In the reaction $2Br^- + X_2 \to Br_2 + 2X^-$,what is $X_2$?

How do you determine the chemical reactivity of a compound? Provide an example.

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R.$
Assertion $A :$ Metallic character decreases and non-metallic character increases on moving from left to right in a period.
Reason $R :$ It is due to increase in ionisation enthalpy and decrease in electron gain enthalpy,when one moves from left to right in a period.
In the light of the above statements,choose the most appropriate answer from the options given below :

As the atomic number increases in a group,the atomic density .........

The elements with the following atomic numbers belong to the same group:

Which of the following is most acidic?