SAMPLE EXERCISE 2.9 Identifying Ionic and Molecular Compounds
Which of the following compounds would you expect to be ionic: N 2O, Na2O, CaCl2, SF4?
Solution We would predict that Na2O and CaCl2 are ionic compounds because they are composed of a metal
combined with a nonmetal. The other two compounds, composed entirely of nonmetals, are predicted (correctly)
to be molecular compounds.
PRACTICE EXERCISE
Which of the following compounds are molecular: CBr4, FeS, P4 O6, PbF2 ?
Answer: CBr4 and P4 O6
, SAMPLE EXERCISE 2.10 Using Ionic Charge to Write Empirical Formulas for
Ionic Compounds
What are the empirical formulas of the compounds formed by (a) Al3+ and Cl– ions, (b) Al3+ and O2 – ions, (c)
Mg2+ and NO3– ions?
Solution (a) Three Cl– ions are required to balance the charge of one Al3+ ion. Thus, the formula is AlCl3.
(b) Two Al3+ ions are required to balance the charge of three O2 – ions (that is, the total positive charge is 6+
and the total negative charge is 6–). Thus, the formula is Al2O3.
(c) Two NO3– ions are needed to balance the charge of one Mg3+. Thus, the formula is Mg(NO3)2. In this
case the formula for the entire polyatomic ion NO3– must be enclosed in parentheses so that it is clear that the
subscript 2 applies to all the atoms of that ion.
PRACTICE EXERCISE
Write the empirical formulas for the compounds formed by the following ions: (a) Na+ and PO43– , (b) Zn2+ and
SO42– , (c) Fe3+ and CO32–.
Answers: (a) Na3PO4, (b) ZnSO4, (c) Fe2(CO3)3
Which of the following compounds would you expect to be ionic: N 2O, Na2O, CaCl2, SF4?
Solution We would predict that Na2O and CaCl2 are ionic compounds because they are composed of a metal
combined with a nonmetal. The other two compounds, composed entirely of nonmetals, are predicted (correctly)
to be molecular compounds.
PRACTICE EXERCISE
Which of the following compounds are molecular: CBr4, FeS, P4 O6, PbF2 ?
Answer: CBr4 and P4 O6
, SAMPLE EXERCISE 2.10 Using Ionic Charge to Write Empirical Formulas for
Ionic Compounds
What are the empirical formulas of the compounds formed by (a) Al3+ and Cl– ions, (b) Al3+ and O2 – ions, (c)
Mg2+ and NO3– ions?
Solution (a) Three Cl– ions are required to balance the charge of one Al3+ ion. Thus, the formula is AlCl3.
(b) Two Al3+ ions are required to balance the charge of three O2 – ions (that is, the total positive charge is 6+
and the total negative charge is 6–). Thus, the formula is Al2O3.
(c) Two NO3– ions are needed to balance the charge of one Mg3+. Thus, the formula is Mg(NO3)2. In this
case the formula for the entire polyatomic ion NO3– must be enclosed in parentheses so that it is clear that the
subscript 2 applies to all the atoms of that ion.
PRACTICE EXERCISE
Write the empirical formulas for the compounds formed by the following ions: (a) Na+ and PO43– , (b) Zn2+ and
SO42– , (c) Fe3+ and CO32–.
Answers: (a) Na3PO4, (b) ZnSO4, (c) Fe2(CO3)3
