Final answer:
The least abundant gas in atmospheric air, among the options provided, is CO₂ (carbon dioxide), which is present at approximately 0.04% of the atmosphere.
Step-by-step explanation:
Of the following choices, the least abundant gas in atmospheric air is CO₂(carbon dioxide). The least abundant gas in atmospheric air is carbon dioxide (CO₂). Although it makes up only a small percentage of the atmosphere (~0.04%), it is still present. The major components of air include nitrogen (N₂) and oxygen (O₂), which make up approximately 78.6% and 20.9% of the atmosphere respectively. Water vapor (H₂O) varies with humidity and is also present in higher amounts compared to CO₂. Carbon dioxide, while essential for life processes like photosynthesis, is only present in the atmosphere at about 0.04% concentration, making it the least abundant of the gases listed in the question.