Question

Which of the following are ionic, and which are molecular? (a) \(\mathrm{PF}_{5}\), (b) \(\mathrm{NaI},(\mathrm{c}) \mathrm{SCl}_{2}\), (d) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2},(\mathrm{e}) \mathrm{FeCl}_{3}\) (f) \(\mathrm{LaP},(\mathrm{g}) \mathrm{CoCO}_{3}\) (h) \(\mathrm{N}_{2} \mathrm{O}_{4}\)

Short answer

(a) PF5 is a molecular compound. (b) NaI is an ionic compound. (c) SCl2 is a molecular compound. (d) Ca(NO3)2 is an ionic compound. (e) FeCl3 is an ionic compound. (f) LaP is an ionic compound. (g) CoCO3 is an ionic compound. (h) N2O4 is a molecular compound.

Step-by-step solution

(a) PF5:

This compound is composed of phosphorus (a non-metal) and fluorine (a non-metal). Since both elements are non-metals, PF5 is a molecular compound.

(b) NaI:

This compound is composed of sodium (a metal) and iodine (a non-metal). Since we have a metal and a non-metal, NaI is an ionic compound.

(c) SCl2:

This compound is composed of sulfur (a non-metal) and chlorine (a non-metal). Since both elements are non-metals, SCl2 is a molecular compound.

(d) Ca(NO3)2:

This compound is composed of calcium (a metal) and nitrate (NO3, a polyatomic ion). Ca(NO3)2 is an ionic compound because it is formed between a metal and a non-metal.

(e) FeCl3:

This compound is composed of iron (a metal) and chlorine (a non-metal). Since we have a metal and a non-metal, FeCl3 is an ionic compound.

(f) LaP:

This compound is composed of lanthanum (a metal) and phosphorus (a non-metal). Since we have a metal and a non-metal, LaP is an ionic compound.

(g) CoCO3:

This compound is composed of cobalt (a metal) and carbonate (CO3, a polyatomic ion). CoCO3 is an ionic compound because it is formed between a metal and a non-metal.

(h) N2O4:

This compound is composed of nitrogen (a non-metal) and oxygen (a non-metal). Since both elements are non-metals, N2O4 is a molecular compound.

Key concepts

Chemical Bonding

Understanding the types of chemical bonds that form between different elements is essential in the study of chemistry. There are primarily two types of chemical bonds: ionic and covalent. Ionic bonding occurs when there is a transfer of electrons from one atom to another, typically between a metal and a non-metal, resulting in the formation of ions. These ions attract each other due to opposite charges, creating an ionic compound.

On the other hand, covalent bonding happens when two atoms, usually non-metals, share electrons to achieve stability in their outer electron shells. This sharing leads to the formation of molecules, hence the term molecular compounds. The character of the bond—whether ionic or covalent—determines the physical properties and behaviors of the compound it forms. Identifying the types of bonds in compounds can be done by examining the elements involved and their position in the periodic table.

Identification of Compounds

The identification of whether a compound is ionic or molecular is critical for understanding its properties and reactions. To identify the type of compound, one must look at the elements involved. If the compound consists of both metals and non-metals, it is typically ionic. Metals tend to lose electrons and form positively charged ions, called cations, while non-metals tend to gain electrons and form negatively charged ions, called anions.

For instance, sodium iodide (NaI) is an ionic compound because sodium (Na) is a metal that loses an electron to iodine (I), a non-metal that gains the electron. However, if a compound is made up of only non-metals, like phosphorus pentafluoride (PF₅), it forms covalent bonds, making it a molecular compound. By identifying the nature of the elements in a compound, one can predict the type of bond and, consequently, the type of compound.

Metal and Non-metal Chemistry

The interaction between metals and non-metals underpins much of inorganic chemistry. Metals, found on the left side of the periodic table, are characterized by their ability to conduct electricity and heat, their shiny appearance, and their tendency to lose electrons during chemical reactions. Non-metals, on the other hand, are located on the right side of the periodic table and are poor conductors of heat and electricity, with a greater tendency to gain or share electrons.

When a metal and non-metal react, the metal usually donates electrons to the non-metal, resulting in the formation of an ionic compound, as seen in the cases of sodium iodide (NaI) and iron(III) chloride (FeCl₃). This electron transfer forms ions with opposite charges that attract each other, creating a strong ionic bond. Understanding the chemistry between metals and non-metals is vital for predicting the behavior of compounds during chemical reactions.

Polyatomic Ions

Polyatomic ions are charged entities consisting of two or more atoms bonded together, acting as a single ion. These ions cannot be simply classified as metals or non-metals; they are groups of atoms that collectively hold a charge. Common polyatomic ions include sulfate (SO₄^2-), nitrate (NO₃^-), and carbonate (CO₃^2-).

In the identification of compounds, it's important to recognize that the presence of polyatomic ions usually indicates the formation of an ionic compound, even if the polyatomic ion itself contains only non-metal elements. For example, calcium nitrate, Ca(NO₃)₂, is ionic because it consists of calcium ions (a metal) and nitrate ions bonded together. Similarly, cobalt(II) carbonate, CoCO₃, is ionic, comprising cobalt ions (a metal) and carbonate ions. Recognizing polyatomic ions in a chemical formula is crucial for properly classifying the compound.