1.3.3Additional lessons
Energy Levels, Sublevels and Orbitals — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 10 of 172
Part 1 of 4Main Energy Levels
Electrons are arranged in shells numbered n = 1, 2, 3 and so on, with n = 1 closest to the nucleus.
Energy
- The energy of the levels increases with n, and the gaps between levels become smaller as n increases.
Capacity
- The maximum number of electrons in main level n is 2n².
Examples
- The first level holds 2, the second holds 8 and the third holds 18 electrons.
Sublevels
- Each main level is divided into sublevels labelled s, p, d and f.
Part 2 of 4Sublevels and Orbitals
Sublevels and Orbitals
Orbital
- A region of space where there is a high probability of finding an electron;
- each holds a maximum of two electrons.
Spin
- Two electrons in the same orbital must have opposite spins.
s and p shapes
- An s orbital is spherical, while a p orbital has two lobes and there are three at right angles to each other.
Availability
- Level 1 has only s, level 2 has s and p, level 3 has s, p and d, and level 4 has s, p, d and f.
| Sublevel | Orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Part 3 of 4Filling Order
Sublevels do not fill strictly in order of shell number.
1
Electrons fill the lowest energy sublevel available first, which is the aufbau principle.
2
The order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s.
3
The 4s sublevel is filled before 3d because it has slightly lower energy in the neutral atom.
4
Within a sublevel, electrons occupy separate orbitals singly before pairing, as stated in Hund's rule.
5
No orbital may hold more than two electrons, and they must have opposite spins, as stated in the Pauli exclusion principle.
Part 4 of 4Exam Checklist & Tips
1.3.3 – Key Points for the Exam
- Orbital A region of space that can hold up to two electrons with opposite spins.
- Sublevel A subdivision of a main energy level labelled s, p, d or f.
- Aufbau principle Electrons occupy the lowest available energy levels first.
- Pauli exclusion principle No two electrons in an atom can have the same set of quantum numbers, so an orbital holds at most two electrons.
Common Mistakes
- Saying that a p sublevel holds 3 electrons, when it contains 3 orbitals holding 6 electrons.
- Believing that 3d fills before 4s in atoms.
- Describing orbitals as circular paths followed by electrons.
- Forgetting that the electrons in one orbital have opposite spins.
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