Final answer:
The NH₄ ion, also known as the ammonium ion, is a monatomic ion because it consists of a single ammonium (NH₄) molecule with a positive charge.
Step-by-step explanation:
The correct answer is 2) NH₄.
Monatomic ions are ions that consist of a single atom with a positive or negative charge. The NH₄ ion, also known as the ammonium ion, is a monatomic ion because it consists of a single ammonium (NH₄) molecule with a positive charge.
The other compounds listed are not monatomic ions. CN- is a polyatomic ion, CI- is not a correct ion symbol, and SO₃²⁻ is a polyatomic ion.