What you will learn

• Apply the Aufbau principle to select the next subshell to occupy.
• Apply the Pauli exclusion principle within each orbital.
• Apply Hund’s rule across orbitals of equal energy.
• Determine and check ground-state arrangements using electron counts.
• Recognise the limits of the simple filling sequence, including chromium and copper.

A1.2.2: “rules to determine the electronic structure of atoms”. Verified against Pearson AAQ Applied Science, Issue 7 (May 2026), Unit 2, printed page 36. Pearson’s scheme of work names Aufbau, Pauli and Hund. Examples here explain the rules; the full atom-and-ion notation practice belongs to A1.2.3.

1. Start with the electron count

The ground state is the lowest-energy electronic arrangement of an atom. A neutral atom has the same number of electrons as its atomic number, Z. For example, nitrogen has Z = 7, so place exactly seven electrons.

Use three decisions in order: which subshell, which orbital, which spin? Aufbau guides the subshell sequence; Hund guides the distribution across equal-energy orbitals; Pauli limits occupancy and spin within an orbital.

Recall only what you need from A1.2.1: an s subshell has one orbital, p has three and d has five. Every orbital holds at most two electrons. A box represents one orbital and each arrow represents one electron; opposite arrow directions represent opposite spin states, not directions of travel.

2. Aufbau: occupy lower-energy states first

The Aufbau principle describes building the ground-state arrangement by occupying available lower-energy orbitals before higher-energy ones. Do not simply complete every orbital in shell 3 before using shell 4.

Usual building-up sequence to krypton:
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p

The total column is an accounting aid, not a statement that every intermediate atom follows an exception-free pattern. Stop when you reach the atom’s electron count.

The 4s/3d qualification: potassium and calcium occupy 4s before 3d. Once 3d is occupied, the relative energies change; do not say “4s is always lower than 3d”. For transition-metal cations, 4s electrons are removed before 3d electrons. Applying that ion rule is part of A1.2.3.

3. Pauli: at most two, with opposite spins

The Pauli exclusion principle states that no two electrons in an atom have the same set of four quantum numbers. Its practical consequence here is that one orbital contains at most two electrons, and a pair must have opposite spins.

Spin is an intrinsic quantum property. The arrows ↑ and ↓ represent its two allowed spin projections; they do not depict an electron travelling up or down.

Pauli applies to each individual box, whether it belongs to an s, p or d subshell. The six-electron capacity of p comes from three boxes × two electrons, never six in a single box.

4. Hund: singly first, with parallel spins

Hund’s rule: in the ground state, electrons occupy orbitals of equal energy singly with parallel spins before pairing. In an isolated atom without an external field, the three orbitals in a p subshell are equal in energy (degenerate); the five orbitals in a d subshell are also degenerate.

For three p electrons, place one ↑ in each of the three boxes. For the fourth, add a ↓ to one occupied box. You may draw all initial arrows ↓ instead, provided their spins are parallel and any pairs are opposite.

The last row obeys Pauli but is not the ground-state arrangement required by Hund. Hund is a rule for equal-energy orbitals within the same subshell; it does not tell you to put an electron in 2p while 2s still has a lower-energy vacancy.

A singly occupied orbital contains an unpaired electron. Ground-state p² has two unpaired electrons, p³ has three, p⁴ has two, and p⁶ has none.

5. Worked examples: combine the rules

6. Check the limits: chromium and copper

The simple Aufbau sequence is a useful guide, not an exception-free law. For some atoms, the lowest total energy is achieved by a different distribution between closely spaced 3d and 4s states. Within atomic numbers 1–36, remember these ground-state exceptions:

“Half-filled d” for Cr and “filled d” for Cu are useful memory aids. The underlying reason is the total energy of the atom, involving electron interactions as well as orbital energies; do not claim that every atom must transfer an electron to produce a half-filled subshell.

These exceptions do not break Pauli or Hund. Chromium has five singly occupied 3d orbitals plus one 4s electron. Copper has five paired 3d orbitals plus one 4s electron. Writing 3d before 4s in a configuration is a notation convention; it does not assert that 3d was entered first in the building sequence.

7. Common misconceptions and exam tips

• “Fill every third-shell orbital before 4s.” Use the subshell building sequence: 3p → 4s → 3d.
• “Pauli means pair everything immediately.” Pauli constrains a pair; Hund tells you to occupy equal-energy orbitals singly first.
• “Opposite spins belong in separate boxes first.” Initial single occupants of equal-energy orbitals have parallel spins in the ground state.
• “Any Pauli-compliant diagram is a ground state.” Check Aufbau and Hund too. A non-ground-state arrangement can be an excited arrangement rather than an impossible one.
• “4s is always lower than 3d.” Relative energies depend on species and occupancy.
• “All elements follow the simple pattern exactly.” Check Cr and Cu exceptions.

Exam check: electron total → subshell order → maximum 2 per box → opposite-spin pairs → singly first in equal-energy boxes → known exceptions.

When asked to explain, name the relevant rule and connect it to the diagram. For example: “Hund’s rule gives three singly occupied 2p orbitals with parallel spins before pairing.” For an impossible same-spin pair, name Pauli instead.

8. Check your understanding

Choose an answer, select Check Answer, then read the scientific explanation. Try Again clears your choice and feedback. No account is needed; answers are not saved.

Your revision summary

• For a neutral atom, electron number = atomic number.
• Aufbau: occupy available lower-energy states first; use the usual sequence with its known exceptions.
• Pauli: at most two electrons per orbital; a pair has opposite spins.
• Hund: equal-energy orbitals are occupied singly with parallel spins before pairing.
• Cr is [Ar] 3d⁵ 4s¹; Cu is [Ar] 3d¹⁰ 4s¹.

Retrieval challenge: explain why nitrogen has three unpaired electrons but oxygen has two, using Hund and Pauli by name.