Fill equal-energy orbitals before pairing electrons
You will be able to: Apply the Pauli principle and Hund’s rule to orbital diagrams.
Why are three p electrons drawn in three separate boxes?
A p subshell has three orbitals. When we place three electrons in it, the lowest-energy diagram puts one in each orbital with parallel spins before making a pair. The boxes organize quantum states; they are not containers in space.
A useful starting point: Build a ground-state electron configuration →
Words and symbols before equations
- Orbital
- A quantum state described by a spatial wavefunction; a box can represent one orbital.
- Spin arrow
- A symbol for one of two spin states, not a path or literal spinning ball.
- Pauli principle
- At most two electrons occupy one orbital, and they have opposite spins.
- Hund’s rule
- Degenerate orbitals are singly occupied with parallel spins before pairing in a ground-state diagram.
What this picture assumes
Three equal-energy p orbitals in a ground-state occupancy diagram. Arrows denote spin states, not paths or literal rotation. The choice of which equivalent box pairs first is arbitrary.
Read the picture in three steps
- Identify the chemical species and the quantities each label or axis represents. Read the units and any scale assumptions before comparing values.
- p⁴: occupancies 2, 1, 1; 2 unpaired electrons.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
An s subshell has one orbital and capacity 2. A p subshell has three orbitals and capacity 6. Put one arrow in each p box before adding opposite-spin partners.
For p⁴ the occupancy is 2,1,1: one pair and two unpaired electrons. Which equivalent box is drawn paired first is a diagram choice, not a unique physical orientation.
An arrangement can satisfy Pauli yet fail to be the lowest-energy arrangement under Hund’s rule. Orbital diagrams refine configurations by revealing pairing. Detailed quantum-number assignments are not required here.
A worked example, step by step
Draw the 2p part of oxygen, 2p⁴, and count unpaired electrons.
- Draw three equal-energy p boxes.
- Place one up arrow in each: three electrons assigned.
- Add a down arrow to one box: four total.
- The occupancies are 2,1,1, leaving two unpaired electrons.
Do not pair in one p orbital while an equal-energy p orbital is still empty in the ground state.
Does p³ mean three electrons must share one orbital?
Compare with an explanation
No. They occupy the three p orbitals singly in the ground-state diagram.
Predict. Change one thing. Explain.
Fill a p subshell from zero to six electrons. Predict when the number of unpaired electrons starts decreasing.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
p⁴: occupancies 2, 1, 1; 2 unpaired electrons.
Three equal-energy p orbitals in a ground-state occupancy diagram. Arrows denote spin states, not paths or literal rotation. The choice of which equivalent box pairs first is arbitrary.
Explain what you noticed: Which quantity changed? Which stayed fixed? Use particle counts, mass or charge balance, electron structure, or nuclear attraction to justify your prediction. Separate an observation from an explanation.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionCompare p² and p⁴ ground-state orbital diagrams. Give occupancies and unpaired counts for each, then distinguish the two filling rules.
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Compare with the answer and four-point rubric
- 1 point: p² has occupancies 1,1,0, with two unpaired electrons.
- 1 point: p⁴ has occupancies 2,1,1, also with two unpaired electrons.
- 1 point: Pauli permits at most two opposite-spin electrons per orbital.
- 1 point: Hund favors singly occupying equal-energy orbitals with parallel spins before pairing.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1How many orbitals are in p?
Three.
RECALL 2What is p³’s unpaired count?
Three in the ground state.
RECALL 3Are box diagrams pictures of electron paths?
No.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Fill equal-energy orbitals before pairing electrons
- One orbital holds at most two opposite-spin electrons.
- Three p orbitals hold at most six electrons.
- For p¹–p⁶, unpaired counts are 1,2,3,2,1,0.
Remember: Do not pair in one p orbital while an equal-energy p orbital is still empty in the ground state.
Conditions: Three equal-energy p orbitals in a ground-state occupancy diagram. Arrows denote spin states, not paths or literal rotation. The choice of which equivalent box pairs first is arbitrary.
Refresh Kid · AP Chemistry Unit 1 · Objectives 1.5.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 1.5, objectives 1.5.A. CED effective Fall 2024, current official file checked September 16, 2026, together with the published clarifications. This is Unit 1: Atomic Structure and Properties, Topics 1.1–1.8. The topic mapping identifies a framework area; focused lesson titles are our own teaching sequence. Molecular-formula scaling is an application of empirical composition. Models explicitly distinguish atom counts, molecule counts, mass fractions and electron structure. Spectra marked schematic are not measured data. Mass spectra here use single-element, singly charged monatomic ions. Configurations avoid Aufbau exceptions and individual quantum-number assignments. Qualitative attraction and size indices are not exact atomic predictions. The optional NaCl-type spatial block supplements complete charge-balance explanations. The lesson breakdown and questions are original Refresh Kid work, not official topic subdivisions.
Implementation and automated checks are separate from independent teacher review and observation of students. Both human review stages remain pending. This is a review edition, not a certified or validated assessment.
Optional further resource: College Board’s released questions and scoring guides. Papers can combine units; this link is an archive, not an assignment of every question to this lesson.
Our learn, explore, practice and recall sequence is informed by the IES learning guide. The exact Refresh Kid implementation has not been evaluated for learning effectiveness.
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