# Mendeleev arranged elements in his early periodic table based on 'atomic weights'. Explain why, in some cases, this led to elements being placed in the wrong order.

> WJEC GCSE Chemistry (Wales) — 2.1 Bonding, structure and properties · Explain · 4 marks

> When Mendeleev published his periodic table, he arranged elements in order of increasing atomic weight. He also left gaps in his table where he predicted undiscovered elements would fit.

## Mark scheme (4 marks)

1. Different atoms of the same element can have different numbers of neutrons
2. These atoms are called isotopes
3. Isotopes have different masses / atomic weights, so the average atomic weight of an element is affected by which isotopes are present and their abundance
4. This meant the order based on atomic weight did not always match the correct order based on atomic number (number of protons), placing some elements in the wrong position / group

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