Answer
(a) K$_2$S
(b) K$_2$Se
(c) Rb$_2$SO$_4$
(d) RbNO$_2$
(e) MgSO$_4$
Work Step by Step
(a) K$_2$S
This compound is a simple binary ionic compound. Potassium is the cation with a valence of 1+, and it's elemental symbol is "K." Sulfur is the anion with a valence of 2-, and it's elemental symbol is "S." We will need two potassium atoms for every one sulfur atom to balance the charges in this compound.
(b) K$_2$Se
This compound is a simple binary ionic compound. Potassium is the cation with a valence of 1+, and it's elemental symbol is "K." Selenium is the anion with a valence of 2-, and it's elemental symbol is "Se." We will need two potassium atoms for every one selenium atom to balance the charges in this compound.
(c) Rb$_2$SO$_4$
This compound is a compound containing an oxoanion. Rubidium is the cation with a valence of 1+, and it's elemental symbol is "Rb." Sulfate is the oxoanion with a valence of 2-, and it's symbol is "SO$_4$." We will need two rubidium atoms for every one sulfate oxoanion to balance the charges in this compound.
(d) RbNO$_2$
This compound is a compound containing an oxoanion. Rubidium is the cation with a valence of 1+, and it's elemental symbol is "Rb." Nitrite is the oxoanion with a valence of 1-, and it's symbol is "NO$_2$." The cation and anion combine in a 1:1 ratio. We will need one rubidium atom for every one nitrite oxoanion to balance the charges in this compound.
(e) MgSO$_4$
This compound is a compound containing an oxoanion. Magnesium is the cation with a valence of 1+, and it's elemental symbol is "Mg." Sulfate is the oxoanion with a valence of 2-, and it's symbol is "SO$_4$." The cation and anion combine in a 1:1 ratio. We will need one magnesium atom for every one sulfate oxoanion to balance the charges in this compound.
