§ 28. Nascent State. The necessity of distinguishing between atoms and molecules of elements has been but slowly recognised; it has proved of great service in providing an explanation of certain apparently inexplicable phenomena. It has frequently been observed that many elements which, as a rule, do not readily enter into combination easily unite if brought together at the moment of their liberation from other compounds. In this specially active condition the elements are said to be in the nascent state.' The peculiar behaviour of elements in the nascent state is accounted for by assuming that they are then present as isolated atoms. Naturally these isolated atoms are more ready to enter into combination than they would be if they were already united to similar atoms in the form of molecules.

Hydrogen offers a striking example of the activity of elements in the nascent state. It is only at a high temperature that free hydrogen burns in oxygen, forming water, but both elements will unite at the ordinary temperature, or even at a lower temperature, at the moment of their liberation from other compounds. It is more difficult to combine free nitrogen with oxygen or hydrogen, but if the elements are in the nascent state combination readily takes place. It is easy to understand that isolated atoms at once unite when they meet each other, but when an atom is united to one or more atoms to form a molecule, it must first of all be detached from this molecule before it can form a new compound. In the case of nitrogen the tendency of the two atoms to combine and form the free molecule appears to be very strong.

§ 29. Determination of the Stochiometric Values.-Having considered the grounds on which the determination of the atomic weights is based, we must now proceed to the description of the methods employed in the exact determination of these highly important values. The process is far from simple. In the first place it is necessary to know, with the utmost degree of accuracy, the proportions by weight with which the given element unites with other elements. This knowledge can only be acquired by careful analyses or syntheses of compounds. But all our methods of analysis and synthesis are vitiated by certain errors, which can never be entirely avoided, but must be

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45

reduced to the narrowest limits. Those methods alone are to be used which can be carried out with the minimum amount of error. In analysis a certain definite weight of a compound is decomposed and the weight of its constituents determined. A distinction is made between partial and complete analyses, according as one or all the constituents are determined; and a similar distinction is drawn between partial and complete syntheses. When practicable, complete or total analyses or syntheses are preferred, as in these cases we have a guarantee that nothing has been lost or gained during the operations, when the sum of the weight of the constituents is equal to the weight of the compound. In many cases it is only possible to make a partial analysis or synthesis, as some substances cannot be brought into a form in which their weight can be ascertained with a sufficient degree of accuracy.

As to the means for determining the weight and therewith the mass of a body, the balance and weights have been developed to a point of such great accuracy that the error has been reduced to 100000, or even 1000000 But such accuracy can only be attained in weighing stable bodies, which occupy a very small space in proportion to their weight, and do not possess a very large surface; for large volumes and large surfaces increase the possible errors in weighing.

As weighings are generally made in atmospheric air, the substance weighed appears lighter than it really is by the weight of air it displaces. This loss of weight can be calculated and allowed for, but the error increases as the volume of air displaced increases. Air and other gases and moisture condense on the surfaces of the body weighed as well as of the vessels containing it, and in this way the error of weighing increases with the surface. This source of error can be diminished, but cannot be entirely avoided.

In atomic weight determinations we avoid, as far as possible, weighing gases or liquids on account of the error introduced by the use of large vessels for holding them. This can be accomplished by measuring instead of weighing these bodies, if the weight of the unit of volume, i.e. the density, has been once determined.

The use of substances which easily oxidise, absorb moisture

from the atmosphere, or in other ways change, should be avoided if possible; if it is necessary to employ them they must be weighed in air-tight vessels, which have either been exhausted by the air-pump or filled with an indifferent gas.

It frequently occurs that an element in the free state is unsuitable for weighing. In this case it is converted into a suitable compound, which is weighed, the amount of the element in the compound having been previously accurately determined. Chlorine is weighed as silver chloride, sulphur as barium sulphate, &c.

Great care must be taken to insure the purity of the substance investigated and of the other substances used in the various operations, in order that the bodies which are weighed may really have the composition they are supposed to possess. If these precautions are neglected very grave errors will follow.

