Question

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

Short answer

The compounds can be classified as follows: - Molecular: \(\mathrm{PF}_{5}\), \(\mathrm{SCl}_{2}\), and \(\mathrm{N}_{2}\mathrm{O}_{4}\). - Ionic: \(\mathrm{NaI}\), \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\), \(\mathrm{FeCl}_{3}\), \(\mathrm{LaP}\), and \(\mathrm{CoCO}_{3}\).

Step-by-step solution

(a) \(\mathrm{PF}_{5}\)

Phosphorus (P) and fluorine (F) are both nonmetals, so they will form a molecular compound through covalent bonding. Therefore, \(\mathrm{PF}_{5}\) is a molecular compound.

(b) \(\mathrm{NaI}\)

Sodium (Na) is a metal and iodine (I) is a nonmetal, so they will form an ionic compound. Sodium will lose an electron to become a cation, while iodine will gain an electron to become an anion. Therefore, \(\mathrm{NaI}\) is an ionic compound.

(c) \(\mathrm{SCl}_{2}\)

Sulfur (S) and chlorine (Cl) are both nonmetals, so they will form a molecular compound through covalent bonding. Therefore, \(\mathrm{SCl}_{2}\) is a molecular compound.

(d) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)

Calcium (Ca) is a metal and nitrate \(\left(\mathrm{NO}_{3}^{-}\right)\) is a polyatomic anion, which is a group of nonmetals covalently bonded with a net negative charge. Therefore, calcium nitrate is an ionic compound.

(e) \(\mathrm{FeCl}_{3}\)

Iron (Fe) is a metal and chlorine (Cl) is a nonmetal, so they will form an ionic compound. Iron can have multiple oxidation states, but in this case, it forms an ionic compound with a charge of +3. Therefore, \(\mathrm{FeCl}_{3}\) is an ionic compound.

(f) \(\mathrm{LaP}\)

Lanthanum (La) is a metal and phosphorus (P) is a nonmetal, so they will form an ionic compound. Lanthanum will lose electrons to become a cation, while phosphorus will gain electrons to become an anion. Therefore, \(\mathrm{LaP}\) is an ionic compound.

(g) \(\mathrm{CoCO}_{3}\)

Cobalt (Co) is a metal and carbonate \(\left(\mathrm{CO}_{3}^{2-}\right)\) is a polyatomic anion, which is a group of nonmetals covalently bonded with a net negative charge. Therefore, cobalt carbonate is an ionic compound.

(h) \(\mathrm{N}_{2}\mathrm{O}_{4}\)

Nitrogen (N) and oxygen (O) are both nonmetals, so they will form a molecular compound through covalent bonding. Therefore, \(\mathrm{N}_{2}\mathrm{O}_{4}\) is a molecular compound.

Key concepts

Ionic Bonding

Ionic bonding is a type of chemical bond formed through the electrostatic attraction between oppositely charged ions. When a metal, like sodium (Na), meets a nonmetal, such as iodine (I), it forms an ionic compound.
In this process, the metal loses electrons to become a positively charged cation, while the nonmetal gains electrons to become a negatively charged anion. For example, in NaI, sodium donates an electron to iodine, resulting in the formation of Na⁺ and I⁻ ions.
Ionic compounds generally have high melting and boiling points due to the strong interactions between the ions in a lattice structure.
They often dissolve in water and conduct electricity when in molten or dissolved form.

Covalent Bonding

Covalent bonding occurs when two nonmetals share pairs of electrons to achieve full outer electron shells.
In the example of \(\mathrm{PF}_5\), phosphorus (P) and fluorine (F) are both nonmetals and engage in covalent bonding.
This allows them to form a stable molecular compound by sharing electrons rather than transferring them completely. \(\mathrm{SCl}_2\) and \(\mathrm{N}_2\mathrm{O}_4\) are other examples of molecular compounds created through covalent bonds.
Covalent compounds generally have lower melting and boiling points compared to ionic compounds. They are often gases or liquids at room temperature.

Metal and Nonmetal Interactions

Understanding how metals and nonmetals interact helps in identifying whether compounds are ionic or molecular.
Typically, when a metal reacts with a nonmetal, the tendency is to form an ionic compound due to electron transfer. Metals like calcium (Ca) and lanthanum (La) lose electrons, while nonmetals like chlorine (Cl) or phosphorus (P) gain electrons to form ionic bonds.
This results in compounds like \(\mathrm{Ca}(\mathrm{NO}_3)_2\) and \(\mathrm{LaP}\), where electrostatic forces hold the compounds together.
Examination of the elements involved provides insight into the bonding nature and helps predict compound types.

Polyatomic Ions

Polyatomic ions are charged entities composed of two or more atoms covalently bonded together. They act as a single unit with a net charge, playing a significant role in ionic compounds.
For example, nitrate (\(\mathrm{NO}_3^{-}\)) and carbonate (\(\mathrm{CO}_3^{2-}\)) are common polyatomic ions.
These ions can combine with metals like calcium in \(\mathrm{Ca}(\mathrm{NO}_3)_2\) or cobalt as in \(\mathrm{CoCO}_3\), resulting in ionic compounds. These compounds exhibit properties typical of ionic substances, such as high melting points and electrical conductivity in aqueous or molten states.
Recognizing polyatomic ions is essential for understanding the composition and behavior of various compounds.