<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.
Physical. When you simply desolve something with water, to don't actually change it. This action can be undone.
An example of a chemical reaction is when the cells within completely change. You you burn wood, you can't go back in time and un-burn it. Does that make sense?
A chemical reaction is the process by which a CHANGE takes place (?)
5. 1 hydrogen, 1 Nitrogen, 3 oxygen