Answer:
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Explanation:
<u>Answer: </u>The correct statement is X is the effective nuclear charge, and it increases across a period.
<u>Explanation:</u>
We are given that:
X = number of protons − number of core electrons
Effective nuclear charge is defined as the actual nuclear charge (Z = number of protons) minus the screening effect caused by the electrons present between nucleus and valence electrons. These electrons are the core electrons.
The formula used for the calculation of effective nuclear charge given by Slater is:

where,
= effective nuclear charge
Z = atomic number or actual nuclear charge or number of protons
= Screening constant
The effective nuclear charge increases as we go from left to right in a period because nuclear charge increases with no effective increase in screening constant.
Hence, the correct answer is X is the effective nuclear charge, and it increases across a period.
The 3 indicates the third electron shell. (Which has only 1 electron in it in this configuration)
Hope this helps! :)
Open system ( exchanges energy and matter with the surroundings)
Closed system ( exchanges only energy with the surroundings)
Isolated system ( does not exchange energy or matter)
Adiabatic system ( does not allow any heat to be transferred into or out of the system)