V. Oral practice.

Exercise 12 . Explain :

- why oxygen is the most important element to man ;

-why oxygen is necessary for steelmaking .

VI. Reading and comprehension.

Exercise 13. Read text B “How many hydrogens are there on earth ?” without a dictionary for 6 min.

Text b. How many hydrogens are there on earth?

It was thought previously that there was only one hydro­gen on Earth, that with an atomic weight of one. Murphy and his colleagues discovered a second hydrogen, twice as heavy. This was the hydrogen isotope having the atomic weight two. Isotopes are varieties of atoms with the same charge but different atomic weights. In other words, the nuc lei of isotopic atoms contain an equal number of protons, but different numbers of neutrons. Isotopes are known for all the chemical elements: some of them exist in nature; others have been obtained artificially by means of nuclear reactions.

The hydrogen isotope whose nucleus is a bare proton is called protium and its symbol is H.¹ This is the only atomic nucleus which contains no neutrons at all. (Another unique property of hydrogen!) Add a neutron to this single proton and the result is the nucleus of the heavy hydrogen isotope called deuterium (H² or D²). Protium is far more abundant in nature than deuterium, constituting over 99 per cent of all the hydrogen.

But there is a third variety of hydrogen, with two neutrons in its nucleus; this is tritium (H³ or T³). It forms only to disappear again rather quickly. It is radioactive and decays into a helium isotope (helium-3). Tritium is a very rare ele­ment: its content in all the atmosphere of the Earth is only 6 grams. There is only one atom of tritium in every 10 cubic centimetres of air. Just recently still heavier isotopes of hydrogen H⁴ and H⁵ have been obtained artificially, but they are unstable.

The fact of its having isotopes does not distinguish hy­drogen among the chemical elements. What does distinguish it is that hydrogen isotopes differ noticeably in properties, primarily in physical properties. Isotopes of the other ele­ments are almost indiscriminable.

Each variety of hydrogen has its own personality and behaves differently in chemical reactions. For example, pro-tium is more active than deuterium. By studying the behav­iour of hydrogen isotopes, scientists started an 'entirely new branch of science, known as isotope chemistry. Chemis­try has to do with elements as a whole, with all the isotopes of each, taken together. But isotope chemistry deals with separate isotopes. It enables investigators to look into the most intricate details of various chemical processes.

Exercise 14. Answer the following questions:

1. Who discovered a second hydrogen ?

2. What is called protium ?

3. What is called deuterium?

4. How does tritium form ?

5. How did scientists create isotope chemistry?

6. What does isotope chemistry deal with ?

VII. Oral practice.

While doing the exercises given below use some of these conversational formulas:

1. To begin with – почнемо з того, що

2. It is interesting to note – цікаво зазначити

3. It is well known that – добре відомо, що

4. It should be added – необхідно додати

5. To cut a long story short – коротше кажучи

6. It is worth pointing out – слід підкреслити

7. We cannot but mention – ми не можемо не згадати

8. In conclusion I would like to stress – на закінчення, мені б хотілось наголосити

Exercise 15. Prove:

a). that tritium is a very rare element.

Exercise 16 . Compare protium with deuterium.