Selenides elemental halogens

Nitrogen and sodium do not react at any temperature under ordinary circumstances, but are reported to form the nitride or azide under the influence of an electric discharge (14,35). Sodium siHcide, NaSi, has been synthesized from the elements (36,37). When heated together, sodium and phosphoms form sodium phosphide, but in the presence of air with ignition sodium phosphate is formed. Sulfur, selenium, and tellurium form the sulfide, selenide, and teUuride, respectively. In vapor phase, sodium forms haHdes with all halogens (14). At room temperature, chlorine and bromine react rapidly with thin films of sodium (38), whereas fluorine and sodium ignite. Molten sodium ignites in chlorine and bums to sodium chloride (see Sodium COMPOUNDS, SODIUM HALIDES). 

Selenium resembles sulphur in its variations in valency, compounds being known in which the element is definitely bivalent, quadrivalent and sexavalent, respectively. The tendency to quadrivalency, however, is greater than in the case of sulphur, and the compounds formed in which this valency is exhibited are relatively more stable in the case of selenium than in the case of sulphur.5 Thus, whilst the bivalent selenium in organic selenides readily combines with a molecular proportion of chlorine or other halogen,5 sexavalent selenium in the selenates is easily reduced to the quadrivalent selenious condition. As with sulphur, it has been possible to demonstrate that in quadrivalent selenium the four valencies are disposed in three dimensions of space, since substances of the type... 

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