Trends of Period 3 Elements: First Ionisation Energy (AQA A-Level Chemistry): Revision Notes

2.1.3 Trends of Period 3 Elements: First Ionisation Energy

What is Ionisation Energy?

$$X(g) \rightarrow X^+(g) + e^-$$

Trend Across Period 3 (Sodium to Argon)

As you move across Period 3 from sodium (Na) to argon (Ar), the first ionisation energy generally increases.

Reasons for the Trend

Increasing Nuclear Charge

• As you move across Period 3, the number of protons in the nucleus increases, leading to a higher nuclear charge.
• This increased charge results in a stronger electrostatic attraction between the nucleus and the outer electrons, making it more difficult to remove an electron.

Similar Shielding

• Although the nuclear charge increases, the electrons are all being added to the same energy level (third shell).
• There is no significant increase in shielding because the number of inner electron shells remains constant across the period.

Decreasing Atomic Radius

• As the nuclear charge increases, the atomic radius decreases because the electrons are pulled closer to the nucleus.
• A smaller atomic radius results in a stronger attraction between the nucleus and the outer electrons, requiring more energy to remove an electron.

Increased Energy Required

• With the increased nuclear attraction and decreasing atomic radius, more energy is required to remove the outer electron, leading to higher first ionisation energies.

Exceptions in the Trend

Drop in Group 3 (Aluminium)

The first ionisation energy of group 3 elements (like aluminium) is lower than expected due to:

• The outer electron in group 3 elements is in a p sub-shell, while in group 2, it is in an s sub-shell.
• The p sub-shell is at a higher energy level and further from the nucleus than the s sub-shell, so the outer electron in group 3 experiences more shielding and is less strongly attracted to the nucleus, making it easier to remove.

Drop in Group 6 (Sulphur)

The first ionisation energy of group 6 elements (like sulphur) is also lower than expected due to:

• Group 6 elements have a p4 electron configuration, where two electrons are in the same p orbital.
• The repulsion between the two electrons in the same orbital makes it easier to remove one of them, leading to a lower first ionisation energy.