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PART III

THE SYSTEMATIC STUDY OF THE ELEMENTS, BASED UPON THE PERIODIC CLASSIFICATION.

CHAPTER I

THE ELEMENTS OF GROUP VII. (FAMILY B.)

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THE first to be discovered, and the most important element of the group, is chlorine, which is a constituent of sea salt (sodium chloride). The term halogen, signifying sea salt producer, has been applied to this family of elements, on account of the close resemblance between their sodium salts and sea salt. This family exhibits, in a marked manner, many of the features which are found to exist in most chemical families of elements.

In their general behaviour they strongly resemble one another, and readily displace each other in combinations without producing any very marked change upon the character of the compounds. They each unite with hydrogen, giving rise respectively to hydrofluoric acid, HF; hydrochloric acid, HC1; hydrobromic acid, HBr; hydriodic acid, HI.

These hydrogen compounds are all colourless gases, which fume strongly in the air; they are extremely soluble in water, and are strongly acid in character. In combination with potassium and with sodium, the halogens form a series of compounds, which are similarly constituted, and which closely resemble each other in their habits. Their similarity of composition is expressed in the following formula

Compounds with potassium, KF, KCl, KBr, KI.
Compounds with sodium, NaF, NaCl, NaBr, Nal.

The physical properties of the elements exhibit a regular gradation with increasing atomic weight; thus, fluorine and chlorine are gases, bromine is liquid, while iodine is solid, at ordinary temperatures. In their chemical activity they also show the same gradual change; thus, in the case of their combination with hydrogen, when fluorine and hydrogen are brought together, combination instantly takes place with explosion, even in the dark. Chlorine and hydrogen do not combine in the dark, but in diffused daylight they unite slowly, and in direct sunlight their combination takes place suddenly with explosion.

Bromine vapour and hydrogen do not combine even in direct sunlight, but a mixture of the two gases ignites in contact with a flame, yielding hydrobromic acid, while iodine vapour and hydrogen require to be strongly heated in contact with spongy platinum to effect their combination. This difference in the activity of the halogens towards hydrogen is seen by a comparison of the heats of formation of their hydrogen compounds, thus—

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The heat of formation of hydrofluoric acid has not yet been determined, but there can be no doubt that it is considerably greater than that of hydrochloric acid.

Although a strong resemblance exists between all the members of the halogen family, the element fluorine, which is the typical member (see page 114), stands marked off from the others in many of its attributes. Thus fluorine exhibits a great tendency to form double salts which have no counterpart among the compounds of the other elements of the family, and at temperatures below 32° the molecule of hydrofluoric acid consists of two atoms of hydrogen and two of fluorine, having the composition H2F 2.

FLUORINE.

Symbol, F. Atomic weight=19.

History. This element, the first of the halogen series, was the most recent to be discovered, it having baffled all attempts to * This value refers to bromine in the liquid state.

Iodine as solid.

isolate it until the year 1886, when Moissan succeeded in solving the problem.

Occurrence.-Fluorine occurs in considerable quantities in combination with calcium in the mineral fluor spar (CaF2), which is found in cubical crystals. On account of the occurrence of this mineral in large quantities in Derbyshire it is frequently termed Derbyshire spar. It is a constituent also of cryolite, NaAlF6, fluorapatite, 3P0Ca,,CaF, and many others. In small quantities fluorine is found in bones, in the enamel of teeth, and also in certain mineral waters.

Mode of Formation.—When an electric current is passed into an aqueous solution of hydrochloric acid, the acid is decomposed into its elements, chlorine being liberated at the positive electrode, while hydrogen is evolved at the negative. When aqueous hydrofluoric acid is treated in the same way, the water only is decomposed, oxygen and hydrogen being liberated. Davy found that the more nearly the acid approached the anhydrous condition, the less easily did it conduct electricity; and that in the perfectly pure state, that is, entirely free from water, hydrofluoric acid was a nonconductor. Moissan's recent success in the isolation of fluorine depends upon the discovery that a solution of the acid potassium fluoride, HF,KF, in anhydrous hydrofluoric acid is an electrolyte, and that by the passage of an electric current through this solution fluorine is disengaged at the anode, or positive electrode, and hydrogen is evolved at the cathode.

The primary decomposition taking place is the breaking up of the acid potassium fluoride

HF,KF=2F+H+K.

The atom of potassium, in contact with the free hydrofluoric acid present, is then converted into potassium fluoride with the elimination of an equivalent of hydrogen

K+HF=KF+H.

And the normal potassium fluoride then unites with a molecule of the acid, to regenerate the acid salt

KF+HF=HF,KF.

The reaction is performed in a U-tube made of an alloy of platinum and iridium, a material which is less acted upon by the

liberated fluorine than platinum alone. The apparatus has two side-tubes (Fig. 89), which can be either closed with a screw cap, C, or connected to platinum delivery tubes by means of the union D. The two limbs of the tube are closed by means of stoppers made of fluor spar, shown in section at S, and which can be securely screwed into the tube. These serve to insulate the electrodes, which are constructed of the same platinum-iridium alloy. The anhydrous hydrofluoric acid is introduced into the apparatus, and about 25 per cent. of its weight of the acid potassium fluoride is added, which really dissolves in the liquid. The tube is immersed in a bath of methyl chloride (M, Fig. 90), which boils at -23°; the

D

FIG. 89.

supply being continuously replenished from the reservoir B, while the vapour is drawn away by the pipe C. On passing a current from 20 to 25 Grove's cells through the apparatus, fluorine is evolved at the positive electrode, and hydrogen is liberated at the negative.*

Properties. Fluorine is, of all known elements, the most chemically active. It is on account of its intense chemical affinities that it so long resisted all attempts to isolate it, as when liberated from combination it instantly combined with the materials of the vessels in which the reactions were made. It is impossible to collect this gas by any of the usual methods, for it decomposes water and instantly combines with mercury. It also attacks glass, so that it can only be collected by displacement of air in vessels of platinum. Fluorine is a pale yellowish-coloured gas, appearing almost colourless when viewed in small quantities. The smell of the gas is very characteristic-it is irritating to the mucous membranes, and is not unlike the odour of the mixture of chlorine and chlorine peroxide, evolved from potassium chlorate and hydro

* Fluorine has recently been obtained by Brauner (June 1894) by heating potassium fluorplumbate, 3KF, HF, PbF4. At 200° this salt gives off hydrofluoric acid, HF, and when heated to 230°-250° fluorine is evolved.

chloric acid. Whether the smell actually perceived is the true smell of fluorine is doubtful, for when fluorine comes into contact with the moisture in the nostrils water is decomposed, with the formation of ozonised oxygen and hydrofluoric acid.

Fluorine not only decomposes potassium iodide, with liberation of iodine, but also displaces chlorine from sodium chloride.

It combines directly with a large number of elements with intense energy; in contact with hydrogen it instantly explodes. Iodine, sulphur, and phosphorus first melt, and then take fire in fluorine. Crystals of silicon, when brought into the gas, spontane

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ously inflame, and burn with brilliancy. All of the metals are acted upon by fluorine; some, when finely divided, undergoing spontaneous inflammation when thrown into the gas. Even gold and platinum are attacked by fluorine, especially if gently warmed; its action upon the latter metal being seen by the corrosion of the apparatus, and especially the positive electrode employed in its preparation. Organic compounds are attacked by fluorine with violence, and often inflamed.

When fluorine is cooled to a temperature about - 185° (i.e. a few

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