aufbau principle
/ OWF-bow /
Imagine pouring water into a bumpy landscape of basins. The water always settles into the lowest dips first, only spilling into higher ones once the low ones are full. Building up an atom's electrons works the same way: electrons fill the lowest-energy orbitals first, and move to higher ones only once the lower seats are taken. 'Aufbau' is German for 'building up'.
The aufbau principle says that in the ground state of an atom, electrons are added one by one to the available orbitals in order of increasing energy. The usual order is captured by the Madelung (n + l) rule: orbitals are filled by the sum of their principal and angular quantum numbers, and where that sum ties, the lower principal number goes first — giving the familiar sequence 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.
Together with the Pauli exclusion principle (at most two electrons per orbital) and Hund's rule (spread out before pairing), the aufbau principle lets you predict any element's ground-state configuration. The honest caveat is that it is a useful rule of thumb, not an exact law: a handful of elements, especially among the transition and heavier metals, deviate because energy gaps there are tiny and subtle effects tip the order.
Following the rule, potassium's nineteenth electron goes into 4s rather than 3d, even though 3d has a smaller principal number — because 4s sits slightly lower in energy in a many-electron atom. That gives potassium the configuration [Ar] 4s¹ and places it neatly at the start of period 4.
Fill the lowest-energy orbitals first — water settling into the deepest basins.
Famous exceptions include chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹), which borrow an electron from 4s to reach a more stable half-filled or full d subshell.