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<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.
<u>Answer:</u> The atomic mass of these species is different and atomic number remains same.
<u>Explanation:</u>
Isotopes are the chemical species of the same element having different number of neutrons.
- Atomic number is equal to the number of protons or electrons present in that element.
Atomic Number = Number of electrons = Number of protons
- Atomic mass is defined as the sum of number of protons and neutrons contained in an atom.
Atomic Mass = Number of protons + Number of neutrons
For isotopes, as the number of neutrons differ, the atomic mass also differs.
For Example: Carbon has 3 naturally occurring isotopes: . The atomic number remains the same but atomic mass differs.
Hence, for isotopes, the atomic mass of these species is different and atomic number remains same.