Complete Text (Part 10)

the chlorine atom is — 1, for negative chlorine is always a monad ; and the highest combining number of chlorine, shown in KC104, is +7, because the four oxygen atoms together have 8 negative bonds and hence the K and CI must together have 8 positive bonds, of which only 1 belongs to the potassium. The same range is seen in iodine. The lowest algebraic combining number of sulphur is —2, which is its value in all sulphides ; and its highest algebraic combining number is -f 6, as in S03. The lowest algebraic combining number of nitrogen, as shown in ammonia (H3N) and in all ammonium compounds, is -3 ; and its highest combining value, as shown in N205 and in the nitrates, is +5. The nitrogen has an algebraic combining number amount- ing to —3 in all ammonium compounds, as may be shown here by one example. The molecule of ammonium chloride is H4NC1; hence the nitrogen atom here has four negative bonds which hold the positive hydrogen atoms in combination and one positive bond holding the negative chlorine atom, and the sum of —4 and +1 is —3. The lowest algebraic combining value of carbon is -4, as shown in H4C; its highest combining value is +4, as shown in C02. In all the cases referred to it will be seen that the total range, from highest to lowest, of the algebraic combining value of each element is just 8 units. That this is not mere chance but the result of natural law may be inferred from the fact that the quantity of any oxidizing agent required to increase the algebraic combining THE ALGEBRAIC COMBINING NUMBERS OF ATOMS 95 number of any atom from +1 to 4-5 is the same as the quantity required to increase it from —4 to 0, or from —1 to +3, or from —2 to +2, or from —3 to +1, or from +2 to +6, or from +3 to +7, and it is twice as great as the quantity of oxidizing agent required to raise the algebraic combining number of any atom from —4 to —2, or from —2 to 0, or from 0 to -i-2, or from +1 to +3, etc. To increase the algebraic combining number of nitrogen from —3 to +5, as when ammonia is converted into nitric acid, requires just 8 units of oxidizing power, and to change H2S into H2S04 also requires just 8 units of oxidizing power, because the value of the sulphur in H2S is evidently —2 and that of the sulphur in H2S04 is evidently +6 (see next paragraph). 198. As one half of all the bonds of all the atoms com- posing any molecule are positive bonds and the other half negative bonds, it follows that the algebraic sum of all must in every case be 0. If, therefore, the algebraic combining number of two out of three elements in any molecule be known, the combining number of the third is easily found, for it must be in every case the difference between 0 and the algebraic sum of the combining numbers of the other two. In H2S04 we have eight negative oxygen bonds, because each of the four oxygen atoms has two negative bonds ; the two hydrogen atoms have together two positive bonds ; the sulphur atom must, therefore, have six positive bonds, for —8 and -i-2 and +6 added together make the sum of 0. 199. The algebraic combining number of any free atom is of course 0. It cannot be known how many bonds any atom has, nor what its polarity is, except from its actual com- binations, for we have seen that many elements have a variable valence and at least thirteen elements are positive in some compounds and negative in others. Thus, a free chlorine atom is neither positive nor negative and its actual combining number is 0. If it enters into chemical com- 96 A CORRESPONDENCE COURSE IN PHARMACY bination with hydrogen, or with any metal, it then assumes negative polarity, and its algebraic combining number will be —1 ; but if it enters into direct combination with oxygen in the formation of molecules of NaOCl, the chlorine atom assumes positive polarity and its value will be +1, and if it forms KC103 the chlorine assumes an algebraic combining number of +5. The carbon atom in any carbon compound usually has 4 bonds. In the free or uncombined state its algebraic com- bining number is 0. When it is united to four hydrogen atoms, H4C, the carbon atom has a combining number of —4; if it holds three (positive) hydrogen atoms and one (negative) chlorine atom, H3CC1, its value is —2; if it holds two (posi- tive) hydrogen atoms and two (negative) chlorine atoms, H2CC12, its value is 0; if it holds one hydrogen atom and three chlorine atoms, its value is —2. 200. Whenever two atoms of the same element are directly united to each other it must be assumed that they are held to each other by bonds of opposite polarities. Hence it follows that one of the hydrogen atoms in a molecule of hydrogen (composed of two atoms) must be positive and the other negative. In a molecule of oxygen containing two atoms of that element we must conclude either that each atom has one positive and one negative bond or that one has two positive bonds and the other two negative bonds, for the algebraic sum of all the bonds in any molecule must always be zero. 201. In the molecule HN3 it is impossible to escape the /N" < . conclusion that the atomic linking must be H — Nv 1 1 , which shows that the hydrogen atom with its one positive bond is united to one of the nitrogen atoms by one negative nitrogen THE ALGEBKAIC COMBINING NUMBEKS OF ATOMS 97 bond, while of the other six nitrogen bonds three are positive and the other three negative. In HO OH we assume that the algebraic