Which of the following chemical formulas represents a molecular (covalent) compound rather than a monatomic gas or an ionic compound?

Introduction / Context:
Chemical compounds can be broadly classified as molecular compounds, which are made of discrete molecules with covalent bonds, and ionic compounds, which consist of ions arranged in extended lattices. Some substances are also monatomic, like noble gases, and are not compounds at all. This question asks which formula represents a molecular compound, so you must distinguish between a covalent molecular substance, an ionic salt and a single atom element.

Given Data / Assumptions:

• Ar represents argon, a noble gas element.
• NaI, NaCl and MgO are salts formed between metals and non metals.
• N2O4 represents a compound composed only of non metals nitrogen and oxygen.
• Molecular compounds usually involve only non metals sharing electrons in covalent bonds.

Concept / Approach:
Ionic compounds form when metals transfer electrons to non metals, producing cations and anions that arrange into a crystal lattice rather than discrete molecules. Examples include sodium iodide and sodium chloride. Molecular compounds are typically formed between non metals and consist of distinct molecules with fixed numbers of atoms held together by covalent bonds. Dinitrogen tetroxide, N2O4, is an example of such a molecular compound. Monatomic gases such as argon consist of single atoms and, although they exist as molecules in the very loose sense, they are treated as elements, not compounds.

Step-by-Step Solution:
Step 1: Consider Ar. This symbol stands for argon, a noble gas that exists as isolated atoms. It is an element, not a compound, so it cannot be a molecular compound. Step 2: Consider NaI. Sodium is a metal and iodine is a halogen non metal; NaI is an ionic compound formed by transfer of electrons, not a covalent molecular compound. Step 3: Consider N2O4. This formula shows two nitrogen atoms and four oxygen atoms. Both nitrogen and oxygen are non metals and form covalent bonds with one another in discrete molecules of dinitrogen tetroxide. This is a classical molecular compound. Step 4: Consider NaCl and MgO. In both cases, a metal (sodium or magnesium) combines with a non metal (chlorine or oxygen) to form ionic salts with extended lattices, not discrete covalent molecules. Step 5: Therefore, N2O4 is the only formula that clearly represents a molecular compound.

Verification / Alternative check:
You can cross check by thinking about physical properties. Ionic compounds such as NaCl and NaI have high melting points and conduct electricity when molten or dissolved, indicating the presence of mobile ions. Molecular compounds like N2O4 often have lower melting and boiling points and may exist as gases or liquids at room temperature. Dinitrogen tetroxide, for example, is a gas or liquid depending on conditions and consists of individual N2O4 molecules. Argon is a noble gas element, not a compound, and does not fit the category of molecular compound by definition.

Why Other Options Are Wrong:
- Ar: Represents a single atom of argon, a noble gas element, not a compound formed by bonding two or more different elements.
- NaI: An ionic compound composed of Na+ and I- ions arranged in a lattice, not discrete covalent molecules.
- NaCl: Another ionic compound made of Na+ and Cl- ions, again arranged in an extended crystal structure.
- MgO: Magnesium oxide is an ionic compound made of Mg2+ and O2- ions, not a molecular covalent substance.

Common Pitfalls:
Students sometimes treat any formula as a molecular formula and forget the distinction between ionic and covalent bonding. Another common confusion is to assume that if a formula has two different symbols it must be molecular, which is not true for salts. A useful rule is that compounds containing a metal and a non metal are usually ionic, while those containing only non metals are often molecular. Applying this rule here quickly points to N2O4 as the molecular compound among the options.

Final Answer:
The formula that represents a molecular compound is N2O4.