Electronic Configuration (CIE A Level Chemistry)

• The electron configuration gives information about the number of electrons in each shell, sub-shell and orbital of an atom
• The sub-shells are filled in order of increasing energy

Representing electronic configurations

The electron configuration shows the number of electrons occupying a sub-shell in a specific shell

• Electrons can be imagined as small spinning charges which rotate around their own axis in either a clockwise or anticlockwise direction
  • The spin of the electron is represented by its direction

Electron spin diagram

Electrons can spin either in a clockwise or anticlockwise direction around their own axis

• Electrons with similar spin repel each other which is also called spin-pair repulsion
• Electrons will therefore occupy separate orbitals in the same sub-shell to minimise this repulsion and have their spin in the same direction

Electron configuration: three electrons in a p sub-shell

When there are three electrons in a p sub-shell, one electron will go into each p_x, p_y and p_z orbital

• Electrons are only paired when there are no more empty orbitals available within a sub-shell in which case the spins are the opposite spins to minimise repulsion

Electron configuration: four electrons in a p subshell

When there are four electrons in a p subshell, one p orbital contains 2 electrons with opposite spin and two orbitals contain one electron only

• The principal quantum number indicates the energy level of a particular shell but also indicates the energy of the electrons in that shell
  • A 2p electron is in the second shell and therefore has an energy corresponding to n = 2

• Even though there is repulsion between negatively charged electrons (inter-electrons repulsion), they occupy the same region of space in orbitals
• This is because the energy required to jump to successive empty orbital is greater than the inter-electron repulsion
• For this reason, they pair up and occupy the lower energy levels first

• The electron configuration can also be represented using the electrons in boxes notation
• Each box represents an atomic orbital
• The boxes are arranged in order of increasing energy from lowest to highest
• The electrons are represented by opposite arrows to show the spin of the electrons
  • Eg. the box notation for titanium is shown below
  • Note that since the 3d sub-shell cannot be either full or half full, the second 4s electron is not promoted to the 3d level and stays in the 4s orbital

Arrangement of electrons in titanium

The electrons in titanium occupy the lowest energy levels first before filling those with higher energy

Free radicals

• A free radical is a species with one or more unpaired electron
• The unpaired electron in the free radical is shown as a dot
  • Eg. a chlorine free radical has the electron configuration 1s²2s²2p⁶3s²3p⁵
  • Two of the three p orbitals have paired electrons whereas one of them has an unpaired electron

Arrangement of electrons in a chlorine radical

One of the p orbitals has unpaired electrons in a chlorine radical