Based on the expression given in the question, X is the effective nuclear charge, and it increases down a group.
What is an effective nuclear charge?
Effective nuclear charge is the net nuclear charge that an electron in an atom experiences, after subtracting the nuclear charge shielded by other electrons.
The effective nuclear charge is denoted by Zeff and can be calculated by subtracting the number of shielding electrons from the atomic number.
Therefore, based on the expression given in the question, X is the effective nuclear charge, and it increases down a group.
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