Answer;
X is the effective nuclear charge, and it remains constant down a group.
Explanation;Effective nuclear charge is the actual nuclear charge (atomic number) minus the number of inner-core electrons which "shield" the valence electrons from feeling the full attraction.Effective nuclear charge increases left to right across a period, because atomic number is increasing while inner-core electrons are constant
.The effective nuclear charge remains constant down a group, because atomic number increases, but so do the number of core electrons