The variable X is the effective nuclear charge, and it remains constant down a group.
Discussion:
The effective nuclear charge is referred to as the actual amount of positive (nuclear) charge experienced by an electron in a polyelectronic atom.
The effective nuclear charge experienced by an electron is otherwise called the core charge.
It is mathematically evaluated as the difference in the atomic number and the core electrons. and since elements in the same group have equal number of Valence electrons, the effective nuclear charge is constant down the group.
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Answer:
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