Final answer:
The gaseous non-metal that constitutes 78% of the Earth's atmosphere and is a key component of proteins and fertilizers is Nitrogen (N).
Step-by-step explanation:
The gaseous non-metal that makes up 78% of the Earth's atmosphere and whose compounds are important components of proteins, fertilizers, and many explosives is Nitrogen (N). Atmospheric nitrogen exists mostly in the form of nitrogen gas (N₂). It plays a crucial role in diluting oxygen in the atmosphere, which prevents rapid combustion. Nitrogen is also essential for living organisms, as it is a building block of proteins and nucleic acids like DNA and RNA. However, most living things cannot directly use atmospheric nitrogen in its gaseous form; they require it to be 'fixed' into a more accessible form, such as ammonia, by natural processes or industrial methods, which is then used to produce fertilizers and other chemicals.