Understanding the Composition of Earth's Atmosphere

The Earth's atmosphere is a vital layer of gases surrounding our planet. It is a mixture of many different gases, dust particles, and water vapor. The composition of the atmosphere is not uniform throughout, but the main gases are present in relatively consistent proportions near the Earth's surface.

The question asks about the percentage of oxygen present in the atmosphere. To answer this, we need to know the primary gases that make up the air we breathe and their approximate proportions.

Major Gases in the Atmosphere

The two most abundant gases in the Earth's dry atmosphere are nitrogen and oxygen. Together, they make up about 99% of the total volume of air. Other gases, like argon, carbon dioxide, and trace gases, make up the remaining 1%.

Here is a breakdown of the approximate percentages of the main gases in dry air:

From the table, we can clearly see that oxygen makes up approximately 21% of the Earth's atmosphere by volume.

Analyzing the Options

Let's look at the given options:

• 39%
• 79%
• 10%
• 21%

Comparing these options with the known composition of the atmosphere, the percentage of oxygen is approximately 21%.

Option 1 (39%) is incorrect.

Option 2 (79%) is close to the percentage of nitrogen, not oxygen, so it is incorrect.

Option 3 (10%) is significantly lower than the actual percentage of oxygen, so it is incorrect.

Option 4 (21%) matches the approximate percentage of oxygen in the atmosphere.

Therefore, the correct percentage of oxygen present in the atmosphere is 21%.

Revision Table: Atmospheric Gases

Additional Information: Why Oxygen Matters

Oxygen is a crucial component of the atmosphere for most life on Earth. It is essential for:

• Respiration: Most living organisms, including humans and animals, use oxygen to convert food into energy. This process is called aerobic respiration.
• Combustion: Oxygen is required for burning fuels. This is why fire cannot exist without a sufficient supply of oxygen.
• Ozone Layer: Oxygen molecules (O₂) can be converted into ozone (O₃) in the upper atmosphere. The ozone layer absorbs harmful ultraviolet (UV) radiation from the sun, protecting life on the surface.

While nitrogen is the most abundant gas, oxygen is indispensable for many biological and chemical processes that are fundamental to life as we know it on Earth.