Ionization energies vary from left to right across the periodic table. Factors that contribute to this variation include which of the following?

P) Changes in the nuclear charge (Q) Differences in shielding by valence electrons (R) Differences in shielding by core electrons

As we know Ionization energies vary from left to right across the periodic table due to which their is  changes in the nuclear charge and differences in shielding by valence electrons.

Because of poor electron screening by other electrons in the same main shell, atomic radii fall from left to right across a row as effective nuclear charge increases.

The shielding effect felt by the outermost electrons is stronger the more electron shells there are. As a result, the screening or shielding effect increases in a group as the number of shells grows from top to bottom, but it decreases in a period due to an increase in atomic number without a corresponding change in the number of shells.