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CHAPTER IV THE ELEMENTS OF GROUP I. (FAMILY A.) This family comprises the following five elements, known as the alkali metals-
Melting points. Lithium (Li) . . . . 7.03 . . . . 180° Sodium (Na) . . . . 23.05 . . . . 95.6° Potassium (K) , . . 39.15 . . . . 62.5° Rubidium (Rb) . . 85.4 . . . . 38.5°
Caesium (Cs). . . . 133 . . . . 26.5° The most important and the most abundant of these elements are potassium and sodium, which also were the first to be discovered, having been isolated by Davy in the year 1807. The element lithium, although widely distributed in nature, is for the most part found only in minute quantities; the element was first isolated by Bunsen in the year 1855. The two remaining elements are still rarer substances, usually met with in very minute quantities accompanying sodium and potassium. Both of these elements were discovered by Bunsen by means of the spectroscope-caesium in 1860 and rubidium in the following year.
All these elements are soft, silvery-white metals, which may be readily cut with a knife, and which rapidly tarnish in the air. They all decompose water at the ordinary temperature. The members of this family exhibit that gradation in properties which is met with in all similar families. Thus, their melting-points gradually decrease as their atomic weights rise, as will be seen from the figures given above. Their chemical activity also steadily increases as we pass from lithium to caesium. Thus, in the case of their behaviour in contact with water : potassium, when thrown upon cold water, decomposes that liquid with sufficient energy to cause the ignition of the hydrogen which is evolved ; sodium under the same conditions melts and floats about upon the surface, but the action is not sufficiently energetic to effect the inflammation of the gas, unless the water be previously heated ; while with lithium, even with boiling water, the temperature produced by the reaction does not rise to the ignition-point of hydrogen. The same is also seen in the spontaneous oxidation of these elements when they are exposed to the air. Thus, lithium when cut with a knife, although it is soon covered with a film of oxide, nevertheless retains its bright metallic surface for some seconds ; sodium tarnishes so much more quickly, that the film of oxide appears almost to follow the knife. When potassium is cut the bright surface can scarcely be seen, so rapid is the oxidation, and if left exposed a fragment of the metal soon begins to melt by the heat of its own oxidation, and frequently spontaneously ignites. With rubidium and caesium the oxidation is even more rapid, and a fragment of these metals freely exposed to the air very rapidly takes fire spontaneously.
The electro-positive character of these elements gradually increases from lithium to caesium, which is the most electro-positive of all the known elements.
The term alkali, applied to metals of this family, was originally used (before any distinction was made between potash and soda) to denote the salt obtained by treating the ashes of plants with water. Later on, in order to distinguish between this substance and what became known as the volatile alkali (i.e. ammonium carbonate), it was termed the fixell alkali. The first distinction between potash and soda was based upon the erroneous belief that the former was entirely of vegetable origin, while the latter was only to be found in the mineral kingdom ; hence the names vegetable alkali and mineral alkali were used to denote these two substances, both of which were regarded as elementary bodies until 1807, when Davy showed that they contained the two metals potassium and sodium.
The resemblance between the different members of this family and between their compounds is very close ; so much so, that in the case of sodium, potassium, rubidium, and caesium, there are scarcely any ordinary chemical reactions by which they can be distinguished. They are all readily identified, however, by means of the spectroscope. When a minute quantity of a lithium salt is introduced upon a loop of platinum wire into the non-luminous Bunsen flame, the latter is tinged a brilliant crimson-red colour ; a potassium salt similarly treated colours the flame a delicate lilac, while a sodium compound gives a brilliant daffodil-yellow colour. The colour imparted to a flame by rubidium and caesium salts is indistinguishable by the eye from that given by potassium compounds; and, moreover, when any of these are mixed with a sodium salt the intense yellow emitted by the latter completely masks the colours given by the others. By means of the spectroscope, not only are the apparently similar colours given by potassium, rubidium, and caesium readily distinguished, but the presence of any or all of them is easily detected, even when admixed with sodium salts. Spectrum analysis is based upon the fact that light of different colours has different degrees of refrangibility, and therefore when passed through a prism the different coloured rays are bent out of their straight course at different angles. Ordinary white light
is composed of rays of all degrees of refrangibility, i.e. rays of all colours ; hence, when a beam of such light is passed through a prism, the various coloured rays are separated and become spread out in the order of their refrangibility, from the least refrangible red at the one extreme to the deep violet at the other. This familiar “ rainbow" coloured band of light is termed the continuous spectrum.
