Factors Affecting Ionisation Energies

Chemistry: AQA A Level, topic 1.1.7

The main factors affecting ionisation energies are the nuclear charge, the distance from the nucleus and electron shielding.

The greater the number of protons in the nucleus, the greater the attraction of the electron to the nucleus.
A greater attraction of the electron means more energy is needed to remove the electron.
This means that ionisation energy is greater.

Electrostatic attraction decreases sharply with distance.
This means that less energy is needed to remove electrons which are further away.
This means that as distance increases, ionisation energy decreases.
In practice, this means that the higher the principal quantum number of an electron, the lower its ionisation energy.

The greater the number of electrons between the nucleus and the outer electrons, the lower the effective nuclear charge.
This is because the positive charge felt by the electron is reduced by the electrons in between.
This means that the greater the number of electrons, the lower the ionisation energy.

Experimental Evidence for Shells

Ionisation energies can be used to show the presence of shells in atoms.

Above shows the trend in successive ionisation energies of sodium and forms a crucial piece of evidence for the shell structure of an atom.

Consider the sodium atom of electron configuration 1s²2s²2p⁶3s¹
Its first ionisation energy corresponds to the removal of an electron from the n = 3 shell.
Its second ionisation energy corresponds to the removal of an electron from the n = 2 shell.
Between the third and ninth ionisation energies, an electron is being removed from the n = 2 shell of an increasingly positive ion.
Its tenth ionisation energy corresponds to the removal of an electron from the n = 1 shell.

Ionisation energy increases with proximity to the nucleus.
The n = 1 shell will have a higher ionisation energy than n = 2 shell.
The n = 2 shell will have a higher ionisation energy than n = 3 shell.
- Within the n = 2 shell, successive ionisation energies will rise as the ion becomes more positively charged.

Which of the following would decrease the ionisation energy?
We see large jumps in the trend at the 2nd ionisation and the 10th ionisation. Why is this?
What is the lowest ionisation energy of sodium?
Increasing Ionisation Energy (most energy on the top)
What is the general trend of successive ionisation energies for sodium?