Which one among the following statements is/are not correct?
$I$. $Ba$ compounds,generally are more covalent than $Be$ compounds.
$II$. The electron gain enthalpy of $He$ is positive.
$III$. The oxidation state of $O$ in $OF_2$ and $Na_2O$ are same.
$IV$. The radius of $Na^{+}$ ion is smaller than that of $F^{-}$ ion.

Which of the following orders is not correct about the property shown against it?

The statement that is not correct for the periodic classification of elements is

Match the following elements,given as hints to their first,second ionization enthalpies and electron gain enthalpies.

List-$I$	List-$II$
$i)$ Most reactive non-metal	$A$. $\Delta H_1=419, \Delta H_2=3051, \Delta_{Eg} H=-48$
$ii)$ Most reactive metal	$B$. $\Delta H_1=1681, \Delta H_2=3374, \Delta_{Eg} H=-328$
$iii)$ Least reactive element	$C$. $\Delta H_1=738, \Delta H_2=1451, \Delta_{Eg} H=-40$
$iv)$ Metal forming binary halide	$D$. $\Delta H_1=3272, \Delta H_2=5251, \Delta_{Eg} H=+48$

Consider the following elements $In$,$Tl$,$Al$,$Pb$,$Sn$ and $Ge$. The most stable oxidation states of elements with highest and lowest first ionisation enthalpies,respectively,are

Which of the following statements are correct?
$(i)$ First ionisation enthalpy of $He < $ Second ionisation enthalpy of $Li$.
$(ii)$ $Li$ has the highest second ionisation enthalpy.
$(iii)$ All $d$-block elements are transition elements.
$(iv)$ The only alphabet not found in the periodic table is the letter '$J$'.
$(v)$ Francium concentration is $\sim 10^{-18} \ ppm$ on Earth.