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1.3.5—Orbitals and electron spin

Syllabus
First assessment 2025
Objective
1.3.5
Level
SL

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.

A spherical s orbital and three perpendicular dumbbell-shaped p orbitals are linked to the orbital hierarchy and periodic-table blocks.
Orbital boxes fill from lower to higher energy, singly before pairing and with opposite paired spins, with separate corrected rows for chromium and copper.
Orbital boxes fill from lower to higher energy, singly before pairing and with opposite paired spins, with separate corrected rows for chromium and copper.
Orbital boxes fill from lower to higher energy, singly before pairing and with opposite paired spins, with separate corrected rows for chromium and copper.
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

[ 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

Drawing canvas

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?