Here's why:

* Effective nuclear charge (Zeff) is the net positive charge experienced by an electron in a multi-electron atom. It's always smaller than the actual nuclear charge (Z).

* Reason: The electrons in an atom repel each other. This electron-electron repulsion partially shields the outer electrons from the full attraction of the nucleus.

* Example: In a lithium atom (Li), the nucleus has a charge of +3. However, the two inner electrons (1s electrons) shield the outer valence electron from the full +3 charge. As a result, the effective nuclear charge experienced by the valence electron is less than +3.

In summary: The effective nuclear charge is a concept that takes into account the shielding effect of inner electrons. It's always smaller than the actual nuclear charge.