Answer
Effective nuclear charge $(Z_{eff})$ is a representation of an average of the electrical field experienced by a single electron, which is the average of the attraction between that electron and the nucleus and the repulsions between that electron and other electrons.
Work Step by Step
As we have witnessed in Chapter 6, an electron in a many-electron atom does not just experience the simple attraction between the nucleus and the electron, but also experience the electron-electron repulsions among the electrons.
Therefore, effective nuclear charge $(Z_{eff})$ is created to account for this problem. It is a representation of an average of the electrical field experienced by a single electron, which is the average of the attraction between that electron and the nucleus and the repulsions between that electron and other electrons.
$Z_{eff}$ is calculated most simply according to the following formula $$Z_{eff}=Z-S$$
where $Z$ is the nuclear charge of the element and $S$ is the screening constant (the electron-electron repulsions that reduce the attraction between the nucleus and the electron).
