No, valence electrons do not directly contribute to the shielding effect in an atom. The shielding effect is primarily determined by the inner shell electrons, specifically those in the filled inner energy levels or shells closer to the nucleus.
The shielding effect occurs when inner shell electrons repel or shield the outer electrons from the full attractive force of the positively charged nucleus. These inner electrons create an electron cloud around the nucleus, which reduces the effective nuclear charge experienced by the valence electrons. As a result, the valence electrons are less strongly attracted to the nucleus, and this affects the size and chemical behavior of an atom.
Valence electrons themselves do not shield each other significantly because they are located in the same energy level and have similar energy and proximity to the nucleus. It is primarily the inner shell electrons, which are closer to the nucleus, that contribute to the shielding effect.