# Mendeleev arranged the elements in order of atomic weights and left gaps in his periodic table. Explain why Mendeleev left gaps in his periodic table and why his ordering of some elements was not quite correct.

> WJEC A-Level Chemistry (Wales) — 2.3 Transition metals (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. Mendeleev left gaps because some elements had not yet been discovered / found
2. The gaps allowed elements with similar chemical properties to be placed in the same group / column
3. Mendeleev was able to predict the properties of the missing / undiscovered elements using the gaps
4. The ordering was not quite correct because atoms of the same element can have different numbers of neutrons — these are called isotopes
5. Because isotopes exist, the average atomic weight of an element does not always place it in the correct position when ordered by atomic weight alone

## 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 "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-in-order-dcf9d865) · Published by Druglandscape Ltd.