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Q.
Effective nuclear charge $\left(Z_{eff} \right)$ for a nucleus of an atom is defined as
Structure of Atom
Solution:
Effective nuclear charge is the net positive charge experienced by the electrons from the nucleus. Due to presence of electrons in the innermost shells the electrons in the outermost shell do not experience the full positive charge from the nucleus. This is known as shielding of the outermost shell electrons from the nucleus by the innermost shell electrons.