Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table and is a highly reactive nonmetal and oxidizing agent that readily forms compounds (notably oxides) with most elements. By mass, oxygen is the third-most abundant element in the universe, after hydrogen and helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O 2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. However, monitoring of atmospheric oxygen levels show a global downward trend, because of fossil-fuel burning. Oxygen is the most abundant element by mass in the Earth's crust as part of oxide compounds such as silicon dioxide, making up almost half of the crust's mass.

Many major classes of organic molecules in living organisms, such as proteins, nucleic acids, carbohydrates, and fats, contain oxygen, as do the major inorganic compounds that are constituents of animal shells, teeth, and bone. Most of the mass of living organisms is oxygen as it is a part of water, the major constituent of lifeforms. Oxygen is used in cellular respiration and released by photosynthesis, which uses the energy of sunlight to produce oxygen from water. It is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form (allotrope) of oxygen, ozone (O 3), strongly absorbs UVB radiation and consequently the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation, but is a pollutant near the surface where it is a by-product of smog. At even higher low earth orbit altitudes, sufficient atomic oxygen is present to cause erosion for spacecraft.

Answer the following questions:
1. What is the article about?
2. What is its symbol?
3. it's numeral?
4. is it abundant?
5. how does it rank?
6. What is more abundant?
7. What makes up O2?
8. What seems to be lowering earths levels of O2?
9. Does O2 exist inside our planet?
10. How is O2 released by plants? What process?
11. What is O3?
12. Where is this found?
13. Is it found near the face of the planet?
14. What is this called?
15. Does smog cause this?
16. What can erode space vehicles?
17. At what level?
1. Oxygen
2. O
3. Eight
4. Yes
5. third
6. hydrogen and helium
7. two atoms of oxygen
8. fossil-fuel burning
9. Yes
10. photosynthesis
11. ozone
12. high-altitude
13. Yes
14. a pollutant
15. Yes
16. atomic oxygen
17. higher low earth orbit altitudes