Ionisation energy

Full Notes

Ionisation energies have been covered in more detail here.
This page is just what you need to know for CIE A-level Chemistry :)

Definition of First Ionisation Energy

The first ionisation energy (IE₁) is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Equation: X(g) → X⁺(g) + e⁻

It is always measured in the gaseous state to ensure no influence from intermolecular forces.

Equations for Successive Ionisation Energies

It is possible to carry out successive ionisation, where one moles worth of electrons are removed from one moles worth of positively charged ions. We can write equations them as:

• 2nd ionisation energy: X⁺(g) → X²⁺(g) + e⁻
• 3rd ionisation energy: X²⁺(g) → X³⁺(g) + e⁻

Each successive ionisation energy is greater than the previous one, as it is harder to remove an electron from a more positively charged ion.

Matt’s exam tip

Remember that successive ionisation energies are always for one electron being removed at a time. For example, 2^nd ionisation energy isn't simply removing 2 electrons from an atom, it is first removing one electron (1^st IE) and then removing another (2^nd IE).

Trends Across a Period

Ionisation energy increases across a period (e.g. Na to Ar) as a trend because:

• Nuclear charge increases (more protons)
• Electrons are added to the same shell, so shielding stays similar
• Outer electrons are pulled in more strongly

Exceptions:

• Aluminium has a lower IE than magnesium because its outer electron is in a higher-energy 3p orbital.
• Sulfur has a lower IE than phosphorus because of electron pair repulsion in the 3p orbital, making the paired electron easier to remove.

Trends Down a Group

Ionisation energy decreases down a group (e.g. Group 2: Be to Ba) because:

• Atomic radius increases
• Shielding increases (more inner electron shells)
• Outer electrons are further from the nucleus and experience weaker attraction

This makes outer electrons easier to remove, even though nuclear charge increases.

Successive Ionisation Energy Patterns

Each extra electron removed requires more energy: as electrons are removed, the ion becomes more positive, so the remaining electrons are held more tightly.

• A large jump in ionisation energy indicates that the next electron is being removed from a new inner shell, closer to the nucleus.

Why Ionisation Energy Varies

Several factors influence ionisation energy:

• Nuclear charge: More protons = stronger attraction.
• Atomic radius: Greater distance from the nucleus = lower attraction.
• Electron shielding: Inner electrons reduce the attraction between the nucleus and outer electrons.
• Sub-shell structure: Electrons in higher-energy sub-shells (e.g. 3p vs 3s) are easier to remove.
• Spin-pair repulsion: Paired electrons in the same orbital repel each other, making them slightly easier to remove.

Because of this, by analysing where large jumps occur in successive ionisation energies, we can determine:

• How many electrons are in each shell
• The group number (number of outer electrons)
• The full electronic configuration

Deducing Element Position from IE Data

• Number of gradual increases = number of outer electrons = group number
• Number of total electrons = period (using full electron configuration)
• A large jump shows the start of a new shell

This links directly to periodicity and electron configuration.

Summary

• IE is the energy to remove an electron from gaseous atoms/ions; measured in the gas phase.
• IE generally increases across a period and decreases down a group, with noted sub-shell and pairing exceptions.
• Successive IE data show big jumps when an inner shell is reached, helping deduce configuration and position in the Periodic Table.
• Key factors: nuclear charge, atomic radius, shielding, sub-shell energy, and spin-pair repulsion.