Question

how to calculate effective atomic number in chemistry

Answer 1

Effective atomic number, number that represents the total number of electrons surrounding the nucleus of a metal atom in a metal complex. It is composed of the metal atom’s electrons and the bonding electrons from the surrounding electron-donating atoms and molecules.
The effective atomic number rule is often referred to as the “18-electron rule” since, if one counts only valence electrons (6 for Co3+ and 2 × 6 = 12 for 6 NH3), the total number is 18.