1.3 Electrons, Energy Levels and Atomic Orbitals

Cambridge AS Level Chemistry — 9701

2026 📋 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Understand the terms: shells, sub-shells, orbitals, principal quantum number (n), and ground state
  2. Describe the number of orbitals in s, p, and d sub-shells, and how many electrons each can hold
  3. Describe the order of increasing energy of sub-shells within the first three shells and the 4s and 4p sub-shells
  4. Describe electronic configurations showing electrons in each shell, sub-shell, and orbital
  5. Explain electronic configurations in terms of electron energy and inter-electron repulsion
  6. Determine electronic configurations of atoms and ions using full and shorthand notation
  7. Understand and use the electrons-in-boxes notation
  8. Describe and sketch the shapes of s and p orbitals
  9. Describe a free radical as a species with one or more unpaired electrons

📖 Main Content

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Section 1: Shells, Sub-shells, and Orbitals

What is a Shell?

Electrons do not float randomly around the nucleus. They are arranged in layers called shells (also called principal energy levels). Think of them like the different floors of a building — each floor is at a different height (energy level), and electrons "live" on specific floors.

Each shell is given a number called the principal quantum number, written as n.

  • n = 1 → first shell (closest to the nucleus, lowest energy)
  • n = 2 → second shell
  • n = 3 → third shell
  • n = 4 → fourth shell (furthest out, highest energy)

Key rule: The higher the principal quantum number, the further the shell is from the nucleus and the higher the energy of electrons in that shell.

Each shell can hold a maximum number of electrons:

Shell (n)Maximum electrons
n = 12
n = 28
n = 318
n = 432

What is a Sub-shell?

Each shell is divided into smaller sections called sub-shells. Sub-shells are labelled using the letters s, p, and d (and f, but you do not need to worry about f for AS Level).

  • Shell n = 1 contains: only an s sub-shell → written as 1s
  • Shell n = 2 contains: s and p sub-shells → written as 2s and 2p
  • Shell n = 3 contains: s, p, and d sub-shells → written as 3s, 3p, and 3d
Shell (n)Sub-shells present
1s
2s, p
3s, p, d
4s, p, d (and f — not needed at AS)

Within any shell, the energy of electrons in sub-shells increases in the order: s < p < d This means s-electrons have the lowest energy within a shell, and d-electrons have the highest.

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