The Earth’s Atmosphere

The Earth is surrounded by atmosphere, which is a relatively thin collection of gases that surrounds the Earth. These gases are essential to life on earth. These are the layers off the atmosphere, in order from closest to the Earth’s surface to furthest from the Earth’s surface.


The area of the atmosphere closest to the Earth is called the troposphere, which is where most of the weather activity happens. The troposphere is the most dense part of the atmosphere.


Above the troposphere is the stratosphere, which is contains the ozone layer and also where aircrafts travel. The ozone layer plays a key role in maintaining and protecting life on Earth because it absorbs harmful uv radiation from the sun.


The layer above the stratosphere is the mesosphere. In this area of the atmosphere, meteors usually burn out. However, meteors that pass through the mesosphere and eventually through the troposphere are called meteorites (if it is actually able to hit the Earth’s surface).


Above the mesosphere is the thermosphere. Which is the area where many of our satellites are.


The layer above the thermosphere is the exosphere and is the outermost layer. The exosphere is the least dense part of the atmosphere.

Note the order

How high Part of the Atmosphere
7 km Troposphere
50 km Stratosphere
80-85 km Mesosphere
640 Thermosphere

The air is most dense in the troposphere as the air here is made of a mixture of gases. As you go further up the atmosphere, away from earth, the air gets less dense.

Composition of the Atmosphere

In the atmosphere the three main gases are nitrogen, oxygen and argon. Below is a table showing the percentage of the gases present in the atmosphere. 

Gas Abundance
Nitrogen (N2) 78%
Oxygen (O2) 21%
Carbon dioxide 0..04%
Other gases (e.g. argon) <1%

Nitrogen and oxygen, which exist as molecules (groups in sets of 2 atoms), make up 99% of the atmosphere. The other gases are found in smaller amounts.

Calculating the Percentage of Oxygen in the Air

This is how the percentage of oxygen has changed overtime:

We can calculate the percentage of oxygen in the air by reacting it with copper.

  1. Place pieces of copper into a test tube, on top of a Bunsen burner
  2. Heat the pieces of copper strongly with the Bunsen burner
  3. Syringes are used to pass air over the hot copper

The oxygen in the air reacts with the copper to form copper oxide, in the reaction:

Oxygen + Copper ⮕ Copper oxide

or more specifically

O2 + 2Cu ⮕ 2CuO

At the end, there should be no oxygen left in the air that was passed over the hot copper.

If you were to do a test, this is what you would typically find:

Starting volume of the air in cm3 100
Ending volume of the air in cm3 79
Oxygen Volume in cm3 (Volume of air at the start – Volume of air at the end) 21

To calculate the percentage of oxygen, the equation is:

Volume of oxygen ÷ Volume of air × 100

= 21 ÷ 100 × 100

= 21%