Periodic Properties

Step 1: Understanding the comparison.
Hydrogen shows similarities with both alkali metals and halogens due to its unique position in the periodic table. Some of its properties resemble halogens.

Step 2: Ionisation enthalpy comparison.
Halogens possess high ionisation enthalpy values, and hydrogen also has a relatively high ionisation enthalpy compared to alkali metals. This similarity is one of the reasons hydrogen is sometimes compared with halogens.

Step 3: Analysis of options.
(A) Ionisation enthalpy: Correct — hydrogen and halogens both have high ionisation enthalpy values.
(B) Formation of unipositive ion: Incorrect — this is a property similar to alkali metals.
(C) Electronic configuration: Incorrect — hydrogen has , while halogens have .
(D) Electron gain enthalpy: Incorrect — halogens have very high electron gain enthalpy, much higher than hydrogen.

Step 4: Conclusion.
Ionisation enthalpy is the property of hydrogen that is similar to the halogen family.