Question

The M.P. and B.P. are very low for?
A.Ne
B.He
C.Kr
D.Ar

Answer 1

Hint: Boiling point and melting point increases down the group in case of noble gases. The answer has application of being a cooling medium for the LHC, and the superconducting magnets in MRI scanners and NMR spectrometers.

Complete step by step answer:
-Here, we can see in the question that all the options belong to the noble gas group. If we put them in accordance with their atomic number we have helium, neon, argon and krypton with atomic number 2, 10, 18 and 36 respectively.
-From the above arrangement we can see that the atomic number is increasing. When we move down the group, atomic mass and atomic radii rises. This happens because the atoms expand in atomic size down the group and the electron clouds of these non-polar atoms become increasingly polarized. This results in weak Van Der Waals forces among the atoms.
-The boiling points of noble gases are lower in comparison to other substances. Therefore, the formation of solids and liquids is more readily attainable for these heavier elements because of their boiling and melting points.
-The Van Der Waals force decreases with the decrease in the size of the atom and thus, generally, the boiling point decreases from Radon to Helium. Helium has smaller mass than neon, argon and krypton and also has smaller dispersion forces. Because of smaller size the outer electrons are more tightly held in smaller atoms so that instantaneous dipoles are not easily induced which results in lesser interaction between atoms. Hence, the boiling point of helium is -268.93 which is less than the boiling point of all the other three noble gases.
-The temperature at which solid changes its state to liquid at atmospheric pressure is known as the melting point of that liquid. Similar to the boiling point, melting point also increases down the group in case of noble gases for the same reasons. Therefore, Helium has the lowest boiling point from all the options mentioned.

Hence, the correct answer is (B).

Note:
Due to the low solubility of helium, it is used as a component of breathing gases. It is useful as the other gases are absorbed in the blood and tissues while scuba diving under pressure. Thus, helium causes a decrease in the narcotic effect at far depths. It is also used in gas chromatography.