aufbau principle

Imagine seating guests in a theatre that fills cheapest seats first: people take the front, lowest rows before climbing to the back. The aufbau principle says electrons do the same — they fill the lowest-energy orbital available first, then the next lowest, and so on, until all of the atom's electrons are placed. "Aufbau" is German for "building up", and that is the picture: you build a heavier atom by adding electrons one at a time into the cheapest empty seat.

The catch is that orbital energies do not climb in a simple 1, 2, 3 order. The 4s orbital sits slightly lower in energy than 3d for the early part of a row, so 4s fills before 3d; 5s fills before 4d; and the 4f orbitals slot in only after 6s and just before 5d. A handy memory aid is the Madelung (or n+l) rule: orbitals fill in order of increasing n+l, and for a tie the lower n goes first. So the order runs 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, and so on — and you can read this same order straight off the rows of the periodic table.

This is why the periodic table has the exact shape it does: the s-block is two columns wide, the p-block six, the d-block ten (after a one-row delay, giving the transition metals their own slab), and the f-block fourteen (pulled out below). Be honest about its limits, though: the aufbau order is an approximation that breaks for chromium, copper, and several heavier elements where half-filled or filled subshells win out, and where relativistic effects shuffle the energies. It is the right place to start, not the last word.