# Mendeleev arranged the elements into a periodic table. Explain two reasons why Mendeleev's arrangement of the elements was not fully accepted by other scientists at the time.

> WJEC GCSE Chemistry (Wales) — 2.5 The production of important chemicals · Explain · 4 marks

## Mark scheme (4 marks)

1. Mendeleev left gaps in his table (where elements had not yet been discovered/found)
2. Other scientists were not convinced by / did not accept the idea of leaving gaps for undiscovered elements, as there was no evidence those elements existed
3. In some cases Mendeleev changed the order of elements (based on atomic weights/mass) so that elements with similar chemical properties were placed in the same group
4. The ordering by atomic weight was sometimes incorrect because different isotopes of an element have different masses, making the atomic weight unreliable as a basis for ordering

## Key terms

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

## Related

- [Revision notes for WJEC GCSE Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/mendeleev-arranged-the-elements-into-a-b7efde6f) · Published by Druglandscape Ltd.