1. Have you ever ridden on a balloon? Many tourist spots offer balloon rides in
order for people to see the beauty of a place from above. A balloon contains a
noble gas called helium(氦). Formerly, balloons contained hydrogen but hydrogen
is very flammable and dangerous when uncontrolled. Therefore, people shifted to
helium, which is safer. Helium is safe because it has the properties of the
noble gases. 2. People once believed that noble gases couldn't
chemically react at all. For this reason, they were called inert gases(惰性气体).
They were also listed under Group 0 in the old periodic table because scientists
believed that the gases have zero valence (价) electrons in their outer shell.
This was later proven to be untrue when some noble gas compounds were
discovered. 3. The gases are elements, which share similar
properties. These properties include being monoatomic, colorless, odorless,
being able to conduct electricity, and having low chemical reactivity. Noble
gases include Helium, Neon, Argon, Krypton, Xenon and Radon. These are all found
in Group 18, in the rightmost column of the periodic table. If you look at the
periodic table, you will notice that these elements are the only ones, which do
not have a charge. Helium has the lowest molecular (分子的) weight while Radon is
the heaviest. 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 low chemical reactivity of these said
gases. 5. Because of their properties, noble gases have many
important applications. They are widely used in 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
person's 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.
填空题
paragraph 2 ______.
A. What are noble gases
B. What is the periodic table
C. What causes the low chemical reactivity of noble gases
D. How were noble gases discovered
E. How were noble gases understood in the past
F. What are the applications of noble gases
填空题
paragraph 3 ______.
填空题
paragraph 4 ______.
填空题
paragraph 5 ______.
填空题
Noble gases are not very chemically ______.
A. reactive B. lightest C. important
D. complete E. flammable F. unnecessary
填空题
Among the elements of noble gases Helium is the ______.
填空题
The required number of electrons in noble gases' outer shell is ______.