# Argon (Ar), potassium (K) and calcium (Ca) are three elements found in Period 3 and Period 4 of the periodic table. Explain how the properties of these three elements reflect the periodic pattern of the periodic table, including reference to their group positions and typical chemical behaviour.

> OCR A-Level Chemistry A (H432) — 3.1 The periodic table and periodicity · Explain · 5 marks

> When Mendeleev arranged elements in order of atomic mass, argon appeared to be in the wrong group. Modern periodic tables arrange elements in order of atomic number, placing argon in Group 0, potassium in Group 1 and calcium in Group 2.

## Mark scheme (5 marks)

1. Argon is in Group 0 (the noble gases) and is unreactive / inert because its outer shell is full
2. Potassium is in Group 1 and reacts vigorously / readily, losing one electron to form a 1+ ion
3. Calcium is in Group 2 and loses two electrons to form a 2+ ion, reacting less vigorously than potassium
4. Elements in the same group have the same number of outer-shell electrons, giving them similar chemical properties / a repeating (periodic) pattern across the table
5. The modern periodic table is arranged in order of atomic number, which correctly places argon before potassium, resolving the problem Mendeleev faced when ordering by atomic mass

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry A (H432)](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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Source: [GradeNine](https://www.gradenine.co.uk/q/argon-ar-potassium-k-and-calcium-b26b2102) · Published by Druglandscape Ltd.