Read the question, and then use the drop-down menus to complete the paragraph. What is Earth’s atmosphere made of? The most abundant gas in Earth’s atmosphere is , followed by , which is needed to burn wood and fuel. The third-most abundant gas is . The amount of in the atmosphere is very small, making up just 0.033% of the atmosphere, but it is very important because plants use it to make simple sugars. Another important gas is water vapor, which condenses around in the atmosphere to form clouds and then rain.