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How do alkali metals react with sulfur and phosphorus?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

Alkali metals are highly electropositive elements that react readily with a variety of nonmetals, including those in Groups 15 and 16 of the periodic table.12

Reactions with Sulfur

Alkali metals react directly with elemental sulfur to form a variety of sulfides. These compounds are primarily ionic and are among the few metal sulfides that exhibit appreciable water solubility. When these water-soluble sulfides are heated in an aqueous solution with additional elemental sulfur, they form polysulfides, which consist of anions such as $S_{4}^{2 -}$ and $S_{3}^{2 -}$ .13

Reactions with Phosphorus

Phosphorus combines with alkali metals to form phosphides with the general formula $M_{3} P$ . The bonding in these alkali metal phosphides is very close to ionic bonding. A notable chemical property of these specific phosphides is that they react readily with water or dilute acid to produce phosphine ( $P H_{3}$ ).14