Read the given expression. x = number of protons − number of core electrons which of the following explains the identity of x and its trends across a period? x is the effective nuclear charge, and it remains constant across a period. x is the screening constant, and it remains constant across a period. x is the effective nuclear charge, and it increases across a period. x is the screening constant, and it increases across a period.

The correct statement is X is the effective nuclear charge, and it increases across a period.

Explanation:

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.