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Electron Shells, Sub-Shells & Orbitals

Chemistry: AQA A Level, topic 1.1.4

Electronic Structure

The placement of electrons in atoms is complex and there are several levels of organisation.

Electron shells

Electron shells

  • As per the Bohr model, electrons are arranged in shells.
  • These shells are defined by the principal quantum number, given the symbol 'n'.
  • The lowest energy shell has n = 1.
  • Other shells have higher energy and higher n.
  • The higher the n of an electron, the further from the nucleus it orbits.
Sub-shells

Sub-shells

  • For atoms with more than one electron, shells are split into sub-shells that have slightly different energies.
  • The difference in energy between sub-shells is much less than the difference in energy between shells.
  • A shell with a given n will have n sub-shells.
  • E.g. the n = 3 electron shell has three sub-shells.
Orbitals

Orbitals

  • Sub-shells are composed of orbitals.
  • Orbitals in the same sub-shell have the same energy.
  • Each orbital can hold a maximum of two electrons.
Bringing it all together

Bringing it all together

  • Electrons occupy orbitals.
  • An orbital can hold a maximum of two electrons.
  • A set of orbitals of the same energy is called a sub-shell.
  • Sub-shells make up different shells of electrons.
  • Different sub-shells do not have the same energy.

Focusing on Orbitals

The concept of an orbital is key to understanding many chemical properties. One orbital can only contain up to two electrons.

Types of orbitals

Types of orbitals

  • Orbitals are labelled by letters.
  • The first three orbitals are called s, p and d:
  • An s sub-shell is made of one s orbital.
  • A p sub-shell is made of three p orbitals.
  • A d sub-shell is made of five d orbitals.
  • Orbitals of exactly the same energy are called 'degenerate'.
Shells and sub-shells

Shells and sub-shells

  • The n = 1 shell has one sub-shell.
  • It is an s sub-shell.
  • The n = 2 shell has two sub-shells.
  • They are an s sub-shell and a p sub-shell.
  • The n = 3 shell has three sub-shells.
  • It has an s sub-shell, a p sub-shell and a d sub-shell.
Filling orbitals

Filling orbitals

  • Orbitals are filled from the lowest energy to the highest energy.
  • The usual energy order is from s ⇒ p ⇒ d.
  • The transition metals are an anomaly. The 4s sub-shell is of a lower energy than the 3d sub-shell, and so here the filling order is: 3s ⇒ 3p ⇒ 4s ⇒ 3d.

Test yourself on Electron Shells, Sub-Shells & Orbitals

  • Which of the following is true of orbitals in the same sub-shell?

  • Which of the following are true of orbitals?

  • Electronic Structure

  • Which kind of sub-shell contains five orbitals?

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