Final answer:
The formula NH4Cl is the only one among the options that represents an ionic compound, due to the combination of the ammonium ion (NH4+) and the chloride ion (Cl-).
Step-by-step explanation:
The formula that represents an ionic compound among the given options is NH4Cl. This is because NH4+ is an ammonium ion, which is a polyatomic ion, and Cl- is a chloride ion. When these two ions combine, they form an ionic compound due to the electrostatic attraction between the positively charged ammonium ion and the negatively charged chloride ion.
None of the other options represent an ionic compound: H2 is a diatomic molecule, CH4 (methane) is a covalent compound, and CH3OH (methanol) is also a covalent compound.
Ionic compounds are typically formed between metals and nonmetals, or can involve polyatomic ions, such as ammonium. They are described by empirical formulas that indicate the simplest ratio of ions in the compound, reflecting its crystal lattice structure where cations and anions are arranged in a repeating pattern.