# Mendeleev developed an early version of the periodic table. Describe how Mendeleev arranged the elements in his periodic table and explain two ways in which his arrangement differed from the modern periodic table.

> WJEC A-Level Chemistry (Wales) — 2.1 Periodic table · Describe and Explain · 5 marks

## Mark scheme (5 marks)

1. Mendeleev arranged elements in order of increasing atomic mass / 'atomic weights'
2. Elements with similar chemical properties were placed in the same group / column
3. Mendeleev left gaps in his table where no known element fitted / where elements had not yet been discovered
4. The modern periodic table is arranged in order of increasing atomic number (number of protons), not atomic mass
5. In Mendeleev's table the order was sometimes not correct because different isotopes of the same element have different masses / atomic masses of some elements did not fit the expected order

## Key terms

- [periodic table](https://www.gradenine.co.uk/glossary/periodic-table)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe and 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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