Which of the following correctly describes an ammonium ion?

The ammonium ion is indeed a positively charged polyatomic ion, which can be represented as NH₄⁺. Its positive charge arises from the fact that it has one more proton than it has electrons, resulting in an overall positive charge. The ammonium ion is formed when an ammonia molecule (NH₃) accepts a proton (H⁺).

This molecular structure allows ammonium to participate in various chemical reactions, especially in acid-base chemistry where it acts as a weak acid. It is commonly encountered in various ammonium salts, which are solid at room temperature.

When considering the other options, they do not accurately describe the properties of the ammonium ion. The ammonium ion does contain hydrogen atoms, as noted in its chemical formula, and it is not negatively charged. Additionally, while it can be found in gaseous states under certain conditions, it more commonly exists in aqueous solutions or solid salts rather than always as a gas.