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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
Argon is a noble gas (Group 0). Its outer shell is full, so it is chemically inert and does not react with other elements.
Potassium is a Group 1 alkali metal. It has one valence electron, which it loses readily to form a 1+ ion, giving it a very vigorous reactivity.
Calcium is a Group 2 alkaline‑earth metal. It has two valence electrons, which it loses to form a 2+ ion. Its reactivity is high but less vigorous than potassium.
Elements in the same group have the same number of outer‑shell electrons, so they show similar chemical behaviour. This gives the periodic pattern observed across the table.
The modern periodic table is arranged by increasing atomic number, which places argon (Z=18) before potassium (Z=19) and calcium (Z=20). This correct ordering resolves the anomaly Mendeleev encountered when arranging by atomic mass.
Potassium is a Group 1 alkali metal. It has one valence electron, which it loses readily to form a 1+ ion, giving it a very vigorous reactivity.
Calcium is a Group 2 alkaline‑earth metal. It has two valence electrons, which it loses to form a 2+ ion. Its reactivity is high but less vigorous than potassium.
Elements in the same group have the same number of outer‑shell electrons, so they show similar chemical behaviour. This gives the periodic pattern observed across the table.
The modern periodic table is arranged by increasing atomic number, which places argon (Z=18) before potassium (Z=19) and calcium (Z=20). This correct ordering resolves the anomaly Mendeleev encountered when arranging by atomic mass.
Examiner tips
- Use the command word ‘Explain’ – describe the pattern and give specific examples.
- Mention the group numbers and the typical ionisation behaviour for each element.
- Show the link between outer‑shell electrons and periodicity.
- Explain why the modern table uses atomic number.”]
Mark scheme (5 marks)
- Argon is in Group 0 (the noble gases) and is unreactive / inert because its outer shell is full
- Potassium is in Group 1 and reacts vigorously / readily, losing one electron to form a 1+ ion
- Calcium is in Group 2 and loses two electrons to form a 2+ ion, reacting less vigorously than potassium
- 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
- 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 in this question
periodic table · group · period
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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