Which pair of elements with the given atomic numbers is expected to have similar properties?

In $s-$block,on moving down the group,the reactivity of metals with water and air increases. Which of the following factors has no significance for this trend of reactivity?

$Be$ and $Al$ show similarities in properties due to the diagonal relationship except in the property $X$ given below. What is $X?$

The increasing order of reactivity among group-$1$ elements is $Li < Na < K < Rb < Cs$ whereas that among group-$17$ elements is $F > Cl > Br > I$. Explain.

Observe the following data ($ \Delta_i H_1, \Delta_i H_2 $ and $ \Delta_{eg} H $ represent the first,second ionisation enthalpies and electron gain enthalpy respectively).

Element	$ \Delta_i H_1 $ $( kJ \ mol^{-1} )$	$ \Delta_i H_2 $ $( kJ \ mol^{-1} )$	$ \Delta_{eg} H $ $( kJ \ mol^{-1} )$
$ I $	$ 520 $	$ 7300 $	$ -60 $
$ I.I $	$ 490 $	$ 3051 $	$ -48 $
$ III $	$ 1681 $	$ 3374 $	$ -328 $
$ IV $	$ 2372 $	$ 5251 $	$ +48 $

Using the data,identify the most reactive metal.

Identify the correct order of acidic strengths of $CO_2, CuO, CaO, H_2O:-$