Question

explain the rule according to which electrons are filled in the various energy level

Answer 1

electrons are filled according to aufbau principle into various energy levels..it's also called (n+l) rule..where n=principle quantum number and l is azimuthal quantum number

Answer 2

"The Aufbau principle states that, hypothetically, electrons orbiting one or more atoms fill the lowest availableenergy levels before filling higherlevels (e.g., 1s before 2s). In this way, the electrons of an atom, molecule, or ion harmonize into the most stableelectron configuration possible"