2 Answers

To start the air around the earth is 78% Nitrogen 21% Oxygen 0.93% argon 0.4% carbon dioxide and are called the atmospheric gases. It was created when the Earth was formed as a ball of hot gases revolving around the Sun. As the Earth cooled, it solidified, except for water, which remains liquid above zero degrees Celsius and the atmospheric gases.
These gases were created by plants which absorb carbon dioxide and then create oxygen as a by-product.
When the oceans were formed and microbes developed, these microbes were first chemical in nature, not relying on sunlight to grow. But they eventually adapted to the light and also took C02 to grow. Creating Oxygen. The oxygen floated to the surface and over thousands of years filled the sky.
Hope that helps.
religion school -

Only place we got  to live on  so we better take care of it.