The Aufbau principle (from the German aufbauen, meaning "to build up") is one of the foundational rules used to determine the ground-state electronic configuration of atoms. It states that electrons occupying the orbitals of a multi-electron atom are filled into those orbitals in order of increasing energy — that is, the lowest-energy orbitals available are filled first, before electrons occupy higher-energy orbitals. This reflects the natural tendency of any physical system to settle into its most stable, lowest-energy configuration first.

Typical filling order (as energy increases): 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d …

Notice that this order is not simply increasing principal quantum number (n); for instance, the 4s orbital is filled before the 3d orbital, even though 3d has a lower n, because 4s happens to be at a slightly lower energy in many atoms. This ordering is systematically explained by the (n + l) rule (also called the Madelung rule): orbitals are filled in order of increasing (n + l) value; when two orbitals share the same (n + l) value, the one with the lower value of n is filled first. For example, 4s has n + l = 4 + 0 = 4, while 3d has n + l = 3 + 2 = 5, so 4s (lower n+l) fills before 3d.

The Aufbau principle does not act alone — it works together with two other essential rules to build up correct electron configurations:

• The Pauli Exclusion Principle, which limits each orbital to a maximum of two electrons, and only if their spins are opposite (paired).
• Hund's Rule of Maximum Multiplicity, which states that electrons will singly occupy all degenerate (equal-energy) orbitals within a subshell (like the three 2p orbitals) with parallel spins before any orbital gets a second, paired electron — this minimizes electron-electron repulsion and gives the most stable (lowest-energy) arrangement.

Together, these three principles allow us to systematically predict the electronic configuration of any element in its ground state, which in turn underlies the structure of the periodic table and explains trends in atomic size, ionization energy, and reactivity.

The alternative statements are incorrect because filling is neither random nor unaffected by energy considerations, and specifically, higher-energy orbitals are never filled before lower-energy ones are available — that would contradict the entire principle of building up the most stable (lowest-energy) configuration first.