Which of the following has the minimum ionization energy?

Identify the elements $X$ and $Y$ using the ionisation energy values given below :

Element	Ionization energy $1^{st}$ $(kJ/mol)$	Ionization energy $2^{nd}$ $(kJ/mol)$
$X$	$495$	$4563$
$Y$	$731$	$1450$

Ionization energy of gaseous $Na$ atoms is $495.5 \ kJ \ mol^{-1}$. The lowest possible frequency of light that ionizes a sodium atom is $(h = 6.626 \times 10^{-34} \ J \ s, N_A = 6.022 \times 10^{23} \ mol^{-1})$.

Would you expect the first ionization enthalpies for two isotopes of the same element to be the same or different? Justify your answer.

Which of the following represents the correct decreasing order of the first ionization enthalpy of $Ba, Sr, Ca,$ and $Mg$?

Which of the following orders of ionization energy is correct?