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

(a) K$_{2}$S (b) K$_{2}$Se (c) Rb$_{2}$SO$_{4}$ (d) RbNO$_{2}$ (e) MgSO$_{4}$
(a) Potassium (K) is an alkali metal with an oxidation number of +1 whereas sulfur (S) has an oxidation number of -2. For a neutral compound, we need two potassium 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, we merely change the ending to $−ide$. The molecular formula for the compound potassium sulfide is K$_{2}$S. (b) Potassium (K) is an alkali metal with an oxidation number of +1 whereas selenium (Se) is a part of group 16 and has an oxidation number of -2. For a neutral compound, we need two potassium atoms for every atom of selenium 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, we merely change the ending to $−ide$. The molecular formula for the compound potassium selenide is K$_{2}$Se. (c) Rubidium (Rb) is an alkali metal with an oxidation number of +1 whereas the sulfate ion (SO$_{4}^{2-}$) has an oxidation number of -2. For a neutral compound, we need two rubidium atoms for every sulfate ion to keep the charge neutral. For ionic compounds, we don't need a prefix for the first element, which is a metal. For the polyatomic ion, we keep the name as-is. The molecular formula for the compound rubidium sulfate is Rb$_{2}$SO$_{4}$. (d) Rubidium (Rb) is an alkali metal with an oxidation number of +1 whereas the nitrite ion (NO$_{2}^{-}$) has an oxidation number of -1. For a neutral compound, we need one rubidium atom for every nitrite ion to keep the charge neutral. For ionic compounds, we don't need a prefix for the first element, which is a metal. For the polyatomic ion, we keep the name as-is. The molecular formula for the compound rubidium nitrite is RbNO$_{2}$. (e) Magnesium (Mg) is an alkaline earth metal with an oxidation number of +2 whereas the sulfate ion (SO$_{4}^{2-}$) has an oxidation number of -2. For a neutral compound, we need one magnesium atom for every sulfate ion to keep the charge neutral. For ionic compounds, we don't need a prefix for the first element, which is a metal. For the polyatomic ion, we keep the name as-is. The molecular formula for the compound magnesium sulfate is MgSO$_{4}$.