Explain how first and successive ionisation energies provide evidence for shells and subshells.

In the subtopic Electron configuration and ionisation energy, this AQA A-Level Chemistry 7405 learning objective focuses on explain how first and successive ionisation energies provide evidence for shells and subshells. It belongs to Atomic structure, so revision should stay anchored to this exact subtopic rather than drifting into a generic GCSE-level chemistry summary. Approved keywords to use include shell, subshell. Ionisation energy. means the energy required to remove an electron from an atom or ion in its gaseous state Avoid the mistake of students often confuse the trends in first and successive ionisation energies, failing to recognize that successive ionisation energies increase due to the removal of electrons from an increasingly positive ion; instead, to fix this, students should focus on understanding that as electrons are removed, the remaining electrons experience a greater effective nuclear charge, leading to higher ionisation energies. Reviewing the concepts of electron shielding and effective nuclear charge can also help clarify these trends For exam answers, when studying ionisation energies, focus on how the first and successive ionisation energies increase across a period. This trend provides evidence for the arrangement of electrons in shells and subshells