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That collocation of symbols which expresses the composition of a compound is called the formula of that compound. The formulae Bao, B.O, Cr, Cl, 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 ~ 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. We 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 4.6 2.8, 3-5, 4:6, 7, 14 Phosphorus 10-3 6-2, 10:3

6-2, 10:3 Iron 18.6, 21, 28

186, 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 : s. 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

11 Arsenic

25

75 Bromine 80 80 Barium 68.5 137 Cadmium

112 Beryllium 4.5 9 Caesium 133 133

3.6

56

Calcium
Carbon
Cerium
Chlorine
Chromium
Cobalt
Copper
Didymium
Erbium
Fluorine
Gallium
Germanium
Gold
Hydrogen
Indium
Iodine
Iridium
Iron
Lanthanum
Lead
Lithium
Magnesium
Manganese
Mercury
Molybdenum
Nickel
Niobium
Nitrogen
Osmium

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

1 1 37.8 113.4

127 96.3 192.6 28 56 46.6 139 103.5 207

7 7 12 24 27.5 55 200 200 48 96 29.3 58.6 47 94 14 14 96.5 193

Oxygen
Palladium
Phosphorus
Platinum
Potassium
Rhodium
Rubidium
Ruthenium
Scandium
Selenion
Silicon
Silver
Sodium
Strontium
Sulphur
Tantalum
Tellurium
Terbium
Thallium
Thorium
Tin
Titanium
Tungsten
Uranium
Vanadium
Yttrium
Ytterbium
Zinc
Zirconium

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 46 184 60 240 51.2 51.2 29.6 89 57.6 173 32.5 65 45 90

127

80

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 one 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

81

of the elements, stated as a certain number of combining
weights of each element, which combine to form a specified mass
of the compound A number placed beneath (or sometimes
above) the symbol of an element in the formula of a compound
tells that the symbol is to be multiplied by this number. A
number placed at the beginning of the formula of a compound
multiplies the whole of the formula, or if a full stop occurs in
the formula the number multiplies all as far as that stop i
sometimes the formula is put in brackets and the multiplier is
placed outside the bracket. The following formulae will
illustrate these points.

Fe = 56, 0=16, S = 32.
FeO means 56 + 16 = 72 parts by weight of a compound called

ferrous oxide; this formula also tells that one c. w. of iron

combines with one c. w. of oxygen to form ferrous oxide. Fe,0, means (56 x 2) + (16 x 3) = 160 parts by weight of a

compound called 'ferric oxide ; also that 2 c. ws. of iron

combine with 3 c. ws. of oxygen to form ferric oxide.
FeSO, means 56 + 32 + (16 x 4) = 152 parts by weight of a

compound called ferrous sulphate ; also that one c. w. of
iron, one c. w. of sulphur, and four c. ws. of oxygen, combine

to form ferrous sulphate.
Fe,.380, or Fe (SO2), means (56 x 2) + 3 (32 + 64) = 400 parts

by weight of a compound called ferric sulphate ; also that two c. ws. of iron, three c. ws. of sulphur, and twelve c. ws.

of oxygen, combine to form ferric sulphate. 3Fe,350, or 3Fe (SO2), means 3{(56 x 2) + 3(32 +64)} = 1200

parts by weight of ferric sulphate.

Chemical changes are also expressible in formulae, so far 82 at least as the composition of the elements or compounds before and after such changes is concerned. Thus, we have learned that

(1) Sulphur and iron combine when heated in the ratio 1:1.75, to form iron sulphide;

(2) Hydrogen and oxygen combine in the ratio 1:8 to form water. These chemical reactions may be shortly expressed thus ;(1) S + Fe = Fes.

(2) 2H + 0 = H,O. Fe = 56, S = 32, 0 = 16. The ratio 32 : 56 =1:1.75 ; the, ratio 2 : 16 =1:8.

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The sign + signifies reacts chemically with; the sign signifies with production of. The total mass of matter on one side of the sign

is equal to the total mass of matter on the other side. Let us consider one or two rather more complex reactions.

(1) Na + H2O + Aq=NaOHAq+H.
(2) 3Fe + 4H, O = Fe 0, +8H.
(3) Zn + H 80 Aq = ZnSO, Aq + 2H.

Na = 23,0 = 16, Fe = 56, Zn = 65, S = 32. (1) When sodium and water interact, 23 parts by weight of sodium and 18 parts by weight of water disappear, and there are produced 40 parts of sodium hydroxide, which remains dissolved in the water that has not been changed, and 1 part by weight of hydrogen.

(2) When iron and water interact, 168 parts of iron and 72 of water are changed to 232 parts of iron oxide and 8 parts of hydrogen.

(3) When zinc and a solution in water of sulphuric acid interact, 65 parts by weight of zinc and 98 of the acid are changed into 161 parts of zinc sulphate, which remains in solution, and 2 parts of hydrogen.

These chemical equations, as they are called, also represent the compositions of the compounds before and after the change, expressed as so many combining weights of each elementary constituent of each compound; when elements take part in the reactions, the equations also express the number of combining weights of this or that element which interacts with a certain mass of a compound, or with a certain number of combining weights of another element, and the number of combining weights of this or that element which is produced by the interaction. Thus (1) states, more shortly than can be done in words, the fact that one c. w. of sodium interacts with 18 parts of water to produce 40 parts of sodium hydroxide and 1 c. w. of hydrogen; and (3) states that one c. w. of zinc interacts with 98 parts by weight of sulphuric acid dissolved in water (which 98 parts are composed of 2 c. ws. of hydrogen, 1 c. w. of sulphur, and 4 c. ws. of oxygen) to produce 161 parts of zinc sulphate (composed of 1 c. w. of zinc, 1 c. w. of sulphur, and 4 c. ws. of oxygen) which remain in solution, and 2 c. ws. of hydrogen.

The symbol Aq is used here, and generally in this book, to mean a large indefinite) quantity of water; when placed

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