sum of the bonds of both oxygen atoms together must be —2, because the hydrogen atoms together must be +2 and the total must be 0 ; hence one of the oxygen atoms must have one positive and one negative bond and the other oxygen atom must have two negative bonds. The two bonds by which the two oxygen atoms are held to each other must therefore be one of them positive and the other negative. We must assume that the algebraic sum of the bonds by which any two atoms of the same element are held in direct combination with each other is always zero, because one half of them must be positive and the others negative, 202. To find the algebraic combining number of any atom in any molecule of simple structure is usually an easy task, unless two or more atoms of one and the same element are directly united to each other. The student can readily find the combining numbers of combined elements from the atoms known to have constant values, such as H, K, Na, Li, Ba, Sr, Ca, Mg, Al, B, Ag, Zn, and F; also from the oxygen in any molecule, unless two oxygen atoms are united directly to each other; also from the negative atoms of chlorine, bromine, iodine, sulphur and nitrogen. Since a varying algebraic combining number is possible only to elements having positive polarity, the student should find the algebraic combining numbers of any atom of such an element from the atom or atoms with which it is in direct combination; or, in other words, from any atoms in the molecule the algebraic combining values of which are con- stant and, therefore, known. 203. The algebraic combining number of the acidic element in any inorganic acid composed of that element together with hydrogen and oxygen, is found by deducting 98 A CORRESPONDENCE COURSE IN PHARMACY the algebraic sum of the oxygen and hydrogen bonds from 0. The oxygen atoms in the molecule of such an acid are the only atoms having exclusively negative bonds ; the hydrogen atoms are all positive, and either all or a majority of the bonds of the acidic element are positive. In HN03 the N must have five positive bonds; in H3P04 the P must have five positive bonds. But in Na2AsH03 the As has an algebraic combining number of only +3, although it has five bonds, for the structure of the molecule H is known to be N^0/As= 0 ; and in KPH202 the P has an algebraic combining number of only +1, although it has five H I bonds, because the structure is KO — P = 0. I H 204. In the molecule commonly but erroneously written CaS5 we know that the algebraic sum of all the bonds of the five sulphur atoms must be —2, because Ca (as shown by its position in the periodic system) can never have any other value than +2. No compound of calcium is known in which that metal has any other combining value than 2. It follows that the two positive calcium bonds must be in combination with two negative sulphur bonds and that of all the remain- ing sulphur bonds one-half must be positive and one-half negative. This leads to the structural formula in which the central sulphur atom is acidic and has six positive bonds, while all the other sulphur atoms each have THE ALGEBRAIC COMBINING NUMBERS OF ATOMS 99 two negative bonds, the molecule being perfectly analogous to CaS04, which has the structure There is a molecule erroneously written K2S3 which has the structure K— S— S— S— K. In this molecule the central sulphur atom is acidic and has a combining value of +2, while all the other four sulphur bonds are negative, because the structure is perfectly anal- ogous to that of KOSOK. There is a compound erroneously called ''hyposulphite of sodium," which is commonly represented as Na2S203. But with the aid of our conception of the algebraic combining numbers of the component atoms of molecules we can readily see that the structure must be NaS\Q^0 nr mO\„//8 NaO/%0 or NaO/%0 and chemists now write it Na2S03S, to show that one sulphur atom performs the acidic function while the other performs the same function as any of the oxygen atoms in Na2S04. 205. The algebraic sum of all the carbon bonds in any molecule composed of carbon, hydrogen and oxygen (and a vast majority of organic substances are composed of those elements) is at once found by subtracting the algebraic sum of all the bonds of the atoms of hydrogen and oxygen from 0. It is further known that all the carbon atoms in such com- pounds have each four bonds. The total number of positive LOFa 100 A CORRESPONDENCE COURSE IN PHARMACY carbon bonds and the total number of negative carbon bonds can, therefore, be readily found. It is also known that the algebraic sum of all bonds by which any of the carbon atoms are united to each other must be 0. These facts are helpful in determining the actual atomic linking. Test Questions 1. Define valence. 2. What is the valence of the boron in H3B ? in B203 ? 3. What is the difference arithmetically between the valence of the boron in H3B and in B203 ? 4. What is the difference between the algebraic combining number of the boron in H3B and the boron in B203 ? 5. What are the respective valences of the elements form- ing a binary hydrogen compound ? 6. How do you find the valence of each of the two elements in any oxide ? 7. Can you name an element having a valence of 10 ? 8. What is the number of chlorine atoms in the chloride of an element having a valence of 8 ? 9. What is the number of hydrogen atoms in the hydride of an element having a valence of 6 ? 