Periodic Trends
Section: 8. The Periodic Table | Syllabus: Cambridge IGCSE Chemistry (0620)
Why Elements in the Same Group Behave Similarly
Elements in the same group have the same number of outer-shell electrons. Since chemical properties are determined mainly by outer-shell electrons, elements in a group react in similar ways and form compounds with similar formulas.
| Group | Outer electrons | Shared behaviour |
|---|---|---|
| I | 1 | all form 1+ ions, react vigorously with water |
| VII | 7 | all form 1− ions, exist as diatomic molecules |
Predicting Properties from Position
Because properties change in a regular way down a group (or across a period), the position of an element can be used to predict its properties from the known properties of its neighbours - without needing to memorise data for every element.
Worked Example: Predicting a Group I Trend
- Question: rubidium (Group I) has a density of 1.53 g/cm³ and a melting point of 39°C. Caesium is the next element down Group I. Which of these is most likely for caesium: (A) density 1.20 g/cm³, m.p. 29°C (B) density 1.20 g/cm³, m.p. 49°C (C) density 1.90 g/cm³, m.p. 29°C (D) density 1.90 g/cm³, m.p. 49°C? [1]
- Answer: C. Down Group I, density increases (so caesium's must be > 1.53) and melting point decreases (so caesium's must be < 39°C) - only option C satisfies both trends.
Given data for known elements in a group, extend the trend to predict an unknown element's value
Common Mistakes
- ❌ Assuming every physical property increases down every group
- ✓ check the specific trend for that property and group - e.g. density increases down Group I, but melting point decreases down the same group
- ❌ Predicting an unfamiliar element's properties from memory rather than from the given data
- ✓ these questions always supply the data you need (for a neighbouring element) - read the trend from what's given, don't guess from memorised values
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