Which sequence correctly lists the chemical symbols for nitrite, nitrate, unionized ammonia, and ionized ammonia?

• A. NO3-, NH3, NH4+, NO2-
• B. NO2-, NO3, NH3, NH4+
• C. NO3-, NH3, NH4+, CO2
• D. NH3, NH4+, NH2, NH5

Answer: (B)

Recognizing the distinct chemical forms and their charges for nitrogen species and ammonia is what this question tests. Nitrite is the two-oxygen ion NO2-; nitrate is the three-oxygen ion NO3- (both carry a -1 charge). Unionized ammonia is NH3, a neutral molecule. Ionized ammonia is NH4+, the ammonium ion formed when NH3 gains a proton and becomes positively charged. The requested sequence—nitrite, nitrate, unionized ammonia, ionized ammonia—maps to NO2-, NO3-, NH3, NH4+. While nitrate is usually written NO3- in formal notation, the symbol NO3 is commonly understood in many contexts, and this order correctly reflects the progression of these species.