This PowerPoint lesson explains the trends in atomic and ionic radius, helping IBDP Chemistry students understand periodicity through effective nuclear charge and electron shielding. Designed for first assessment 2025, it develops the conceptual depth required to confidently explain and compare atomic and ionic sizes in exam-style questions.
Atomic radius is defined as half the distance between the nuclei of two identical atoms joined by a covalent bond, since the edge of an atom cannot be measured directly in the quantum model. This lesson carefully builds this definition and introduces the concept of effective nuclear charge (Zₑff), explaining how shielding by inner electrons reduces the full nuclear attraction felt by valence electrons.
Students are guided through the periodic trends: atomic radius increases down a group due to additional electron shells and increased shielding, and decreases across a period due to increasing nuclear charge within the same principal energy level. Deviations and measurement differences (covalent vs van der Waals radii) are clearly addressed to strengthen exam accuracy.
The lesson then extends to ionic radii, comparing cations and anions with their parent atoms and explaining size changes in terms of electron–electron repulsion and nuclear attraction. A major focus is placed on isoelectronic species, where students compare ions with the same number of electrons and determine size based on proton number.
Structured explanations, worked examples, periodic table references, and clear summary slides support understanding. Starter activities and practice questions are included for consolidation or assessment.
This resource is suitable for SL and HL students and supports teaching of periodicity in Topic 3.
File type included: PowerPoint (.pptx)
Last updated: January 2026 – explanations refined, isoelectronic comparisons clarified, and content checked against the 2025 IB Chemistry syllabus.
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