Q.Oxygen gas is in negligible quantity at the height of atmosphere:

Think of the air you’re breathing right now. You can’t see it, taste it, or usually feel it, yet it’s pressing on you from every side. That invisible stuff is not one single substance — it’s a mixture, a blend of different gases, each present in its own proportion. That mixture, and the exact proportions of its parts, is what we call the atmospheric composition.

The word “composition” just means “what something is made of and in what amounts.” So when we talk about the atmosphere’s composition, we’re asking a simple question: if you took a big sample of air, what gases would you find inside it, and how much of each? The answer is surprisingly consistent near the Earth’s surface, which is why we can speak of it as a single thing rather than a chaotic jumble.

Here’s the everyday intuition to hold onto. Imagine a fruit punch made by mixing several juices. The punch’s taste, colour, and thickness depend entirely on which juices you poured in and how much of each. Change the recipe — add more lemon, less sugar — and the punch behaves differently. The atmosphere works the same way. Its “recipe” determines everything from the weather to whether life can survive.

Now for the precise meaning. The atmosphere is a layer of gases held around the Earth by gravity. Its composition is usually described by volume — that is, out of every hundred parts of dry air, how many parts are each gas. Two gases dominate overwhelmingly: nitrogen and oxygen. Together they make up roughly 99% of the air you breathe. Nitrogen is the quiet, inert bulk — it doesn’t react easily with other things, so it acts as a stable filler. Oxygen is the active one; it’s what fuels fire and what your body uses to release energy from food.

The remaining 1% is a collection of trace gases, but don’t let the small number fool you. These tiny players have outsized roles:

• Carbon dioxide — present in very small amounts, yet it’s the main gas driving the greenhouse effect and global warming.
• Argon — a noble gas, chemically lazy, just sitting there.
• Water vapour — variable, not constant like the others; it’s the source of clouds, rain, and humidity.
• Ozone — found mostly in the upper atmosphere, where it shields us from harmful ultraviolet radiation.
• Dust particles, pollen, and other aerosols — not gases, but suspended solids that affect cloud formation and air quality.