1.1.4Physical Chemistry
Shells and Orbitals — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 4 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Shells and Sub-shells
Electrons are arranged in energy levels around the nucleus.
Principal shell
- Principal shell Shells are numbered n = 1, 2, 3 and 4, and the shell’s energy increases as the number n gets bigger.
Capacity
- Capacity A shell can hold a maximum of 2n² electrons, which gives 2, 8, 18 and 32 for shells 1 to 4.
Sub-shells
- Sub-shells Each shell contains sub-shells labelled s, p, d and f, with the number of sub-shells equal to the shell number up to four.
Energy order
- Energy order Within a shell the energy order is s < p < d < f, and the 4s sub-shell lies below 3d.
Part 2 of 5Orbitals
An orbital is a region of space that can hold up to two electrons.
Orbitals
Definition
- Definition An orbital is a region around the nucleus that can hold a maximum of two electrons with opposite spins.
s sub-shell
- s sub-shell One s orbital that can hold 2 electrons; its shape is spherical.
p sub-shell
- p sub-shell Three p orbitals that can hold 6 electrons in total;
- each has a dumbbell shape along one axis, at right angles to the others.
d sub-shell
- d sub-shell Five d orbitals hold up to 10 electrons, with more complex shapes that are not required in detail at AS.
| Sub-shell | Orbitals | Maximum electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
| f | 7 | 14 |
Part 3 of 5Filling Order
1
Electrons fill the lowest available energy level first.
2
The order of filling is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
3
The 4s sub-shell fills before 3d because it has slightly lower energy in a neutral atom.
4
Each orbital holds up to two electrons, which must have opposite spins.
5
Electrons occupy separate orbitals of the same sub-shell singly before pairing up.
Part 4 of 5Spin and Notation
Spin
- Spin Two electrons in one orbital spin in opposite directions, which reduces repulsion between them.
Arrows
- Arrows Spin-pair diagrams show orbitals as boxes and electrons as up or down arrows.
Notation
- Notation The notation 2p⁴ means that four electrons occupy the 2p sub-shell.
Check
- Check The sum of the superscripts in any electron configuration must equal the atomic number of the neutral atom.
Part 5 of 5Exam Checklist & Tips
1.1.4 – Key Points for the Exam
- Shell A group of orbitals with the same principal quantum number, n.
- Sub-shell A set of orbitals of one type within a shell, labelled s, p, d or f.
- Orbital A region around the nucleus that holds up to two electrons with opposite spins.
- Spin
- A property of an electron that makes it behave like a tiny magnet;
- two electrons in one orbital have opposite spins.
- A property of an electron that makes it behave like a tiny magnet;
Common Mistakes
- Saying a p sub-shell holds 2 electrons; it has three orbitals and can hold 6.
- Describing an orbital as the path an electron follows, rather than a region where an electron is likely to be found.
- Putting the 3d sub-shell before 4s when writing the order of filling for neutral atoms.
- Forgetting that two electrons in the same orbital must have opposite spins.
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