IB Chemistry HL — Structure 1.3: Electron Configurations

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Why do elements give line spectra, not continuous ones?

This is the Higher Level lesson. The core S1.3.1–S1.3.5 (spectra, levels, orbitals, configurations) is shared with SL; the HL-only statements S1.3.6–S1.3.7 — ionisation energy carry a purple HL badge.

Core — SL & HL

HL AHL only

👆 Click the transitions · drag the slider · click the IE bars

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.

4. Ionisation energy HL

The first ionisation energy is the energy to remove one mole of electrons from one mole of gaseous atoms. Two pieces of evidence at HL:

Convergence of the hydrogen spectrum S1.3.6

The emission lines get closer together (converge) at higher frequency. At the convergence limit the electron has been removed entirely — so this limit corresponds to the ionisation energy. Measuring the frequency at convergence lets you calculate it (E = hf).

Successive ionisation energies S1.3.7

Keep removing electrons one at a time and the energy needed rises — but with big jumps whenever you break into a new, closer main energy level. The pattern of jumps reveals the electron arrangement. Pick an element and click the bars:

Successive ionisation energies (log scale) · click a bar

Click a barEach bar is the energy to remove the next electron. Watch for the big jumps — they mark where a whole energy level has been emptied.

Sodium jumps after the 1st electron (2,8,1) and again after the 9th (2,8,1) — the 1 : 8 : 2 pattern of its shells, read straight off the graph.

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:

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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.

What does a big jump in successive ionisation energies show?✗ The atom has run out of electrons. ✓ That the next electron comes from a closer, lower main energy level — so the jumps reveal the shell structure (e.g. Na 2,8,1). HL

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Electron Configurations