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.
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
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- understand the terms: • shells, sub-shells and orbitals • principal quantum number (n) • ground state, limited to electronic configuration
- describe the number of orbitals making up s, p and d sub-shells, and the number of electrons that can fill s, p and d sub-shells
- describe the order of increasing energy of the sub-shells within the first three shells and the 4s and 4p sub-shells
- describe the electronic configurations to include the number of electrons in each shell, sub-shell and orbital
- explain the electronic configurations in terms of energy of the electrons and inter-electron repulsion
- determine the electronic configuration of atoms and ions given the atomic or proton number and charge, using either of the following conventions:
- understand and use the electrons in boxes notation e.g. for Fe: [Ar]
- describe and sketch the shapes of s and p orbitals
- describe a free radical as a species with one or more unpaired electrons 1.4 Ionisation energy In 1.4 each atom or ion described will be in the ground state. Only the elements hydrogen to krypton will be assessed. Learning outcomes
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 Electrons, energy levels and atomic orbitals
- › Always define relative atomic mass as the weighted average mass of atoms of an element compared to 1/12th the mass of a carbon-12 atom.
- › Specify the exact atomic orbitals involved in hybridisation, such as the sp hybridised orbitals of nitrogen in the N2 molecule.
- › Look for the largest 'jump' in energy; a jump after four electrons indicates the fifth is coming from a closer, full inner shell.
- › Use the syllabus definition: nucleon number is the total number of protons and neutrons in the nucleus of an atom.
- › State that shielding is 'similar' across a period because electrons are added to the same main energy level, while nuclear charge increases.
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.
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.
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.
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.
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.
Use the 'electrons in boxes' notation to show the electronic configuration of iron (Fe), given its atomic number is 26.
Fe: [Ar] 4s² 3d⁶. This shorthand notation uses the preceding noble gas (Argon) to represent the filled inner shells, followed by the filling of the valence shell.
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).
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.
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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