STUDENT A

1. Read the text below and try to complete the missing words.
2. Listen to student B giving you the definitions of the missing words and fill the gaps.
3. Explain the words in bold to student B.

OXYGEN[14]

Oxygen is the element with 1._ _ _ _ _ _ _ _ _ _ _ _ _ and represented by the symbol O. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless, 2._ _ _ _ _ _ _ _, tasteless 3._ _ _ _ _ _ _ _ gas with the formula O2. Oxygen is a member of the 4._ _ _ _ _ _ _ _ _ group on the 5._ _ _ _ _ _ _ _ _ _ _ _ _, and is a highly 6._ _ _ _ _ _ _ _ nonmetallic element that readily forms 7._ _ _ _ _ _ _ _ _ (notably oxides) with almost all other elements. By mass, oxygen is the third most abundant element in the universe after hydrogen and 8._ _ _ _ _ _ and the most abundant element by mass in the 9._ _ _ _ _ _ _ _ _ _ _, making up almost half of the crust's mass. Free oxygen is too chemically reactive to appear on Earth without the 10._ _ _ _ _ _ _ _ _ _ _ _ _ _ action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Elemental O2 only began to accumulate in the atmosphere after the evolutionary appearance of these organisms, roughly 2.5 billion years ago. Diatomic oxygen gas constitutes 20.8% of the 11._ _ _ _ _ _ of air.

Oxygen comprises most of the mass of living organisms (for example, about 12.two-thirds of the human body's mass). All major classes of structural molecules in living organisms, such as 13. proteins, 14. carbohydrates, and fats contain oxygen, as do the major 15. inorganic compounds that comprise animal shells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in 16.cellular respiration for all complex life. Oxygen is 17.toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphere. Another form (allotrope) of oxygen, 18. ozone (O3), helps protect the biosphere from 19. ultraviolet radiation with the high-altitude 20. ozone layer, but is a pollutant near the surface where it is a 21. by-product of smog. At even higher low earth orbit 22. altitudes atomic oxygen is a significant presence and a cause of erosion for spacecraft.

STUDENT B

1. Read the text below and try to complete the missing words.
2. Explain the words in bold to student A.
3. Listen to student A giving you the definitions of the missing words and fill the gaps.

OXYGEN[14]

Oxygen is the element with 1. atomic number 8 and represented by the symbol O. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless, 2. odorless, tasteless 3. diatomic gas with the formula O2 . Oxygen is a member of the 4. chalcogen group on the 5. periodic table, and is a highly 6. reactive nonmetallic element that readily forms 7. compounds (notably oxides) with almost all other elements. By mass, oxygen is the third most abundant element in the universe after hydrogen and 8. helium and the most abundant element by mass in the 9. Earth's crust, making up almost half of the crust's mass. Free oxygen is too chemically reactive to appear on Earth without the 10. photosynthetic action of living organisms, which use the energy of sunlight to produce elemental oxygen from water. Elemental O2 only began to accumulate in the atmosphere after the evolutionary appearance of these organisms, roughly 2.5 billion years ago. Diatomic oxygen gas constitutes 20.8% of the 11. volume of air.

Oxygen comprises most of the mass of living organisms (for example, about 12. _ _ _ -_ _ _ _ _ _ of the human body's mass). All major classes of structural molecules in living organisms, such as 13. _ _ _ _ _ _ _ _, 14. _ _ _ _ _ _ _ _ _ _ _ _ _, and fats, contain oxygen, as do the major 15. _ _ _ _ _ _ _ _ _ compounds that comprise animal shells, teeth, and bone. Elemental oxygen is produced by cyanobacteria, algae and plants, and is used in 16. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ for all complex life. Oxygen is 17. _ _ _ _ _ to obligately anaerobic organisms, which were the dominant form of early life on Earth until O2 began to accumulate in the atmosphere. Another form (allotrope) of oxygen, 18. _ _ _ _ _, helps protect the biosphere from 19. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ with the high-altitude 20._ _ _ _ _ _ _ _ _ _, but is a pollutant near the surface where it is a 21._ _ _ _ _ _ _ _ _ of smog. At even higher low earth orbit 22. _ _ _ _ _ _ _ _ _ atomic oxygen is a significant presence and a cause of erosion for spacecraft.