Question

Which noble gas has the lowest boiling point?
A.$Kr$
B.$He$
C.$Ar$
D.$Ne$
E.$Xe$

Answer 1

Hint: The boiling point is an indication of forces acting between the particles of a matter so we can use the nature and extent of these forces in case of noble gases.

Step by step answer: We can define boiling point as a temperature at which liquid state of a substance is in equilibrium with its vapor state and the phase conversion takes place. As it involves phase change, it will be affected by the forces that are keeping the particles together such as strong ionic bonds between the opposite ions in an ionic solid, hydrogen bonding between the water molecules or weak dispersion forces between the atoms of noble gas. These weak dispersion forces are easily overcome giving very low boiling points to noble gases.

Now among noble gases, they all belong to the same group i.e. group $18$. Being inert they do not combine with other atoms or even that of themselves with some exceptions. So, they exist as mono-atomic gases only with weak dispersion forces between their atoms.

For a group, we can deduce some general trends of properties for them such as their size. Size increases down a group which in turn leads to more interatomic forces and higher boiling point. So, we can say that the topmost element would have the smallest size, weakest interatomic forces and lowest boiling point.
In this group, ${\rm{He}}$ is the topmost element in the group $18$ and thus has the lowest boiling point.

Hence, the correct option is B.

Note: The boiling point of ${\rm{He}}$ is the lowest known for any substance being only $4.2{\rm{ K}}$ as it exists as mono-atomic gas being a noble one and with weakest inter-atomic dispersion forces.