## Chemistry: The Science in Context (4th Edition)

(a) sodium sulfide (Na$_{2}$S) (b) strontium chloride (SrCl$_{2}$) (c) aluminum oxide (Al$_{2}$O$_{3}$) (d) lithium hydride (LiH)
(a) Sodium is an alkali metal with an oxidation number of +1 whereas sulfur has an oxidation number of -2. For a neutral compound, we need two sodium atoms for every atom of sulfur to keep the charge neutral. For ionic compounds, we don't need a prefix for the first element, which is a metal. For the second element (or polyatomic ion), we merely change the ending to $-ide$. The compound between sodium and sulfur is sodium sulfide (Na$_{2}$S). (b) Strontium is an alkaline earth metal with an oxidation number of +2 whereas chlorine is a halogen with an oxidation number of -1. For a neutral compound, we need two chlorine atoms for every atom of strontium to keep the charge neutral. For ionic compounds, we don't need a prefix for the first element, which is a metal. For the second element or polyatomic ion, which would be a nonmetal, we merely change the ending to $-ide$. The compound between strontium and chlorine is strontium chloride (SrCl$_{2}$). (c) Aluminum is a metal with a common oxidation number of +3 whereas oxygen, which is in group 16, has an oxidation number of -2. For a neutral compound, we need two aluminum atoms for every three atoms of oxygen to keep the charge neutral. For ionic compounds, we don't need a prefix for the first element, which is a metal. For the second element or polyatomic ion, which would be a nonmetal, we merely change the ending to $-ide$. The compound between aluminum and oxygen is aluminum oxide (Al$_{2}$O$_{3}$). (d) Lithium is an alkali metal with an oxidation number of +1 whereas hydrogen, with its single electron, can either act as an electron acceptor or an electron donor. In this case, hydrogen will accept an electron from lithium and takes on a charge of -1. For a neutral compound, we need one lithium atom for every atom of hydrogen to keep the charge neutral. For ionic compounds, we don't need a prefix for the first element, which is a metal. For the second element or polyatomic ion, which would be a nonmetal, we merely change the ending to $-ide$. The compound between lithium and hydrogen is lithium hydride (LiH).