Electrons, energy levels and atomic orbitals
Cambridge A-Level Chemistry (9701) · Unit 1: Atomic structure · 9 flashcards
Electrons, energy levels and atomic orbitals is topic 1.3 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 1 — Atomic structure , alongside Particles in the atom and atomic radius, Isotopes and Ionisation energy. In one line: A shell is a region around the nucleus with a specific energy level (principal quantum number, n). A sub-shell is a group of orbitals within a shell that have the same energy level (s, p, d). An orbital is a region of space within a sub-shell that can hold a maximum of two electrons.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 9 flashcards — 3 definitions and 6 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Some cards in this deck are on other syllabus topics: 1.4 Ionisation energy. Each card is labelled with its topic.
The terms 'shell', 'sub-shell', and 'orbital' in the context of atomic structure
A shell is a region around the nucleus with a specific energy level (principal quantum number, n). A sub-shell is a group of orbitals within a shell that have the same energy level (s, p, d). An orbital is a region of space within a sub-shell that can hold a maximum of two electrons.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand what is meant by an orbital, a sub-shell and a shell; by the principal quantum number, n; and by ground state, limited to electronic configuration.
- Describe how many orbitals s, p and d sub-shells each contain, and the maximum number of electrons each of these sub-shells can hold.
- Describe how the sub-shells in shells 1 to 3, plus 4s and 4p, are ordered by increasing energy.
- Describe electronic configurations, showing how many electrons occupy each shell, each sub-shell and each orbital.
- Explain electronic configurations using the energies of the electrons and the repulsion between electrons.
- Determine, from the proton (atomic) number and charge, the electronic configuration of an atom or ion, written either in full (for Fe: 1s²2s²2p⁶3s²3p⁶3d⁶4s²) or in shorthand (for Fe: [Ar] 3d⁶4s²).
- Understand and use 'electrons in boxes' notation, where each orbital is a box and each electron an arrow; for example, Fe is [Ar] followed by five 3d boxes (one with a pair of opposite arrows, four with a single arrow) and a 4s box holding a pair of opposite arrows.
- Sketch s orbitals and p orbitals and describe their shapes.
- Describe what a free radical is: a species with at least one unpaired electron.
Also in this deck: 1.4 Ionisation energy
- Define first ionisation energy (IE) and use the term.
- Construct equations showing the first, the second and further ionisation energies.
- Identify and explain how ionisation energies change going down a group and along a period of the Periodic Table.
- Identify how the successive ionisation energies of one element vary, and explain the pattern.
- Understand that ionisation energy arises from the nucleus attracting the outer electron.
- Explain which factors affect the ionisation energies of elements: nuclear charge, spin-pair repulsion, atomic or ionic radius, and shielding from inner shells and inner sub-shells.
- Use data on successive ionisation energies to deduce an element's electronic configuration.
- Use data on successive ionisation energies to deduce where an element sits in the Periodic Table.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in these topics
- › Look for the largest 'jump' in energy; a jump after four electrons indicates the fifth is coming from a closer, full inner shell.
- › State that shielding is 'similar' across a period because electrons are added to the same main energy level, while nuclear charge increases.
- › Always include gaseous state symbols (g) when writing equations for ionisation energy or electron affinity.
- › Explain the two dips in first ionisation energy across Period 3: from Mg to Al the electron is removed from a 3p orbital, higher in energy and shielded by the 3s electrons; from P to S it is removed from a doubly occupied 3p orbital, where paired electrons repel.
Topic 1.3 · Electrons, energy levels and atomic orbitals
Define the terms 'shell', 'sub-shell', and 'orbital' in the context of atomic structure.
A shell is a region around the nucleus with a specific energy level (principal quantum number, n). A sub-shell is a group of orbitals within a shell that have the same energy level (s, p, d). An orbital is a region of space within a sub-shell that can hold a maximum of two electrons.
Topic 1.3 · Electrons, energy levels and atomic orbitals
What are the number of orbitals in s, p, and d sub-shells, and how many electrons can each sub-shell hold?
An s sub-shell has 1 orbital and can hold 2 electrons. A p sub-shell has 3 orbitals and can hold 6 electrons. A d sub-shell has 5 orbitals and can hold 10 electrons.
Topic 1.3 · Electrons, energy levels and atomic orbitals
Describe the order of increasing energy of the sub-shells within the first three shells, and the 4s and 4p sub-shells.
The order of increasing energy is: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p. Note that the 4s sub-shell fills before the 3d sub-shell due to lower energy.
Topic 1.3 · Electrons, energy levels and atomic orbitals
Write the full electronic configuration for an atom of chlorine (Cl), given its atomic number is 17.
The electronic configuration for chlorine (Z=17) is 1s²2s²2p⁶3s²3p⁵. This notation indicates the number of electrons in each subshell.
Topic 1.3 · Electrons, energy levels and atomic orbitals
Explain why electrons fill the 4s sub-shell before the 3d sub-shell.
Although the 3d subshell belongs to the n=3 shell and the 4s belongs to the n=4 shell, the 4s subshell is at a slightly lower energy level than the 3d subshell. Therefore, the 4s subshell is filled before the 3d.
Topic 1.3 · Electrons, energy levels and atomic orbitals
Use the 'electrons in boxes' notation to show the electronic configuration of iron (Fe), given its atomic number is 26.
Fe: [Ar] then the 3d sub-shell as five boxes: one box with a pair of opposite arrows (↑↓) and four boxes with a single arrow each (↑ ↑ ↑ ↑), then the 4s box with a pair of opposite arrows (↑↓). Written out: [Ar] 3d ↑↓ ↑ ↑ ↑ ↑ 4s ↑↓, which is [Ar] 3d⁶4s². Electrons occupy the 3d orbitals singly before pairing, so Fe has four unpaired electrons.
Topic 1.3 · Electrons, energy levels and atomic orbitals
Sketch the shapes of an s orbital and a p orbital.
An s orbital is spherical and centred around the nucleus. A p orbital has a dumbbell shape, with two lobes along one axis (x, y, or z).
Topic 1.3 · Electrons, energy levels and atomic orbitals
Define a free radical and provide an example.
A free radical is a species with one or more unpaired electrons. Free radicals are highly reactive due to their tendency to pair their unpaired electrons.
Topic 1.4 · Ionisation energy
What is ionisation energy and what state must the atom be in?
Ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms or ions. Each atom or ion described will be in the ground state
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Electrons, energy levels and atomic orbitals sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Electrons, energy levels and atomic orbitals deck
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