Rank the following in order of decreasing

(a) Boiling point: O₂, Br₂, As(s)

(b)${\mathbf{\Delta H}}_{\mathbf{vap}}$: Cl₂, Ar, I₂

In general, the boiling point of an element is defined as the heat energy required to change the element state from the liquid to the vapour phase, which involves breaking bonds. Therefore the element having stronger and higher bonds requires much more energy to break the bonds; on increasing the molar mass of an element, its boiling point also increases.

The boiling point trends in the periodic table vary from group to group.

There is also no regular or continuous trend in boiling point along the period. The boiling point first increases in metals and then decreases as we move from left to right in non-metals.

The enthalpy of vaporisation is the amount or quantity of heat that must be absorbed if a particular amount of liquid is vaporised under constant pressure conditions. It is directly proportional to the boiling point.

(a) As we discussed above, the boiling point depends upon the molecular mass of the atom, as it increases as the molar mass increases and depends upon the intermolecular forces between the atoms. Therefore, arsenic is a solid having strong intermolecular forces which require more energy to change its state. Thus, it has a higher boiling point than oxygen, and also bromine, a gas with weak intermolecular forces. Bromine also has comparatively stronger intermolecular forces than oxygen. So, the order of boiling points is as follows:

_{$As>B{r}_2>O_2$}

(b) As we know, the heat of vaporisation is directly proportional to the boiling point; therefore, the boiling point order of the following compounds is $I_2> C{l}_2>Ar$.

In the periodic trend, the heat of vaporisation decreases as we move down the group and move left to the right along the period in the case of non-metals.

Hence the heat of vaporisation of the given compounds in decreasing order is shown below:

$I_2> C{l}_2>Ar$