The effective nuclear charge experienced by the outermost electron of na is different than the effective nuclear charge experienced by the outermost electron of ne. this difference best accounts for which of the following?
(a) na has a greater density at standard conditions than ne.
(b) na has a lower first ionization energy than ne.
(c) na has a higher melting point than ne.
(d) na has a higher neutron-to-proton ratio than ne.
(e) na has fewer naturally occurring isotopes than ne.

(B) Na has a lower first ionization energy than Ne.

Explanation:

As we know that ionization potential is the energy required to remove one electron from outermost shell. So if we found the ionization potential of Na and then ionization potential of Ne we will find the difference.

Experimentally it is found that Na has smaller value of ionization potential compare to Ne. So we can say the effective charge of Nucleus of Na must be less than the effective charge of nucleus of Ne.

Because if effective charge of nucleus is more than it will have strong attraction on outer electron and it must have more ionization potential.

So here correct answer will be

(B) Na has a lower first ionization energy than Ne.