4. Remember that chemical reactions occur because atoms have valence electrons, which are electrons in their outer shell. When the outer shell is unfilled or the required number of electrons is not yet complete, the atom is more reactive. Noble gases have a full outer shell, meaning that they have complete electrons in their outer shell. This complete number varies. For instance, the outer shell of Helium has 2 valence electrons while the outer shell of Xenon has 8 electrons. Nowadays, there remains to be a few noble gases because of the inherent low chemical reactivity of these said gases.

　　5. Because of their properties, noble gases have many important applications. They are widely used in recreation, medicine and industries. For instance, Liquid Helium is used for superconducting magnets(磁体). These magnets are very important in physics and medicine. When a doctor suspects that a persons brain has been damaged, he might request for Magnetic Resonance Imaging (MRI). MRI allows the doctor to see the brain, without operating on the patient.

　　23. Paragraph 2

　　【答案】：E

　　24. Paragraph 3

　　【答案】：C

　　25. Paragraph 4

　　【答案】：D

　　26. Paragraph 5

　　【答案】：A

　　A. What are the applications of noble gases?

　　B. How were noble gases discovered?

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