The big idea: Electrons in an atom are not just 'in shells' — they sit in a tidy hierarchy of energy levels, sublevels and orbitals.
- Main energy level (n) — the big shell; n = 1, 2, 3, … Higher n means higher energy and further from the nucleus. - Sublevel — within each main level are sublevels named s, p, d, f. They differ slightly in energy (s < p < d < f). - Orbital — a region that holds up to 2 electrons. An s sublevel has 1 orbital, p has 3, d has 5, f has 7.
Oxygen (1s² 2s² 2p⁴). Boxes stack up the energy axis: each main level (n) holds sublevels (s, p), each sublevel holds orbitals (the boxes), each orbital holds up to 2 electrons. The 2p sublevel is higher in energy than 2s.
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Three words, one nesting: Read it inwards: a main level contains sublevels, a sublevel contains orbitals, and each orbital holds 2 electrons maximum.
The first main level (n = 1) has only an s sublevel; from n = 2 a p sublevel appears; from n = 3 a d sublevel appears.
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An orbital is the region around the nucleus where an electron is most likely to be found. Each type of sublevel has its own characteristic shape — this is a favourite Paper 1A 'pick the correct diagram' question.
s = sphere, p = dumbbell: Picture the orbital shapes with the nucleus at the centre:
- An s orbital is spherical — a single round cloud symmetrical around the nucleus, like a ball. - A p orbital is shaped like a dumbbell — two lobes pointing in opposite directions through the nucleus. The three p orbitals point along three perpendicular directions (often written px, py, pz).
s orbital
- Spherical — one round lobe.
- Symmetrical around the nucleus.
- 1 orbital per s sublevel → holds 2 electrons.
p orbital
- Dumbbell — two lobes through the nucleus.
- Three of them lie along perpendicular axes (px, py, pz).
- 3 orbitals per p sublevel → hold 6 electrons.
Pick-the-diagram tip: In the Paper 1A 'choose the correct orbital diagram' question, the right answer shows a round s orbital and a two-lobed dumbbell p orbital — both centred on the nucleus. Reject any option that draws p as a single sphere or s as a dumbbell.
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Because each orbital holds 2 electrons, you can count the maximum electrons in any sublevel or main level just by counting orbitals.
Electrons per sublevel: Multiply the number of orbitals by 2:
- s = 1 orbital → 2 electrons - p = 3 orbitals → 6 electrons - d = 5 orbitals → 10 electrons - f = 7 orbitals → 14 electrons
| Sublevel | Number of orbitals | Max electrons (2 per orbital) |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Max electrons in a main level = 2n²: Add up the sublevels in a main level and you get a neat pattern, 2n²:
- n = 1: 1s → 2 electrons (2 × 1²) - n = 2: 2s 2p → 8 electrons (2 × 2²) - n = 3: 3s 3p 3d → 18 electrons (2 × 3²) - n = 4: 4s 4p 4d 4f → 32 electrons (2 × 4²)
| Main level (n) | Sublevels present | Max electrons (2n²) |
|---|---|---|
| 1 | 1s | 2 |
| 2 | 2s 2p | 8 |
| 3 | 3s 3p 3d | 18 |
| 4 | 4s 4p 4d 4f | 32 |
Electrons fill the lowest-energy sublevel first (the Aufbau idea). The sublevel energies overlap between main levels: 4s is slightly lower than 3d, so 4s fills before 3d. Within a sublevel, electrons spread out singly before pairing up (Hund's rule).
Nitrogen (1s² 2s² 2p³). The three 2p electrons go into separate orbitals with parallel spins first (Hund's rule) before any pairing.
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Filling order (Aufbau): the lowest-energy sublevel fills first. Notice 4s sits just below 3d on the energy axis, so 4s fills before 3d.
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How this is tested: S1.3.2 shows up as quick Paper 1A recall: identify the diagram of the s and p orbital shapes, or state/determine the maximum number of electrons in a given main energy level.
The shape question is pure recognition (s = sphere, p = dumbbell); the 'maximum electrons' question is a one-line 2n² calculation — so know both cold.
Two reliable marks: For 'max electrons in the nth level' use 2n². For shapes, remember s is a sphere and p is a dumbbell — and that there are 3 p orbitals, so the p sublevel holds 6 electrons.
IB-style question — sublevels of the third level
(a) State the sublevels present in the third main energy level (n = 3). (b) Deduce the maximum number of electrons that this level can hold, showing how you arrive at the value. [3]
How to score the marks
- Mark 1 — name the sublevels. The third main level contains the 3s, 3p and 3d sublevels.
- Mark 2 — count the orbitals/electrons. 3s has 1 orbital, 3p has 3, 3d has 5 → 9 orbitals in total; each holds 2 electrons, so 9 × 2 = 18 (equivalently 2n² = 2 × 3²).
- Mark 3 — state the answer. Maximum number of electrons in n = 3 is 18.
Final answer
(a) 3s, 3p and 3d. (b) 18 electrons (9 orbitals × 2, or 2 × 3² = 18).