1.3.3Additional lessons
Energy Levels, Sublevels and Orbitals — IB Chemistry SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 9 of 98
Part 1 of 4Main Energy Levels
Electrons occupy shells numbered n = 1, 2, 3 and so on.
Shell number n
- A larger value of n means a higher energy level and an average position farther from the nucleus.
Capacity
- The maximum number of electrons in shell n is 2n², giving 2, 8 and 18 for the first three shells.
Sublevels
- Each shell is divided into sublevels labelled s, p and d, with energy increasing in that order within a shell.
Spacing
- The energy gap between successive shells gets smaller as n increases, so the levels converge.
Part 2 of 4Sublevels and Orbitals
An orbital is a region of space where an electron is likely to be found, and it holds at most two electrons.
s sublevel
- One orbital that is spherical, holding up to 2 electrons.
p sublevel
- Three orbitals shaped like dumbbells at right angles, together holding up to 6 electrons.
d sublevel
- Five orbitals, together holding up to 10 electrons.
Two per orbital
- Two electrons in one orbital must have opposite spins, according to the Pauli exclusion principle.
| Shell n | Sublevels | Total orbitals | Max electrons |
|---|---|---|---|
| 1 | 1s | 1 | 2 |
| 2 | 2s 2p | 4 | 8 |
| 3 | 3s 3p 3d | 9 | 18 |
Part 3 of 4Order of Filling
Electrons enter the lowest available energy level first.
1
The order of increasing energy begins 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
2
The 4s sublevel is filled before the 3d sublevel because it is lower in energy in neutral atoms.
3
Within the same sublevel, the orbitals have equal energy and are called degenerate.
4
Atoms are most stable when electrons occupy the lowest available levels, called the ground state.
Part 4 of 4Exam Checklist & Tips
1.3.3 – Key Points for the Exam
- Orbital A region of space around the nucleus in which an electron is likely to be found, holding a maximum of two electrons.
- Sublevel A subdivision of an energy level, labelled s, p, d or f.
- Ground state The lowest energy arrangement of the electrons in an atom.
- Degenerate Having the same energy, as with the three p orbitals of one shell.
Common Mistakes
- Saying an orbital is a fixed circular path, when it is a region of probability.
- Stating that a p sublevel can hold only two electrons; it holds six.
- Putting a 3d sublevel in the second shell.
- Forgetting that two electrons in an orbital must have opposite spins.
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