8.45 In the following pairs of binary compounds determine which one is a molecular substance and which one is an ionicsubstance. Use the appropriate naming convention (for ionic or molecular substances) to assign a name to each compound: (a) SiF4 and LaF3, (b) FeCl2 and ReCl6, (c) PbCl4 and RbCl.
The Correct Answer and Explanation is :
(a) SiF4 and LaF3
SiF4 (Silicon tetrafluoride): This compound is a molecular substance. It consists of silicon (Si) and fluorine (F), two nonmetals. In molecular compounds, the atoms are bound together by covalent bonds, formed by the sharing of electrons. Since both elements involved are nonmetals, SiF4 follows the rules for molecular compounds. The name “silicon tetrafluoride” reflects the fact that silicon is bonded to four fluorine atoms.
LaF3 (Lanthanum trifluoride): This compound is an ionic substance. It consists of lanthanum (La), a metal, and fluorine (F), a nonmetal. When a metal combines with a nonmetal, the result is typically an ionic bond, where the metal donates electrons to the nonmetal. Lanthanum, being a metal, forms a cation (La^3+), while fluorine forms an anion (F^-). The name “lanthanum trifluoride” indicates that three fluoride ions are paired with one lanthanum ion.
(b) FeCl2 and ReCl6
FeCl2 (Iron(II) chloride): This compound is an ionic substance. It consists of iron (Fe), a metal, and chlorine (Cl), a nonmetal. Since metals and nonmetals form ionic bonds, FeCl2 is ionic. Iron in FeCl2 has a +2 charge, which balances the -1 charge of each chlorine ion. Therefore, the compound is called “iron(II) chloride,” where the Roman numeral indicates the charge on the iron ion.
ReCl6 (Rhenium(VI) chloride): This compound is also an ionic substance. Rhenium (Re) is a metal, and chlorine (Cl) is a nonmetal. Since it is an ionic bond between a metal and a nonmetal, it follows the ionic naming convention. Rhenium in ReCl6 has a +6 charge, which balances the six -1 charges from the chloride ions. The compound is named “rhenium(VI) chloride.”
(c) PbCl4 and RbCl
PbCl4 (Lead(IV) chloride): This is an ionic substance. Lead (Pb) is a metal, and chlorine (Cl) is a nonmetal. Lead forms a +4 cation, while chlorine forms -1 anions. The compound is ionic because of the electrostatic attraction between the metal cation and nonmetal anions. The name “lead(IV) chloride” reflects the +4 charge on the lead ion.
RbCl (Rubidium chloride): This is also an ionic substance. Rubidium (Rb) is an alkali metal, and chlorine (Cl) is a nonmetal. Rubidium forms a +1 cation, and chlorine forms a -1 anion. The electrostatic attraction between the oppositely charged ions results in an ionic bond. The name “rubidium chloride” reflects the single charge on the rubidium ion.
Explanation:
The key distinction between molecular and ionic compounds lies in the nature of the bonding. Molecular compounds are formed between nonmetals and involve the sharing of electrons (covalent bonds). Ionic compounds, on the other hand, are formed between metals and nonmetals and involve the transfer of electrons from the metal to the nonmetal, resulting in the formation of ions.
- Molecular substances (like SiF4) have covalent bonds and are usually composed of nonmetals.
- Ionic substances (like LaF3, FeCl2, PbCl4, RbCl) have ionic bonds formed between metals and nonmetals.
This distinction determines their chemical properties, including their melting points, conductivity, and solubility.