Electronic Structure (CIE A Level Chemistry)

Shells

• The arrangement of electrons in an atom is called the electronic configuration
• Electrons are arranged around the nucleus in principal energy levels or principal quantum shells
• Principal quantum numbers (n) are used to number the energy levels or quantum shells
  • The lower the principal quantum number, the closer the shell is to the nucleus
  • The higher the principal quantum number, the higher the energy of the shell

• Each principal quantum number has a fixed number of electrons it can hold
  • n = 1 : up to 2 electrons
  • n = 2 : up to 8 electrons
  • n = 3 : up to 18 electrons
  • n = 4 : up to 32 electrons

Principal quantum shells

Electrons are arranged in principal quantum shells, which are numbered by principal quantum numbers

Sub-shells

• The principal quantum shells are split into sub-shells which are given the letters s, p and d
  • Elements with more than 57 electrons also have an f shell
  • The energy of the electrons in the sub-shells increases in the order s < p < d

• The order of sub-shells appear to overlap for the higher principal quantum shells as seen in the diagram below:

Energy levels of the quantum shells

At principal quantum shell n = 3, the energy levels of the sub-shells start to overlap

Orbitals

• Sub-shells contain one or more atomic orbitals
• Orbitals exist at specific energy levels and electrons can only be found at these specific levels, not in between them
  • Each atomic orbital can be occupied by a maximum of two electrons

• This means that the number of orbitals in each sub-shell is as follows:
  • s : one orbital (1 x 2 = total of 2 electrons)
  • p : three orbitals ( 3 x 2 = total of 6 electrons)
  • d : five orbitals (5 x 2 = total of 10 electrons)
  • f : seven orbitals (7 x 2 = total of 14 electrons)

• The orbitals have specific 3-D shapes

Shapes of the electron orbitals

Representation of orbitals (the dot represents the nucleus of the atom) showing (a) spherical s orbitals and (b) p orbitals containing ‘lobes’ along the x, y and z axis

An overview of the shells, sub-shells and orbitals in an atom

s and p orbitals have different shapes

Ground state

• The ground state is the most stable electronic configuration of an atom which has the lowest amount of energy
• This is achieved by filling the sub-shells with the lowest energy first (1s)
• The order of the sub-shells in terms of increasing energy does not follow a regular pattern at n = 3 and higher

Summary of filling sub-shells 

The ground state of an atom is achieved by filling the lowest energy sub-shells first