Which halogen in its reference state will have the greatest absolute entropy per mole?
A) Fluorine
B) Chlorine
C) Bromine
D) Iodine

HintThe elements of group 17 are called halogens. They are highly reactive in nature. We consider entropy as an extensive property of a thermodynamic system. It is the measure of the thermal energy of a system. In a broad sense entropy is the degree of disorder. The degree of disorder is higher in gaseous molecules than in liquid or solid molecules.

Complete step by step solution:
We have to consider the halogen elements in their reference state to find which element has the greatest entropy per mole.

The reference state which is also called the standard state is the pure and stable state of an element at standard temperature and pressure. We usually consider room temperature and pressure for reference states.

We know that Fluorine and chlorine are gaseous in their reference state. Bromine and Iodine exist in a liquid form in their reference state. From the definition we know, entropy is the degree of disorder and gases have more entropy than liquids. Hence we can eliminate bromine and iodine from the options.
Absolute entropy is a function of molar heat capacity, which is a function of molar mass. Therefore, the element with a higher molar mass will have more absolute entropy than the other. Here chlorine has a higher molar mass. So chlorine has higher absolute entropy than fluorine.

The answer is Option (B): Chlorine.

Note: Halogens are not freely found in nature due to their high reactivity. Fluorine is the most reactive among halogens. Though halogen elements resemble each other greatly, their physical and chemical properties vary from chlorine to iodine. Absolute entropy is the change in entropy of a pure substance when its temperature is increased from absolute zero to a given temperature.