1.1.4

# Electron Shells, Sub-Shells & Orbitals

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## Electronic Structure

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

### 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

• 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

• 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

• 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

• 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

• 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

• 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.