# Dmitri Mendeleev arranged elements into a periodic table in 1869. The modern periodic table is arranged in order of atomic number. Explain how the periodic table is arranged and what is meant by periodicity, including how the properties of elements change across a period and down a group.

> OCR A-Level Chemistry B: Salters (H433) — 3.1 The periodic table and periodicity · Explain · 5 marks

> When Mendeleev published his periodic table, he left gaps for undiscovered elements and predicted their properties. The modern periodic table has since been completed and provides chemists with a powerful tool for predicting the behaviour of elements.

## Mark scheme (5 marks)

1. Elements are arranged in order of increasing atomic number
2. Elements are arranged in periods (rows) and groups (columns), where elements in the same group have the same number of outer electrons / similar chemical properties
3. Periodicity refers to the repeating pattern of properties of elements across each period
4. Across a period, the number of outer electrons increases (from 1 to 8) and the elements change from metals on the left to non-metals on the right
5. Down a group, elements have one more electron shell / become more reactive (for Group 1 metals) or less reactive (for Group 7 non-metals) as you go down

## Key terms

- [periodic table](https://www.gradenine.co.uk/glossary/periodic-table)
- [periodicity](https://www.gradenine.co.uk/glossary/periodicity)
- [period](https://www.gradenine.co.uk/glossary/period)
- [group](https://www.gradenine.co.uk/glossary/group)
- [atomic number](https://www.gradenine.co.uk/glossary/atomic-number)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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