The effective nuclear charge for an atom is less than the actual nuclear charge due to

a) shielding.
b) penetration.
c) paramagnetism.
d) electron-pair repulsion.
e) relativity

A = shielding.

Explanation:

The addition of electron causes the atomic size increase from top to bottom due to increase in atomic number.

As the atomic number increased one more electron is added and because of this electron on more electronic shell is added. Thus the electron become more away from the nucleus as many of other electrons are present in the way from nucleus to the outer electrons.

The hold of nucleus becomes weaker. Although nuclear charge is also increased but at the same time other electrons shield the respective electrons. So effective nuclear charge is weaker than the actual nuclear charge.

Because of this shielding it is easy to remove the electrons or we can say ionization energy decreases.

Here, we are required to determine why the effective nuclear charge for an atom is less than the actual nuclear charge.

First, we must know that the effective nuclear charge is the net positive charge exerted on the negatively charged Valence electrons by the nucleus.

The bigger an atom is, the farther are it's Valence electrons from the positively charged nucleus.

The shielding effect of these electrons therefore, reduce the nuclear charge that is felt by the valence electrons and ultimately, the effective nuclear charge is less than the actual nuclear charge.

Read more:

link

just did the assignment and the answer was #1 +2 n