Final answer:
Oxygen (O) is the most abundant element on Earth and is essential for most forms of life, making up roughly 21% of the Earth's atmosphere.
Step-by-step explanation:
The gaseous, non-metal element that is most abundant on Earth and makes up about 21% of the Earth's atmosphere is oxygen (O). Oxygen is essential for most forms of life, playing a critical role in various processes, such as aerobic respiration and combustion. It is also important for making molecules that are essential for life and, by mass, is the most abundant element in the earth's crust, composing about 50% when combined with other elements like silicon. In living organisms, oxygen combines with carbon, hydrogen, and nitrogen, forming a major component of their structure.