Metals are

Why do elements in the same group have similar physical and chemical properties?

The electronic configuration of an atom $A$ is $1s^2, 2s^2 2p^6, 3s^2 3p^6 3d^{10}, 4s^2 4p^3$. The chemistry of $A$ is therefore likely to be similar to that of:

$HClO_4$ is a stronger acid than $H_2SO_4$ due to which of the following reasons?
$1$. Oxidation state of $Cl$ in $HClO_4$ is $+7$ and that of $S$ in $H_2SO_4$ is $+6$.
$2$. $Cl$ is more electronegative than $S$.
$3$. $HClO_4$ is monobasic,whereas $H_2SO_4$ is dibasic.
$4$. $ClO_3$ part of $HClO_4$ can break $O-H$ bond more easily to liberate a proton than $SO_3$ part in $H_2SO_4$.

An element $A$ dissolves in both acid and alkali. It is an example of

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