Build a ground-state electron configuration
You will be able to: Write ground-state configurations and identify core and valence electrons for the first 20 elements.
What does 1s² 2s² 2p⁶ actually count?
An electron configuration is an organized inventory. For neon, 1s² 2s² 2p⁶ tells us where ten electrons are assigned among subshells, rather than showing electrons traveling along tiny circular tracks.
A useful starting point: Count protons, neutrons and electrons separately →
Words and symbols before equations
- Shell
- An energy-level grouping labeled by a principal number such as 1, 2 or 3.
- Subshell
- An s, p, d or f grouping within a shell; s holds at most 2 and p at most 6 electrons.
- Ground state
- Lowest-energy electron arrangement for the species.
- Valence electrons
- For these main-group examples, electrons in the highest occupied shell; the rest are core electrons.
What this picture assumes
Ground-state neutral atoms H through Ca; usual Aufbau sequence only. Orbital energy ordering depends on species. No quantum-number assignments or exceptional configurations are modeled.
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.
- S (Z=16): 1s² 2s² 2p⁶ 3s² 3p⁴. Highest shell n=3; 6 valence and 10 core 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
For the first 20 neutral elements, fill 1s, 2s, 2p, 3s, 3p, then 4s in the usual ground-state sequence. Each superscript is the number of electrons in that subshell.
The sum of superscripts equals the electron count. For a neutral atom it also equals Z. Noble-gas shorthand replaces a filled core: [Ne] represents 1s² 2s² 2p⁶.
These configurations are a quantum description, not tracks. Shells and subshells explain recurring chemical patterns. Exceptions to the simple filling sequence and assignment of individual quantum numbers are beyond this model.
A worked example, step by step
Write the ground-state configuration of sulfur, Z=16, and identify its valence electrons.
- Assign 2 to 1s, 2 to 2s and 6 to 2p: 10 electrons.
- Assign 2 to 3s and the remaining 4 to 3p.
- Configuration: 1s² 2s² 2p⁶ 3s² 3p⁴, or [Ne]3s²3p⁴.
- The highest occupied shell is n=3; it contains 6 valence electrons and leaves 10 core electrons.
Superscripts count electrons; shell numbers are not electron counts or orbit radii.
How many electrons does [Ne]3s²3p¹ contain?
Compare with an explanation
10+2+1=13, the neutral aluminum configuration.
Predict. Change one thing. Explain.
Increase atomic number from 1 to 20. Watch the next subshell fill and explain the change in valence count when a new shell begins.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
S (Z=16): 1s² 2s² 2p⁶ 3s² 3p⁴. Highest shell n=3; 6 valence and 10 core electrons.
Ground-state neutral atoms H through Ca; usual Aufbau sequence only. Orbital energy ordering depends on species. No quantum-number assignments or exceptional configurations are modeled.
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 questionFor neutral phosphorus, Z=15, write full and noble-gas configurations, identify core and valence counts, and check the total.
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Compare with the answer and four-point rubric
- 1 point: Full configuration: 1s²2s²2p⁶3s²3p³.
- 1 point: Shorthand: [Ne]3s²3p³.
- 1 point: There are 10 core and 5 valence electrons.
- 1 point: 2+2+6+2+3=15, matching neutral Z=15.
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 1What does a superscript count?
Electrons in that subshell.
RECALL 2What does [Ne] replace?
The ten-electron 1s²2s²2p⁶ core.
RECALL 3What does ground state mean?
The lowest-energy arrangement for the specified species.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Build a ground-state electron configuration
- Sum of superscripts = total electron count.
- Capacities: s 2, p 6, d 10, f 14.
- For the first 20 neutral atoms: 1s, 2s, 2p, 3s, 3p, 4s.
Remember: Superscripts count electrons; shell numbers are not electron counts or orbit radii.
Conditions: Ground-state neutral atoms H through Ca; usual Aufbau sequence only. Orbital energy ordering depends on species. No quantum-number assignments or exceptional configurations are modeled.
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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