Chapter 2 - Atoms, Molecules, and Ions - Questions and Exercises - Exercises - Page 88: 2.126

(a) Ca$_3$(PO$_4$)$_2$; ionic compound (b) GeO$_2$; molecular compound (c) Fe$_2$(SO$_4$)$_3$; ionic compound (d) PBr$_3$; molecular compound

(a) Ca$_3$(PO$_4$)$_2$; ionic compound Calcium (Ca) is an alkaline earth metal belonging to group 2 of the periodic table and has a valence of 2+. Phosphate (PO$_4$$^{3-}$), according to Table 2.2 on page 65 of our book, gives the valence of phosphate as 3-. We thus need three atoms of calcium for every two phosphate ions to balance the charges in this compound. This compound is ionic because it involves a metal bonded to a nonmetal; in these types of compounds, the electrons are transferred from one atom to another and are not shared. (b) GeO$_2$; molecular compound Germanium (Ge) is a metalloid belonging to group 14 on the periodic table. Germanium has a valence of 4+. Oxygen (O) is in group 16 and has a valence of 2-. In this compound, germanium and oxygen are present in a 1:2 ratio. This compound is molecular because a compound made of two nonmetals or a metalloid and a nonmetal are usually molecular because the electrons are shared between the two atoms. (c) Fe$_2$(SO$_4$)$_3$; ionic compound Iron is a transition metal and has a valence of 3+, according to the Roman numeral III after its name. Sulfate (SO$_4$$^{2-}$), according to Table 2.2 on page 65 of our book, gives the valence of sulfate as 2-. We will need two iron atoms for every three sulfate ions to balance the charges in this compound. This compound is ionic because it involves a metal bonded to a nonmetal; in these types of compounds, the electrons are transferred from one atom to another and are not shared. (d) PBr$_3$; molecular compound Phosphorus (P) is a nonmetal belonging to group 15 on the periodic table. Phosphorus has a valence of 3+. Bromine (Br) is a halogen in group 17 and has a valence of 1-. In this compound, phosphorus and bromine are present in a 1:3 ratio. This compound is molecular because a compound made of two nonmetals are usually molecular because the electrons are shared between the two atoms.