10. How many chlorine atoms are there in the chloride of a heptad ? 11. How many oxygen atoms are there in the oxide of a tetrad and how many in the oxide of a pentad ? 12. Name two octads. 13. How many bonds has a potassium atom ? 14. How many bonds has aluminum ? 15. Name the number of bonds of the zinc atom. 16. Draw a figure showing the atomic linking of As203; C2Ha. THE ALGEBRAIC COMBINING NUMBERS OF ATOMS 101 17. Under what circumstances can the valence of an element vary ? 18. State the number of bonds of each element in the molecule OSbCl. 19. State the number of bonds of each element in (a) KN03; (b) Na4P207; (c) CaS04; (d) CaH2S05; (e) H5P05; (f) H3P04; (g)HP03; (h) H3P03; (i)HP02; (j) HPH202; (k) H2PH03; (1) KMn04; (m) K2Mn04. 20. State which of the following molecular formulas are right and which are wrong: (a) AgCl3; (b) KO; (c) Mg203; (d)H,0; (e)Na2S5. 21. What is the valence of the sulphur in S02 and what is the algebraic combining number of each of the two ele- ments in that molecule ? 22. What is the algebraic combining number of the sulphur in H2S ? 23. What is the difference between the valence of the sulphur in H2S02 and in H2S and what is the difference between the algebraic combining number of the S in those two molecules ? 24. What is the algebraic combining number of the N in HN03 and in H.lSTBr ? 25. What is the algebraic sum of the carbon bonds in C6H10O5? 26. What is the algebraic combining value of the Br in a bromide ? 27. Can bromine under any circumstances have a higher algebraic combining number, and if so, when ? 28. If the valence of sulphur, with negative polarity, is 2, what is the highest possible algebraic combining number of sulphur ? 29. If zinc phosphide is Zn3P2, then what is the highest possible algebraic combining number of phos- phorus ? 102 A CORRESPONDENCE COURSE IN PHARMACY 30. What is the algebraic combining number of uncom- bined carbon ? 31. State the algebraic combining numbers of the three different elements in Na2S203. 32. What is the algebraic sum of the carbon bonds in HC2H302 ? LESSON NINE XIII Chemical Notation 206. We have already made use of several chemical symbols and self-explanatory formulas. Before proceeding further we will now learn something of the principles governing the construction of symbolic formulas. In order to represent at a glance the composition and structure of molecules a system of chemical notation was invented by Berzelius, which is still in use, modified and adapted to correspond to the development of the science of chemistry since his day. 207. Each atom of any given element is represented by a specific symbol unlike the symbol of any other element. The symbol consists of one or two letters which are the initials of the latinic or other names of the elements. Two letters are used for some of the symbols in cases where the names of two or more elements begin with the same letter. The additional letter used is not always the second letter of the name, but one which will best serve to make the symbol distinctive. As the names of chlorine and chromium both begin with Ch, their symbols are made 01 and Or. The first letter of any symbol of two letters is a capital letter; the second is not. 208. The symbol of any element stands not merely for its name, but for one atom of it and for its atomic weight or combining mass. 103 104 A CORRESPONDENCE COURSE IN PHARMACY Thus S, the symbol for sulphur, means one atom of sulphur and also 32 parts of sulphur. 209. Symbolic formulas are constructed out of one. or more symbols together with one or more numerals, or of two or more symbols with or without numerals. KI is a symbolic formula because it is composed of two symbols, K and I ; Cl2 is a formula because it is composed of a numeral as well as a symbol; 2C1 is also a formula for the same reason; 03, H20, OaCl2 and HN03 are also sym- bolic formulas. Symbolic molecular formulas of compounds are so con- structed that they show not only all of the elements com- posing them but the number of atoms of each. 210. The numerals used are of two "kinds — large and small. A large numeral placed in front of a symbol multiplies it, but it also indicates that the atom represented by the symbol is a free or uncombined atom. Thus 90 means nine oxygen atoms not united to one another. A small numeral placed after a symbol also multiplies the atom, but it signifies that all the atoms are in chemical combination either with one another or with other atoms. Thus 03 means three atoms of oxygen in combination with one another, or, in other words, a molecule of ozone. The formula 303 means three molecules of ozone each containing three atoms of oxygen; but 90 means nine single oxygen atoms and not three molecules of ozone. A large numeral in front of any symbolic molecular formula multiplies the whole molecule, but a small numeral is never used to multiply a molecule; the latter is placed to the right of the symbol which it is intended to multiply and a little below the line, as in 03. Thus, the formula O6H10O5 means one molecule composed of six carbon atoms, ten hydrogen atoms and five oxygen atoms, and 2C^H10O5 means two such molecules. CHEMICAL NOTATION" 105 The following examples will suffice to make these things clear : Hg stands for one atom of mercury. Hg also stands for one molecule of mercury, because each molecule of mercury contains but one atom. 2H means two free hydrogen atoms. 3H means three free hydrogen