1.2.3Physical Chemistry
Orbitals — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 7 of 42
Part 1 of 5What an Orbital Is
Orbitals combine to make up each subshell.
Definition
- An atomic orbital is a region of space around the nucleus that can hold up to two electrons.
Spin
- Two electrons in the same orbital must have opposite spins, shown as one arrow up and one arrow down.
Probability
- An orbital shows where an electron is most likely to be found rather than a fixed path.
Count
- Each s subshell has one orbital, each p subshell three, each d subshell five and each f subshell seven.
Part 2 of 5Shapes
Shapes
s orbital
- Spherical and centred on the nucleus, with the 2s orbital larger than 1s.
p orbitals
- Dumbbell-shaped, with lobes on either side of the nucleus and three orientations along the x, y and z axes.
Degenerate
- The three p orbitals in a subshell have equal energy and differ only in direction.
Size
- Orbitals in higher shells have the same shape type but are larger and farther from the nucleus.
| Subshell | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Part 3 of 5Filling Rules
1
The aufbau principle says electrons fill the lowest energy orbitals first.
2
Each orbital holds a maximum of two electrons, and they must have opposite spins.
3
Hund's rule says that orbitals of equal energy are occupied singly first, with parallel spins, before any pairing.
4
This minimises repulsion between electrons, so nitrogen has three 2p electrons in separate orbitals.
5
Oxygen has a fourth 2p electron which must pair with one already present, adding repulsion and lowering its first ionisation energy slightly.
Part 4 of 5Worked Example
Question Draw, in words, the 2p arrangement of nitrogen (Z = 7) and oxygen (Z = 8) and state the number of unpaired electrons in each.
- Nitrogen1s² 2s² 2p³, so three electrons occupy three 2p orbitals.
- By Hund's rule each 2p orbital has one electron with parallel spin, so nitrogen has 3 unpaired electrons.
- Oxygen1s² 2s² 2p⁴, with four electrons in the three 2p orbitals.
- One orbital must hold a pair, so oxygen has 2 unpaired electrons.
Answer Nitrogen has 3 unpaired electrons and oxygen has 2.
Part 5 of 5Exam Checklist & Tips
1.2.3 – Key Points for the Exam
- Atomic orbital A region of space around the nucleus that holds a maximum of two electrons.
- Spin A property that gives two electrons in the same orbital opposite directions, shown by arrows.
- Hund's rule Degenerate orbitals fill singly before any are doubly occupied.
- Degenerate orbitals Orbitals within a subshell that have the same energy.
Common Mistakes
- Saying that each p subshell has one orbital; it has three.
- Drawing two electrons in an orbital with parallel arrows.
- Pairing electrons in a p subshell before each orbital contains one.
- Describing an orbital as an orbit; it is a region where the electron is likely to be found.
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