous pot, the whole standing in a solution of copper sUlphate in which there is also a copper plate. In this the zinc dis. solves as before, but the copper p1::ttc" when negatively charged, attracts not hydrogen ions but copper ions WIlich, when discharged, are deposited on the copper plate as metallic copper. Thus no change in the working of the cell occurs. ' The most important fOl'llllOf cell at the present time is the LcclallchC cell, particillarly in the form of the so-called dry cell, the manu-facture of which for use in wire-less 'receivers and for similarl)Ul"-poses has become an important industry. The qrdinary LedancM cell consists of a carbon rod in a porous pot surrounded by a mix- Zinc ture of powdered carbon and manganese dioxide. The whole stands in a vessel of ammonium chlOlide solution in which is also a zinc rod. The zinc dissolves as before, forming zinc ions, while the ammonium iOllS are dis--Graphite Depolariser -Papep at the carbon After FIG. 80 -Dr Cell, dIscharge they break up mto all1- y monia and hydrogen, the fomler remaining dissolved while the latter is oxidized to water by the manganese dioxide. This oxidation is slow so that the cell WRy polarize if too big a load is put all it; but it will recover if allO\ved to rest, and also it will yield a small current for an almost indefinite period. 204 MODEl"ertie-al -Oryoen tube, C, which ends in a funnel, D. The electrodes E, E, consist of pieces of platinum foil. Pure water is 8. very poor conductor, and so the experiment is carried out by. mUng the apparatus with water to which a little :mlphnric acid has been added. A current is passed through the solution by connect-ing the electrodes to an accumulator FIG. !H.-\Vatcl' Volt:lmctcr. or a primary hattery. Dmillg the passillg of the electric current, buhbles of gas from about the metal plates rise into the glass tubes. More gas is given off at one plate than the other. Thl' gas in each tube can he cxamined by mcans of a lighted taper or othlrwisc. In the one tube, the taper burns with the" Llindin/-; brilliance" characteristic of oxygen; and the gas in the other tube bums with the blue flame dlaracteristic of llrdrogen. Some of the water has disappeared, but no change can be detected in the amollnt of sulphuric acid mixed with the water. Hence it is inferred tlutt the water, not the acid, has been decomposed. The expcrimE;'nt succeeds equally well if a very dilute 208 MODERN INORGANIC CHEMISTRY solution of sodium or pobssium hydroxide be used with nickel or iron electrodes. Here aga.in the water, not the alkali, is decomposed. A mixture of volume of oxygen and two volumes of hydrogen, called electrolytic gas or detonating gas, is often wanted in gas analysis, etc. This is easily provided by placing both electrodes under one receiver. Electrolytic oxygen contains a little ozone and hydrogen peroxide if prepared by the electrolysis of acid lIla ted water, but not if a solution of barium hydroxide be electrolyzed. The information derived from these experiments did not, by itself, establish the formula of water for, as explained in Chapter IV, no relation between the combining volume::; of gases and the number of atoms uniting was then known. Crude attempts were made to discover the of hydrogen and oxygen combining together to form water, by multiplying the volumes fouud by Cavendish, by the densities of the gases. But with the comparatively primitive types of apparatus then available the results were very inaccurate. A determination of a different kind was made by Dumas in 1843, Although its accuracy is inferior to that of more modern experiments. it was far superior to any which preceded it and it wa::; the forerunner of accurate atomic-weight determinations. The experiment illustrates some imlJortant principles, and it is therefore here described in outline. It depends upon the fact, already noted (p. 259), that when the oxides of sllch metals as iron, copper or lead are heated in a current of hydrogen, water is formed and the oxide is reduced to the metal. If a known amount of copper oxide be employed, and the water formed be collected and weighed, the weight of the reduced copper oxide will show how much oxygen has been used in forming a definite amount of water. This was done by J. B. A. Dumas in 1843. The hydrogen was prepared by the action of zinc on sulphuric acid. It might be thought that pure zinc and pure sulphuric acid should be used. Experiment shows, curiously enough, that the action is so very, very slow, that it is often stated that" absolutely pure sulphuric acid, even when diluted. with pure water, has no action on perfectly pure zinc." Moreover, it is exceedingly difficult to prepare pure zinc and pure sulphuric acid. Hence, pure reagents were not used for the preparation of the hydrogen. Accordingly, the gas may contain nitrogen and oxygen derived from the air; sulphur dioxide and hydrogen sulphide derived from the reduction of the sulphuric acid by the hydrogen, carbon dioxide, arsenic hydride (if the acid or the zinc contained arsenic); hydrogen phosphide (if the zinc or the acid contained phosphorus); nitrogen oxides (if the acid contained nitrogen oxides); and water vapour. Accordingly, Dumas. used sulphuric acid, which had been well boiled to get rid of dissolved air, and then passed the hydrogen through a series of 95-con-taining: (1) pieces of glass moistened with lead nitrate to remove hydrogen sulphide; (2) solution of silver sulphate to remove arsenic HYDIWGEN ANn WATEH and phosphorm; cum pounds; (:1) 80lid hydruxirle tu l"l'll1ove sulphur dioxide, carbon dioxide, and llitrogen oxiucs ;'" alld pllOS-Purification 0' Hydro!]en FIG. 95.-Dumas's (abhrevi,ded). -: G phorus pentoside to remove moisture'f' not absorbed bv the solid potassium hydroxide. The phosphorlls pcntoxiclc tubes \vere placed in a freezing mixture. The tube marked (oj) in the diagram contained phosphorus pentoxide. It was weighed before and after the experi-ment. If no change in weight occurred, it was assnrncrl that the hydrogen passing through was quite ' The cxpcl'illltillt.-The puritied hytlrngen was thrOllgh a weighed bulb, A, containing copper oxide, and lleated hy the spirit lamp underneath. Most of the water condensed in the bulb H, and the remainder was absorbed in the U-tube, C" containing soliel potas-sium hydroxide, and in D and E containing phosphorus pc'ntuxicle. The phosphorus pentoxicle tube D was kept cool by a freezing mixture. The three tuhes, C, D and E, anel the bulh B, were weighed beiure and after the csperimt'nt. The last U-tllhe, F, containing phosphorus pcntoxicle, was followed by a cylinder, G, of sulphuric acid through whieh hydrogen escaped. The vessels F and G were not weighed; thpy served to protect the other tubes from the external atmosphere. The results.-The average of nineteen experiments by Dumas gave: Copper oxide lost in wcig-ht 'YVater producer! H:!:! gmms. 4!lin grams. Hydrogen (by ;;';)4 grams. Hence, every 10 parts by weight of oxygen combined with 2004 parts hv weight of hydro[;en to form water. The latter determination of l\[l)rky (sec p, gaye iti : :,:0 J ii2. Thpre b a c1l1'iutls error in Dumas's ., DU1l1n.s used thr(.!e pota,;sium hydroxiclc tul,es, and two pl!o;!J!toru;; pent-oxide tubes. Only one of e