1.3.5—Orbitals and electron spin
- Syllabus
- First assessment 2025
- Objective
- 1.3.5
- Level
- HL
Electron Configurations and Spin
Aufbau fills lower-energy orbitals first. Pauli limits an orbital to two electrons with opposite spins. Hund's rule places electrons singly in degenerate orbitals before pairing.
| Representation | Use |
|---|---|
| Full configuration | Show the complete filling sequence |
| Condensed configuration | Replace the inner electrons with a noble-gas core |
| Orbital-box diagram | Show orbital occupancy and opposite-spin pairing |
For ions, remove 4s electrons before 3d electrons. The exceptions in scope are Cr: [Ar] 4s1 3d5 and Cu: [Ar] 4s1 3d10.
Check total electrons, obey the filling order, apply Hund and Pauli in each sublevel, and treat the Cr/Cu exceptions explicitly rather than forcing the naive pattern.
Build an orbital diagram by checking electron total, energy order, single occupation of equal-energy orbitals, then opposite-spin pairing. For transition-metal ions remove 4s electrons before 3d, and verify Cr and Cu against the stated exceptions rather than forcing the simple filling pattern.
Worked ion check: Fe has 26 electrons and condensed configuration [Ar] 4s² 3d⁶. To form Fe³⁺, remove the two electrons from the highest principal level, 4s, before removing one 3d electron, giving [Ar] 3d⁵. The final superscripts total 23 electrons, matching 26 − 3.
Constructing Electron Configurations
Assessment in practice
1–2 marks in the two selected direct examples marksHow it is assessed
Questions ask students to draw and label a ground-state orbital diagram or select/configure an atom using the filling rules and the Cr/Cu exceptions.
Command terms
draw
What earns marks
Fill orbitals in the stated energy order, place one electron in each degenerate orbital before pairing, pair only opposite spins, remove 4s electrons before 3d for transition-metal ions, and use the accepted Cr/Cu exception configurations.
Watch for
Pairing electrons in a p or d sublevel before singly occupying equivalent orbitals
Representative question
Question 1
[Maximum number: 2]
Draw the orbital diagram of the phosphorus atom in the ground state by adding, filling and labelling the orbitals. Use section 7 of the data booklet.
2s
1s
3p
□
1
3rd shell orbitals must be higher than
2nd shell for M2.
3s
□
2 p
2s □ 1
correct labels
correct electron configuration
Electron Configurations Summary
Retrieve the chain: emission lines reveal discrete levels; capacities, sublevels, orbitals, and spin rules build configurations; first and successive ionization energies then reveal how electrons are held and arranged.
When checking an answer, ask: Did I link a line to a transition? Did I use 2n² and the filling rules? Did I explain an ionization trend or count electrons before a successive-IE jump?