Problem 45 - Chemistry The Central Science

Question

In the following pairs of binary compounds, determine which one is a molecular substance and which one is an ionic substance. Use the appropriate naming convention (for ionic or molecular substances) to assign a name to each compound: (a) \(\mathrm{SiF}_{4}\) and \(\mathrm{LaF}_{3},(\mathbf{b}) \mathrm{FeCl}_{2}\) and \(\mathrm{ReCl}_{6},(\mathrm{c}) \mathrm{PbCl}_{4}\) and \(\mathrm{RbCl} .\)

Step-by-Step Solution

Answer

(a) \(\mathrm{SiF}_{4}\): molecular substance, silicon tetrafluoride \(\mathrm{LaF}_{3}\): ionic substance, lanthanum fluoride (b) \(\mathrm{FeCl}_{2}\): ionic substance, iron(II) chloride \(\mathrm{ReCl}_{6}\): ionic substance, rhenium(VI) chloride (c) \(\mathrm{PbCl}_{4}\): ionic substance, lead(IV) chloride \(\mathrm{RbCl}\): ionic substance, rubidium chloride

Step 1: Identify the Ionic or Molecular Compound of each pair#a# For each compound in the pair, determine whether it is ionic or molecular.

(a) \(\mathrm{SiF}_{4}\): non-metal + non-metal, therefore molecular \(\mathrm{LaF}_{3}\): metal + non-metal, therefore ionic (b) \(\mathrm{FeCl}_{2}\): metal + non-metal, therefore ionic \(\mathrm{ReCl}_{6}\): metal + non-metal, therefore ionic (c) \(\mathrm{PbCl}_{4}\): metal + non-metal, therefore ionic \(\mathrm{RbCl}\): metal + non-metal, therefore ionic

Step 2: Assign the Naming Convention#a# For each compound, apply the correct naming convention based on whether it is ionic or molecular.

(a) \(\mathrm{SiF}_{4}\): molecular, so we use prefixes Name: silicon tetrafluoride \(\mathrm{LaF}_{3}\): ionic, so we use the -ide suffix for the non-metal Name: lanthanum fluoride (b) \(\mathrm{FeCl}_{2}\): ionic, so we use the -ide suffix for the non-metal Name: iron(II) chloride (Roman numeral indicates the charge on the metal) \(\mathrm{ReCl}_{6}\): ionic, so we use the -ide suffix for the non-metal Name: rhenium(VI) chloride (Roman numeral indicates the charge on the metal) (c) \(\mathrm{PbCl}_{4}\): ionic, so we use the -ide suffix for the non-metal Name: lead(IV) chloride (Roman numeral indicates the charge on the metal) \(\mathrm{RbCl}\): ionic, so we use the -ide suffix for the non-metal Name: rubidium chloride Summary: (a) \(\mathrm{SiF}_{4}\): molecular substance, silicon tetrafluoride \(\mathrm{LaF}_{3}\): ionic substance, lanthanum fluoride (b) \(\mathrm{FeCl}_{2}\): ionic substance, iron(II) chloride \(\mathrm{ReCl}_{6}\): ionic substance, rhenium(VI) chloride (c) \(\mathrm{PbCl}_{4}\): ionic substance, lead(IV) chloride \(\mathrm{RbCl}\): ionic substance, rubidium chloride

Key Concepts

Ionic Compounds

Ionic compounds form when atoms transfer electrons from one to another, usually between metals and non-metals. This transfer creates ions, which are charged particles. Metals, which are positive ions (cations), transfer electrons to non-metals, which become negative ions (anions).
The positive and negative charges attract each other, creating a strong bond called an ionic bond. Common properties of ionic compounds include:

• High melting and boiling points due to the strong forces between ions.
• They conduct electricity when dissolved in water, as the ions are free to move.
• They are usually solid at room temperature.

To name ionic compounds, the metal's name comes first, followed by the non-metal, which ends with "-ide". If the metal can have different charges (like transition metals), you specify its charge using Roman numerals. For example, in the compound FeCl₂, iron is in the +2 state, so it is called iron(II) chloride.
This helps distinguish it from iron(III) chloride, where iron would have a +3 charge.

Molecular Compounds

Molecular compounds, also known as covalent compounds, consist of atoms sharing electrons rather than transferring them. These bonds form typically between non-metal atoms. The shared electrons between these atoms hold them together in units called molecules.
This type of bonding results in different properties compared to ionic compounds. Characteristics of molecular compounds include:

• Low melting and boiling points compared to ionic compounds, because the forces between the molecules are weaker.
• They do not conduct electricity because there are no free ions or electrons.
• Many exist as gases or liquids at room temperature.

Naming molecular compounds involves using prefixes to indicate the number of each type of atom present in the compound. For example, SiF₄ is called silicon tetrafluoride where "tetra-" indicates four fluorine atoms. Understanding the system of prefixes (mono-, di-, tri-, tetra-, etc.) is key to proper naming of molecular compounds.

Binary Compounds

Binary compounds are simply those composed of exactly two different elements. These compounds can be either ionic or molecular.
The method of naming them varies depending on the nature of the bonds between the elements. For ionic binary compounds, you name the metal first followed by the non-metal with an "-ide" suffix. If it's a transition metal, remember to add a Roman numeral indicating its charge. For example, CoCl₂ is called cobalt(II) chloride.
For molecular or covalent binary compounds, prefixes are used to specify the number of atoms. For example, in the molecular compound PCl₅, phosphorus is combined with five chlorine atoms, so the compound is named phosphorus pentachloride. In summary, recognizing whether a binary compound is ionic or molecular will guide you to the correct naming convention, aiding in understanding and communicating the compound’s composition effectively.