=

§ 30. Relation of Stochiometric Determinations to each other. As hydrogen has been selected as the unit of equivalent and atomic weights, it is desirable to compare all determinations with this standard. Unfortunately hydrogen only unites with about a dozen other elements, and these compounds are mostly gaseous like hydrogen, and consequently difficult to determine quantitatively. Berzelius determined the atomic weights of nearly all the elements with which he was acquainted with wonderful accuracy, using as his unit the hundredth part of an atom of oxygen, regarding the atomic weight of oxygen as 100. He did this instead of using Dalton's unit, hydrogen 1, on account of the difficulty involved in accurately determining the composition of the gaseous compounds of hydrogen. He also occasionally made use of Dalton's unit, calculating out his results in terms of this standard. At the present day we are frequently compelled to adopt this indirect method. This indirect method involves the knowledge of the proportion by weight in which hydrogen and oxygen unite to form water, and as a natural consequence this determination has been made with the greatest care. The ratio 1: 7.98 has been obtained as the mean of numerous concordant results arrived at by different methods. Water contains 1 part by weight of hydrogen to 7.98 of oxygen, and according to Avogadro's law (§ 19) we

ATOMIC-WEIGHT DETERMINATIONS

47

consider that water contains two atoms of hydrogen, but does not contain two or more atoms of oxygen.

Therefore

H2: 0 = 1 : 7·98, or H: 01:15.96.

There may be an error of one or more units in the second place of decimals: that is, an error of some thousandths of the total value. The practice of representing the atomic weight of oxygen as a whole number, 16, is unwarrantable. Where great accuracy is not necessary the round number may be used as a matter of convenience, and the calculated result will be nearly accurate; but when scientific accuracy is required such arbitrary alterations in the experimental results are not permissible.

Having determined the atomic weight of oxygen in this way, we can now compare a large number of atomic weights of other elements, many metals in particular, with the atomic weight of hydrogen. The amount of oxygen in the oxides is determined by analysis or synthesis. The quantity of the element which unites with an atom of oxygen is equivalent to two atoms of hydrogen. Whether this quantity represents the atomic weight or a multiple or sub-multiple is ascertained by means of Avogadro's law, by the law of Dulong and Petit, or by isomorphism.

An example will explain the method. Berzelius obtained 4.2835 grams of oxide by oxidising 2.9993 grams of pure iron, or 1-42817 gram of oxide from 1 gram of iron, or making the necessary corrections for weighing in air 1-42836 gram of oxide from 1 gram of iron. One part by weight of the metal united with 0-42836 of oxygen. The quantity of metal A oxidised by one equivalent = 7.98 parts by weight of oxygen

is

1 A 0.42836 7.98;
1: =

A = 18.629.

This number cannot be the atomic weight of iron, for on multiplying it by the specific heat of the metal, c = 0·114, it yields the product A. c = 2.13, whilst treble the value, i.e. 55.89, yields 6.4. The latter number also represents the quantity of iron contained in the molecular weight of ferric chloride

(§ 25); this must therefore be regarded as the atomic weight of iron compared with hydrogen as unity. Similar determinations by other chemists yield almost identical results. The mean of the most trustworthy results gives 55.88 as the atomic weight of iron.

The oxides of many elements are difficult to prepare in a state of perfect purity. This is true of many of the light and of some of the noble metals, but the chlorides, bromides, &c. of these elements are admirably adapted for weighing. In such cases the comparison of the atomic weight with that of hydrogen is made by a more indirect method than the preceding. The compounds of silver with chlorine, bromine, and iodine are quite insoluble in water, and are therefore well adapted for analytical determinations. The proportions by weight with which these elements unite with silver have been very carefully estimated. In fact, the most correct of all the stoechiometrical determinations that have ever been made are those which fixed the combining proportions of silver and iodine

Ag: I1: 1.17534.

This determination was carried out by Stas with the utmost care and dexterity; the experimental error is about 1 in 100000. As oxide of silver is too unstable to permit of correct analysis the proportion of silver to oxygen had to be determined by several indirect methods, all of which yielded similar results.

The analysis of potassium chlorate, KClO,, gave the relative quantities of potassium chloride, KCl, and oxygen in the salt : KC1 : 0 = 4·6616 : 1.

By converting weighed quantities of potassium chloride, KCl, into silver chloride, AgCl, the following ratio was obtained: Ag: KCl = 1: 0-69104.

The same result was obtained in a similar way by the synthesis of silver sulphide, Ag2S, and its oxidation to silver sulphate