Explain why the first ionisation energy of beryllium is greater than that of boron.

IB DP Chemistry Higher Level (2023 syllabus) — S1.3 Electron configurations · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Beryllium’s outermost electron occupies a 2s orbital, while boron’s outermost electron is in a 2p orbital.

The 2p orbital lies further from the nucleus and is at a higher energy than the 2s orbital, so the 2p electron is less tightly bound.

In boron the 2p electron is partially shielded by the two 2s electrons, reducing the effective nuclear charge it experiences.

Consequently, less energy is required to remove the outermost electron from boron than from beryllium, giving beryllium a higher first ionisation energy.

Examiner tips

Use the command word ‘Explain’ – give a clear, logical sequence of reasons.
Mention the specific orbitals (2s vs 2p) and the concepts of energy and shielding.
Show the link between weaker attraction and lower ionisation energy.

Common mistakes

Confusing the order of the elements – writing that boron has a 2s electron.
Failing to mention the shielding effect of the 2s electrons on the 2p electron.
Using vague language such as ‘more stable’ without linking to orbital energy.

Mark scheme (4 marks)

Boron has its outermost electron in a 2p orbital, whereas beryllium has its outermost electron in a 2s orbital.
The 2p orbital is at a higher energy (further from the nucleus on average) than the 2s orbital, so the 2p electron is less strongly attracted to the nucleus.
The 2p electron in boron is partially shielded by the 2s electrons, reducing the effective nuclear charge experienced by the 2p electron.
Therefore less energy is required to remove the outermost electron from boron than from beryllium, giving beryllium a higher first ionisation energy.

Key terms in this question

first ionisation energy