« PreviousContinue »
SYMBOLS AND FORMULAE.
It is customary to express the composition of compounds in a kind of shorthand by a method the principle of which is the same as that we are at present illustrating.
A symbol is given to each element; this symbol is formed either of the first letter, or of the first and some other letter, of the name of the element. When the names of several elements begin with the same letter that element which has been longest known and best studied generally gets a symbol formed of the first letter only; but there is no universally applicable rule. Some of the symbols are derived from the names by which the elements were known to the ancients or in the middle ages. The symbols of two elements, potassium (K), and sodium (Na), are derived from the names kalium and natrium by which these elements are known to German chemists. The symbol W is given to the element tungsten, it is derived from the name (Wolfram) of the mineral from which tungsten was first obtained.
It is of the utmost importance to remember that each of these symbols represents a definite mass of the element; it represents either one, two, three, four, five, or six, combining weights, as we are at present using the term combining weight, of the element. The following table gives the names and symbols of the elements.
Mass of element
element Name. Symbol. expressed Name. Symbol. expressed by sym
boll. Aluminium Al 27 Molybdenum Mo 96 Antimony Sb 120 Nickel
Ni 58.6 Arsenic
As 75 Niobium Nb 94 Barium
Ba 137 Nitrogen N 14 Beryllium
9 Osmium Os 193 Bismuth Bi 208 Oxygen
11 Palladium Pd 106 Bromine Br 80 Phosphorus P
31 Cadmium Cd 112 Platinum Pt 194 Caesium Cs
133 Potassium K 39 Calcium Ca 40 Rhodium Rh 104 Carbon
85.4 Cerium Ce 140 Ruthenium Ru
104.6 Chlorine CI
35.5 Scandium Sc 44 Chromium Cr 52.2 Selenion Se 79 Cobalt Co 59 Silicon
Si 28 Copper Cu 63.2 Silver
108 Didymium Di 144 Sodium
23 Erbium Er.
166 Strontium Sr 87 Fluorine F 19 Sulphur
Ga 69.9 Tantalum Ta 182 Germanium Ge 72.2 Tellurium Te 125 Gold
Tr 148 Hydrogen H
1 Thallium ΤΙ 204 Indium
In 113.4 Thorium Th 232 Iodine I 127 Tin
Sn 118 Iridium
Ir 192.6 Titanium Ti 48 Iron
56 Tungsten W 184 Lanthanum La 139 Uranium U 240 Lead
Pb 207 Vanadium V 51.2 Lithium Li
Y 89 Magnesium Mg 24 Ytterbium Yb 173 Manganese Mn 55 Zinc
Zn 65 Mercury
200 Zirconium Zr 90 i The values in this table are given in round numbers ; they are only approximately correct.
That collocation of symbols which expresses the composition of a compound is called the formula of that compound. The formulae Bao, B.Os, Cr,ci,, HI, tell, that barium and
oxygen combine to form barium oxide in the ratio 137:16 by weight, that boron and oxygen combine in the ratio 22:48 (=11 ~ 2:16 x 3), that chromium and chlorine combine in the ratio 104.4 : 213 (= 52.2 x 2 : 35.5 x 6), and that hydrogen and iodine combine in the ratio 1:127.
Or, the facts concerning composition which these formulae express may be thus stated; 153 parts by weight of barium oxide are formed by the combination of 137 parts by weight of barium with 16 parts by weight of oxygen ; 70 parts by weight of boron oxide are formed by the combination of 22 parts by weight of boron with 48 parts by weight of oxygen ; 317-4 parts of chromium chloride are produced by the combination of 104:4 parts of chromium with 213 parts of chlorine ; 128 parts of hydrogen iodide are formed by the union of 1 part of hydrogen with 127 parts of iodine.
· The numbers in the third column of the preceding table are sometimes called the combining weights of the elements. have already given a meaning to the term combining weight (s. par. 74). If that meaning is adopted, the mass of an element expressed by its symbol is seldom the same as the value obtained for the combining weight of that element; but when it is not the same, it is a simple multiple of the combining weight.
We are not yet in a position to go fully into this matter of combining weights. We have already used the expression combining weight to mean, that mass of an element which combines with unit mass of hydrogen, or, in the cases of elements which do not combine with hydrogen, that mass which combines with 8 parts by weight of oxygen, or 16 of sulphur, or 35.5 of chlorine. But when we come to apply this definition we meet with many difficulties. Thus, nitrogen and phosphorus each form one compound with hydrogen; nitrogen forms 5 compounds, and phosphorus 2 compounds, with oxygen. From the composition of each of these compounds a value may be deduced for the combining weight of nitrogen, or for that of phosphorus. Similarly iron forms 3 compounds with oxygen, and 2 with chlorine; from the composition of these, values are found for the combining weight of iron. The values are these.
Combining weights of nitrogen, phosphorus, and iron.
Deduced from composition
of hydrides. of oxides. of chlorides. Nitrogen
2.8, 3.5, 4:6, 7, 14 Phosphorus 10:3 6.2, 10-3
6.2, 10-3 Iron 18·6, 21, 28
18·6, 28 This list might be largely extended ; in very few cases should we find but one value for the combining weight (as defined) of an element.
To adopt several combining weights for each element would introduce endless confusion into our system of representing the composition of compounds. It is absolutely necessary to adopt one value and one value only, not merely for convenience but also for cogent reasons which will be given later. Sometimes the highest value found by the method already stated is adopted, e.g. for nitrogen (N=14; comp. above results with the table in par. 76); sometimes a simple multiple of this highest value is adopted, e.g. for iron (Fe = 56: 8. table in par. 76). If we define combining weight as has been already done, then the definition generally leads to several values for the combining weight of each element. If we call the numbers in the table in par. 76 combining weights, then we cannot accurately define the term combining weight.
The best compromise, at any rate for us at present, is to 79 say, that the actually used combining weight of an element is a number which expresses either the largest mass of the element which combines with 1 part by weight of hydrogen, or 8 parts of oxygen, or 16 of sulphur, or 35.5 of chlorine, or it expresses a simple multiple of this mass.
The following table presents; in column I., the largest mass of each element which is known to combine with either 1 part by weight of hydrogen, or 8 of oxygen, or 16 of sulphur, or 35.5 of chlorine; and in column II., the actually used values for what are generally called the combining weights of the elements. I. II.
I. II. Aluminium 9 27 Bismuth 104 208 Antimony 40 120 Boron
3-6 11 Arsenic 25 75 Bromine
80 80 Barium
68.5 137 Cadmium 56 112 Beryllium 4.5 9 Caesium 133 133
I. II. 20 40 12 12 46.6 140 35.5 35.5 26:1 52.2 29.5
59 63.2 63.2 48 144 58.6 166 19 19 23.3 69.9 36.1 72.2 197 197
I. II. 16 16 106 106 10:3 31 97 194 39 39 52 104 85.4 85.4 52.3 104.6 14.6 44 39.5 79
7 28 108 108 23 23 43.5 87 16 32 45.5 182 62.5 125 49:3 148 204 204 58 232 59 118 24 48 46
240 51.2 51.2 29.6 89 57.6 173 32.5 65 45 90
As we advance in our study of chemical events we shall learn that there is no purely chemical, and general, method, by using which a decision may be arrived at regarding the best value to be given to the combining weight of an element. Each case must be discussed by itself; the result is at best a compromise. But we shall also find that the application of certain physical conceptions to chemical phenomena leads to a generally applicable method, based on definite principle, whereby values may be obtained for what we at present call the combining weights of the elements.
The symbol of an element, then, expresses a definite mass of that element. The formula of a compound expresses the masses