The correct option is **C. Ar (Argon)**. ### Detailed Explanation: 1. **Understanding Rare Gases**: Rare gases, also known as noble gases, are a group of elements in Group 18 of the periodic table. They are characterized by their lack of reactivity due to having a full valence shell of electrons. The common noble gases include Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). 2. **Atmospheric Composition**: The Earth's atmosphere is primarily composed of nitrogen (about 78%) and oxygen (about 21%). The remaining 1% consists of various gases, including rare gases. 3. **Abundance of Rare Gases**: Among the noble gases, Argon is the most abundant in the atmosphere. Here’s a breakdown of their approximate concentrations in the atmosphere: - **Helium (He)**: About 0.0005% (very low concentration) - **Neon (Ne)**: About 0.0018% - **Argon (Ar)**: About 0.934% (the highest concentration among noble gases) - **Krypton (Kr)**: About 0.0001% - **Xenon (Xe)**: About 0.000009% As you can see, Argon is significantly more abundant than the other noble gases. 4. **Why Argon is the Correct Answer**: Given that Argon constitutes approximately 0.934% of the atmosphere, it is the most prevalent noble gas. This makes it the correct answer to the question regarding the most abundant rare gas in the atmosphere. ### Explanation of Other Options: - **A. He (Helium)**: Helium is the second lightest noble gas and is present in very small amounts in the atmosphere (about 0.0005%). Its low abundance makes it an incorrect choice for the most abundant rare gas. - **B. Ne (Neon)**: Neon is slightly more abundant than Helium, at about 0.0018%. However, it is still far less abundant than Argon, making it an incorrect option. - **D. Xe (Xenon)**: Xenon is one of the rarest noble gases in the atmosphere, with a concentration of about 0.000009%. This extremely low abundance makes it the least likely candidate for the most abundant rare gas. ### Common Pitfalls: - Students may confuse the abundance of noble gases with their reactivity or other properties. Remember, abundance in the atmosphere is purely about concentration, not chemical behavior. - It’s important to remember that while Helium and Neon are often discussed due to their uses in balloons and neon signs, they are not abundant in the atmosphere. ### Revision Summary: - The most abundant rare gas in the atmosphere is Argon (Ar), at approximately 0.934%. - Helium (He) and Neon (Ne) are present in very small amounts compared to Argon. - Xenon (Xe) and Krypton (Kr) are the least abundant noble gases in the atmosphere. - Understanding the composition of the atmosphere is crucial for identifying the most abundant gases.