Question

Predict whether each of the following compounds is molecular or ionic: \((\mathbf{a}) \mathrm{BI}_{3}(\mathbf{b}) \mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}(\mathbf{c}) \mathrm{Zr}\left(\mathrm{NO}_{3}\right)_{2}(\mathbf{d}) \mathrm{N}_{2} \mathrm{H}_{4}(\mathbf{e})\) \(\mathrm{OsCO}_{3}(\mathbf{f}) \mathrm{H}_{2} \mathrm{SO}_{4}(\mathbf{g}) \mathrm{HgS}(\mathbf{h}) \mathrm{IOH} .\)

Short answer

(a) Molecular, (b) Molecular, (c) Ionic, (d) Molecular, (e) Ionic, (f) Molecular, (g) Ionic, (h) Molecular.

Step-by-step solution

Understand Compound Types

Compounds can be classified as molecular (covalent) or ionic. Molecular compounds consist of nonmetal atoms sharing electrons, while ionic compounds consist of metal atoms transferring electrons to nonmetals, forming ions.

Analyze Compound (a) - \( \mathrm{BI}_{3} \)

Boron (B) is a metalloid and iodine (I) is a non-metal. The electronegativity difference is insufficient to form an ionic bond, so \( \mathrm{BI}_{3} \) is molecular.

Analyze Compound (b) - \( \mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \)

Nitrogen (N) and carbon (C) are non-metals. Hence, \( \mathrm{N}\left(\mathrm{CH}_{3}\right)_{3} \) involves covalent bonding, making it molecular.

Analyze Compound (c) - \( \mathrm{Zr}\left(\mathrm{NO}_{3}\right)_{2} \)

Zirconium (Zr) is a metal and \( \mathrm{NO}_{3}^{-} \) is a polyatomic ion. This combination of metal and polyatomic ion indicates \( \mathrm{Zr}\left(\mathrm{NO}_{3}\right)_{2} \) is ionic.

Analyze Compound (d) - \( \mathrm{N}_{2}\mathrm{H}_{4} \)

Both nitrogen (N) and hydrogen (H) are non-metals, and the compound shares electrons, making \( \mathrm{N}_{2}\mathrm{H}_{4} \) molecular.

Analyze Compound (e) - \( \mathrm{OsCO}_{3} \)

Osmium (Os) is a metal and \( \mathrm{CO}_{3}^{2-} \) is a polyatomic ion. This metal-polyatomic ion combination means \( \mathrm{OsCO}_{3} \) is ionic.

Analyze Compound (f) - \( \mathrm{H}_{2}\mathrm{SO}_{4} \)

Sulfuric acid consists of hydrogen, a non-metal. The compound is covalent in its pure form, but forms ions in solution. Thus, it's primarily molecular but behaves ionically in solution.

Analyze Compound (g) - \( \mathrm{HgS} \)

Mercury (Hg) is a metal and sulfur (S) is a non-metal. The compound \( \mathrm{HgS} \) is therefore ionic.

Analyze Compound (h) - \( \mathrm{IOH} \)

Iodine (I) and hydroxyl (OH) both involve non-metals, hinting at covalency. \( \mathrm{IOH} \) is molecular.

Key concepts

Electronegativity

Electronegativity is key in determining the bond type within a compound. It measures how strongly an atom attracts electrons in a bond. If two atoms have a large difference in electronegativity (usually more than 1.7), an ionic bond forms as the electron transfer is more favorable.
The atom with higher electronegativity pulls the electrons more towards itself, forming ions. Conversely, a smaller electronegativity difference means the atoms are more likely to share electrons, leading to covalent bonding. Understanding electronegativity helps us predict whether a compound will be ionic or molecular.

Covalent Bonding

Covalent bonding occurs when two non-metal atoms share electrons to fill their valence shells. This sharing creates a strong bond, and the result is a discrete unit called a molecule.
Examples of molecular compounds include:

• Hydrazine (\(N_2H_4\)), where nitrogen and hydrogen, both non-metals, share electrons.
• Trimethylamine (\(N(CH_3)_3\)), where nitrogen and carbon atoms share electrons, forming this molecular compound.

Covalent bonds can be unpredictable because they can vary in strength and length based on sharing unevenness (polarity), but they are generally strong, contributing to the properties of molecular compounds.

Ionic Bonding

Ionic bonding takes place between metal and non-metal elements. In ionic compounds, metals donate electrons, and non-metals accept them, forming positive and negative ions.
These oppositely charged ions attract each other, forming a strong ionic bond, manifested in a lattice structure. Examples of ionic compounds include:

• Zirconium nitrate (\(Zr(NO_3)_2\)), where zirconium donates electrons to nitrate ions.
• Mercury sulfide (\(HgS\)), consisting of mercury donating electrons to sulfur.

Ionic bonds are typically strong, and ionic compounds have high melting and boiling points as a result.

Metalloids

Metalloids, or semimetals, display characteristics of both metals and non-metals. They're strategically positioned between metals and non-metals on the periodic table, enabling them to form various types of bonds.
Boron, a metalloid, forms covalent bonds in a compound like boron triiodide (\(BI_3\)) owing to its distinct properties of interacting with nonmetals like iodine. Understanding metalloids is crucial because their unique position allows for flexibility in the type of bonding, adapting either metallic or non-metallic characteristics.

Polyatomic Ions

Polyatomic ions are composed of multiple atoms that covalently bond to form a charged species. These ions are integral in forming ionic compounds when they interact with metals, given their stable detailed charge.
Examples include:

• The nitrate ion (\(NO_3^-\)), which combines with metals like zirconium in forming ionic compounds.
• The carbonate ion (\(CO_3^{2-}\)), bonding with metals like osmium to form compounds like osmium carbonate.

Polyatomic ions are fascinating because even though they involve covalent bonds, they participate in forming ionic compounds, underscoring the versatility and interplay in chemistry.