Which chemical element exists as a colorless, odorless, and tasteless gas at normal atmospheric temperatures and pressures, comprising about 21% of the earth's atmosphere by volume?

The correct answer is oxygen, which is a chemical element that exists as a colorless, odorless, and tasteless gas at normal atmospheric temperatures and pressures. Oxygen is essential for life on Earth and plays a critical role in various biological processes, particularly respiration in animals and photosynthesis in plants.

In the atmosphere, oxygen is found in a diatomic form (O2) and constitutes about 21% of the total volume of air. This significant proportion is crucial for the sustenance of aerobic organisms, which rely on oxygen for energy production.

It is also important to note that ozone, while made of oxygen, is a triatomic molecule (O3) and exists in the atmosphere primarily in the stratosphere, where it absorbs harmful ultraviolet radiation, rather than being a primary component of the air we breathe. Nitrogen, which is another major component of the atmosphere, makes up approximately 78% of air, but it does not support life in the same way oxygen does. Hydrogen, although it is the most abundant element in the universe, exists in very small quantities in the Earth's atmosphere and is not necessary for respiration. Hence, oxygen stands out as the most relevant answer in this context.