Caesium monosulphide

In properties potassium monosulphide resembles the sodium salt. The anhydrous substance forms small cubic octahedra, isomorphous with the monosulphide of rubidium, but not with that of caesium.1 At the laboratory temperature its density is 1-805.2 The heat of formation of the solid from its elements is 87-1 Cal.,3 and the heat of solution is 22-7 Cal. In dilute solution it has a strongly alkaline reaction, owing to almost complete hydrolysis to the hydroxide and primary sulphide. 

The monosulphide and polysulphides are formed by burning the metals in sulphur vapour, by the action of sulphur on the metals dissolved in liquid ammonia, and by the action of the molten metals on sulphur dissolved in toluene. Hydrates or alcoholates and, in some cases, the anhydrous compounds may be prepared by dissolving sulphur in hot solutions of the hydrosulphides or monosulphides. Potassium, rubidium and caesium give all the sulphides where = 1, 2, 3, 4, 5, or 6 sodium only up to the pentasulphide, and lithium only those for which x = 1, 2 and 4 (Pearson and Robinson, 1931). All the metals form two polysulphides of relatively outstanding stability one is invariably the disulphide, and the other tetrasulphide in the case of lithium or sodium, and pentasulphide in the case of potassium, rubidium or caesium. The amount of water of crystallisation and the solubility decrease with increase in atomic number of the metal, the gradation being most marked between sodium and potassium. 

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