Electrons in an atom tend to occupy shells in increasing order of energies. K < L < M < N.
- The Aufbau principle states that electrons fill atomic orbitals of the lowest available energy levels in the ground state of an atom or ion before occupying higher levels.
- Each electron shell has a different energy level, with those shells closest to the nucleus being lower in energy than those farther from the nucleus.
- The electrons in an atom occupy the lowest available energy level first. This is the shell nearest the nucleus.
- When this shell is full, the electrons begin to occupy the next energy level.