P. 441

1.    Early organisms released it into the air. Water vapor molecules were split apart.

2.    Troposphere, stratosphere, mesosphere, thermosphere, exosphere

3.    There are fewer molecules of oxygen per liter of air molecules

4.    Possible answers: Increased UV radiation reaches Earth, causing more cases of skin cancer; organisms such as algae could be damaged, resulting in the release of less oxygen into the atmosphere.

5.    At night, sunlight no longer interacts with the particles in the ionosphere. This allows AM radio transmissions to be reflected back to Earth at greater distances.

6.    99 percent; 0.96 percent