Final answer:
An outer electron experiences an effective nuclear charge (Zeff), which is a fraction of the full nuclear charge due to the shielding effect of inner electrons. Zeff is always less than the actual nuclear charge (Z) for multi-electron atoms.
Step-by-step explanation:
An outer electron (in an orbital further from the nucleus) experiences an effective nuclear charge of b) A fraction of the nuclear charge. The effective nuclear charge (Zeff) is the charge felt by the electron after accounting for the shielding effects of the electrons between the outer electron and the nucleus. For hydrogen-like atoms or ions with only one electron, there is no shielding, so Zeff is equal to the full nuclear charge (Z). In multi-electron atoms, inner electrons provide shielding, and therefore, the outer electron does not experience the full nuclear charge but rather a reduced effective nuclear charge, which is calculated as Zeff = Z – shielding. As a result, Zeff is always a fraction of the full nuclear charge for these electrons. The value of Zeff depends on the distance from the nucleus and the amount of shielding provided by inner electrons. In summary, the electronegativity and atomic size trends observed across the periodic table result from variations in Zeff.