A simple form of spectroscope is seen in Fig. 129. The light is caused to pass through a narrow slit at the end of the fixed telescope B. If the prism P be removed and the telescope A be moved round so as to be in a continuous line with B, a magnified image of the slit is seen by the observer. When the prism is replaced, and A is moved into such a position that the bent rays fall upon its lens, the continuous spectrum is seen, which is an infinite number of strips of light (corresponding to the image of the slit) of all colours arranged side by side. If the light to be examined, instead of being ordinary white light, were composed of rays all of one degree of infrangibility (i.e. monochromatic light), there would be produced only a single image of the slit, which would fall in that position corresponding to the particular degree of refrangibility of the light. Such a monochromatic light is produced when a sodium salt is heated in a Bunsen flame ; if, therefore, a salt of this metal be introduced upon a loop of platinum wire into the non-luminous flame G, and the light, after passing through the prism, be observed through A, instead of a continuous spectrum, there will be seen a single image of the slit, falling in the brightest yellow part of the spectrum. When the sodium salt is replaced by a lithium salt, it is seen that two images of the slit are obtained, one in the red and the other in the yellow regions of the spectrum. The light emitted from this element consists of rays of two degrees of refrangibility. We say, therefore, that the spectrum of sodium is one yellow line,* and that of lithium consists of one red and one yellow line. In order to distinguish the positions of, for example, the yellow lithium line and that given by sodium, an image of a graduated scale, illuminated by the candle flame F, is also thrown into the telescope A.
If salts of sodium and lithium mixed together be introduced into the flame G, then three images of the slit are seen, namely, the yellow line given by the sodium, the yellow line of the lithium, situated slightly nearer the red, and the lithium red line.
Potassium, like lithium, gives a light of two degrees of refrangibility, forming consequently two images of the slit, one in the deep red and the other in the deep violet ; if, therefore, lithium, sodium, and potassium salts are mixed, and examined by the spectroscope, five lines are seen (Fig. 130), namely, two red (one belonging to lithium and one to potassium), two yellow (one belonging to lithium and one to sodium), and the violet line of potassium.
* In reality, when examined by a higher dispersive power, the sodium line is seen to be a group of lines.
When analysed in this manner, the lights emitted by rubidium and caesium compounds are seen to be totally different from each other, and from potassium. The spectrum of rubidium consists of two prominent lines in the violet (nearer the blue region than that belonging to potassium), two brilliant red lines (very near the potassium red line), and a number of less brilliant lines in the orange, yellow, and green. That of caesium consists of two brilliant blue lines, two bright red lines (near the lithium red line), and a number of less prominent lines in the yellow and green. It will be seen, therefore, that the three elements potassium, rubidium, and caesium may be at once sharply distinguished by this optical method of analysis, although they so closely resemble one another in their chemical behaviour, as to render it highly probable that the separate existence of the two latter would never have been discovered by chemical methods alone.
Indeed, before the discovery of caesium by Bunsen, a rare mineral known as Pollux (now known to contain caesium) was mistaken for a potassium mineral.*
The element lithium, the member of the family that belongs to the Typical series, exhibits certain characteristic differences from the other members. This is seen particularly in the case of the carbonate and phosphate of this element. Lithium carbonate is so little soluble in water, that it is precipitated by the addition of carbonate of either sodium or potassium to a solution of a lithium compound. The phosphates of all the other members are readily soluble in water, while lithium phosphate is almost insoluble, and is precipitated from solutions of a lithium salt by the phosphates of either sodium or potassium. In these two compounds, the car. bonate and phosphate, lithium behaves more like one of the metals of the alkaline earths.
* The student should consult special works on spectrum analysis.