What causes electric shielding?

The shielding effect arises from superposition of the field from the outside charge distribution and the opposing charge distribution that was induced on the copper strips. This induced charge distribution, or "back-field," can be "frozen in place" by removing the wire interconnecting the strips.

Explanation:

The bigger the atom the lesser the ability of the atom to hold on to its valence electrons.

Explanation:

The larger the atom, the lesser the tendency for the atom to withhold its valence electrons.

The size of an atom is usually estimated in terms of its atomic radius. Across the period in the periodic table, the atomic radius decreases progressively from left to right. This is due to the progressive increase in the nuclear charge (i.e., the number of protons in the nucleus) without an attendant increase in the number of electron shells.

Similarly, down the group, the atomic radius increases progressively from top to bottom due to the successive number of electrons' shells.

Thus, the larger the atom's radius, the smaller the Ionization energy because of the increasing shielding effect of inner shell electrons on the electron to be removed from the nuclear attraction.

Finally, we can conclude that as the atom becomes larger, the tendency to withhold the electrons on the outer shell decreases due to distance and the shielding/ screening effect.