what is effective nuclear charge? what are its rules?
The effective nuclear charge, also known as the kernel charge, is the net positive charge experienced by an electron in a multi-electron atom. The term "effective" is used because the shielding effect of negative electrons prevents higher orbital electrons from experiencing the full nuclear charge.
Effective Nuclear Charge for the outermost valence electron remains constant moving down a column or family.
Effective Nuclear Charge increases, going across the Periodic Table. Elements have more protons and the same number of core electrons.
The increase in the electrostatic force provided by the protons is greater than the increase in repulsion due to shielding and distance. Therefore, across the Periodic Table, Effective Nuclear Charge increases.