IB Chemistry SL — Structure 1.3: Electron Configurations

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Why do elements give line spectra, not continuous ones?

This is the Standard Level lesson, covering S1.3.1–S1.3.5. Taking Chemistry at Higher Level? The HL version adds ionisation energy (S1.3.6–S1.3.7) — using the hydrogen spectrum's convergence and successive ionisation energies as evidence for the levels.

👆 Click the electron transitions · drag the atomic-number slider

1. Emission spectra S1.3.1–2

When electrons are excited to higher energy levels and then fall back down, they emit photons of specific energies — giving a line spectrum, not a continuous rainbow. The fixed lines are direct evidence that electrons occupy discrete energy levels. The lines converge (get closer) at higher energy. Click a jump down to level n=2 (the visible Balmer lines):

Hydrogen energy levels · click a transition to n=2

Click a transitionEach downward jump to n=2 emits a photon of visible light. Bigger drop → more energy → shorter wavelength (bluer).

A continuous spectrum is an unbroken rainbow; a line (emission) spectrum is just a few coloured lines on black — the fingerprint of the element.

2. Energy levels, sub-levels & orbitals S1.3.3–4

Each main energy level n holds up to 2n² electrons (n=1 → 2; n=2 → 8; n=3 → 18). Each level is divided into sub-levels — s, p, d, f — of rising energy, made of orbitals. Each orbital holds 2 electrons of opposite spin. An s sub-level has 1 orbital (2 e⁻), p has 3 (6 e⁻), d has 5 (10 e⁻).

Orbital shapes

3. Build an electron configuration S1.3.5

Electrons fill from lowest energy up (Aufbau), one per orbital before pairing (Hund), with opposite spins (Pauli). Drag the slider to change the atomic number and watch the orbitals fill:

Electron configuration builder · Z = 1 to 36

Z = 11

Sodium

Two exceptions you must know: chromium (Cr, Z=24) is [Ar]3d⁵4s¹ and copper (Cu, Z=29) is [Ar]3d¹⁰4s¹ — a half-full or full d sub-level is extra stable, so one 4s electron shifts into 3d.

Going to HL? The HL lesson adds ionisation energy (S1.3.6–7): the convergence limit of the hydrogen spectrum gives the ionisation energy, and the pattern of successive ionisation energies is direct evidence for the main levels and sub-levels. → Open the HL version

Learn Electron Configurations step by step

Free interactive mini-lessons: short screens, worked examples and quick checks. Pick your exam board to open its course map, then tap L on this topic:

HL SL

Common mistakes examiners see

Why do elements give line spectra, not continuous ones?✗ Because electrons can have any energy. ✓ Electrons occupy discrete energy levels, so only fixed energy jumps are possible — giving specific lines.

Which fills first, 4s or 3d?✗ 3d, because 3 is less than 4. ✓ 4s fills before 3d because it is slightly lower in energy (Aufbau). But when forming ions, 4s electrons are removed first.

What is the electron configuration of chromium (Z=24)?✗ [Ar]3d⁴4s². ✓ [Ar]3d⁵4s¹ — a half-filled d sub-level is more stable, so one 4s electron moves to 3d.

How many electrons fit in a p sub-level?✗ 2. ✓ 6 — a p sub-level has 3 orbitals, each holding 2 electrons.

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Electron Configurations