Caesium chemical properties

The alkali metals form a homogeneous group of extremely reactive elements which illustrate well the similarities and trends to be expected from the periodic classification, as discussed in Chapter 2. Their physical and chemical properties are readily interpreted in terms of their simple electronic configuration, ns, and for this reason they have been extensively studied by the full range of experimental and theoretical techniques. Compounds of sodium and potassium have been known from ancient times and both elements are essential for animal life. They are also major items of trade, commerce and chemical industry. Lithium was first recognized as a separate element at the beginning of the nineteenth eentury but did not assume major industrial importance until about 40 y ago. Rubidium and caesium are of considerable academic interest but so far have few industrial applications. Francium, the elusive element 87, has only fleeting existence in nature due to its very short radioactive half-life, and this delayed its discovery until 1939. 

The Group 1 elements are soft, low-melting metals which crystallize with bee lattices. All are silvery-white except caesium which is golden yellow "- in fact, caesium is one of only three metallic elements which are intensely coloured, the other two being copper and gold (see also pp. 112, 1177, 1232). Lithium is harder than sodium but softer than lead. Atomic properties are summarized in Table 4.1 and general physical properties are in Table 4.2. Further physical properties of the alkali metals, together with a review of the chemical properties and industrial applications of the metals in the molten state are in ref. 11. 

Chemical properties.—Sodium chloride is necessary for the proper performance of the physiological functions of the body the other alkali chlorides are said to be poisonous 64 with small animals. According to C. Richet, the maximum dose per kilogram of animal, with subentaneous injections, is 01 grm. with lithium chloride 0 5 grm. with potassium chloride l O grm. with rubidium chloride and 0 5 grm. with caesium chloride. Lithium chloride is very hygroscopic sodium chloride is less hygroscopic, but it takes up O 5 to 0> 6 per cent, moisture on exposure... 

Potassium hydride, KH.—Moissan5 prepared the hydride by a method analogous to that employed by him for the corresponding sodium derivative, the excess of potassium being dissolved by liquid ammonia. Ephraim and Michel6 passed hydrogen into potassium at 350° C., and found the reaction to be promoted by the presence of calcium. The hydride forms white crystals of density 0-80. The vapour-tension for each temperature-interval of 10° between 350° and 410° C. corresponds with the values 56, 83, 120, 168, 228, 308, and 430 mm. respectively.7 In chemical properties potassium hydride resembles the sodium compound, but is less stable. Its stability is greater than that of rubidium hydride or caesium hydride. Carbon dioxide converts it into potassium formate. 

Chemical Properties.—In its chemical character rubidium occupies a position intermediate between potassium and caesium. It combines with atmospheric oxygen and decomposes water more energetically than potassium, and the bright metal ignites spontaneously in dry oxygen. It begins to react with ice at —108° C.18 When dissolved in liquid ammonia, the metal combines with ozone.18 Some of its salts are poisonous. 

Chemical Properties.—In chemical properties caesium is closely related to potassium and rubidium. When brought into contact with air, it undergoes rapid oxidation and the pure metal ignites in dry oxygen at the ordinary temperature.8 It decomposes water energetically, the action on ice beginning at —116° C. Its solution in liquid ammonia reacts with ozone.10... 

Atomic Weight.—The chemical properties of caesium indicate its close relationship to the other alkali-metals. It is univalent, forming compounds of the type CsX, its atomic weight and hydrogen equivalent being the same. Its atomic weight is of the order Cs =133 a value conformed by the specific-heat method (Vol. I., p. 88) by the isomorphism of the caesium compounds with those of potassium, ammonium, and rubidium (Vol. I., p. 74) by the correspondence of the properties of the metal and its compounds with the periodic system by the formation of a univalent cation and by the depression of the freezing-point of bismuth chloride and mercuric chloride produced by caesium chloride. 

Lithium (Li, at. mass 6.94), sodium (Na, at. mass 22.99), potassium (K, at. mass 39.10), rubidium (Rb, at. mass 85.47), and caesium (Cs, at. mass 132.91) form colourless ions, readily soluble in water, and strong bases MOH. Potassium, rubidium, and caesium are very similar in chemical properties. Sodium has somewhat different properties. The properties of lithium are intermediate between those of sodium and calcium. Lithium and sodium have the ability to form weak complexes (e.g., with pyrophosphate and EDTA). 

In caesium (Z = 55) one electron occupies the 6s orbital of the P shell, and in barium (Z = 56) there are two electrons in this orbital. Thereafter the development of the P shell is interrupted, and lanthanum (Z = 57) initiates the third transition series with one electron in the 5 d orbital of the 0 shell. The development of this series, however, proceeds no further at this stage, and in the elements from cerium (Z = 58) to lutecium (Z = 71) electrons are entering the hitherto vacant 4/orbitals of the N shell. These elements constitute the rare earths or elements of the lanthanide series, and the fact that the differentiating electrons are so deep in the electronic structure is responsible for the close similarity of their chemical properties. 

Elements which are similar as regards their chemical properties have atomic weights which are either of nearly the same value (platinum, iridium, osmium), or which increase regularly (potassium, rubidium, caesium). 

Considering adsorption as the first step in catalysis, both physical and chemical adsorption has been observed with intercalates. Watanabe et a/. have shown that selective adsorption based on molecular size occurs with potassium-, rubidium-, and caesium-graphite. The molecular sieve properties are not as distinctive as in zeolites, as a result of the comparatively easy expandability of the layers. Once the gas enters the layers, both chemisorption and chemical reaction is possible. Therefore, for example, adsorption... 

The periodic table of elements is divided into horizontal rows and vertical colunuis. Elements in a particular column have similar chemical behaviom. Looking at the periodic table, the metals are in Row 2 (lithium, beryllium), Row 3 (sodimn, magnesium, aluminium), Row 4 (potassium, K through to gallium, Ga), Row 5 (rubidimn through to tin), Row 6 (caesium to bismuth) and Row 7 (francium to actinium). There are two special series of metals from atomic number 58-71 and 89-103. The first are the rare earth metals and the second the radioactive metals (those beyond 92 do not occur naturalfy). Nos 90 and 92 occur naturally and are used for atomic power. The rest of the elements in the table ate non-metals. Some have some metal-like properties and are called metalloids, e.g. nos 5, 14, 32, 33, 51, 52, 84 and 85. 

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