Dmitri Mendeleev arranged elements into a periodic table in 1869. The modern periodic table is arranged in order of atomic number. Explain how the periodic table is arranged and what is meant by periodicity, including how the properties of elements change across a period and down a group.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When Mendeleev published his periodic table, he left gaps for undiscovered elements and predicted their properties. The modern periodic table has since been completed and provides chemists with a powerful tool for predicting the behaviour of elements.
Model answer (5 marks)
Elements are arranged in order of increasing atomic number.
The table is divided into periods (rows) and groups (columns). Elements in the same group have the same number of outer (valence) electrons and therefore similar chemical properties.
Periodicity is the repeating pattern of element properties that occurs as you move across each period.
Across a period the number of outer electrons rises from 1 to 8, and the elements change from metals on the left to non‑metals on the right.
Down a group the elements have one more electron shell; for Group 1 metals the reactivity increases, while for Group 7 non‑metals the reactivity generally decreases as you go down.
The table is divided into periods (rows) and groups (columns). Elements in the same group have the same number of outer (valence) electrons and therefore similar chemical properties.
Periodicity is the repeating pattern of element properties that occurs as you move across each period.
Across a period the number of outer electrons rises from 1 to 8, and the elements change from metals on the left to non‑metals on the right.
Down a group the elements have one more electron shell; for Group 1 metals the reactivity increases, while for Group 7 non‑metals the reactivity generally decreases as you go down.
Examiner tips
- Use the exact phrases "increasing atomic number", "periods and groups", "same number of outer electrons", "repeating pattern of properties", "metals on the left to non‑metals on the right", "one more electron shell".
- Show the change in outer electrons (1→8) and the trend in reactivity for Group 1 and Group 7.
- Keep the answer concise – one sentence per point to hit all 5 marks.
Common mistakes
- Writing the table in order of atomic mass instead of atomic number.
- Confusing the trend of reactivity down a group – stating it increases for all groups.
- Omitting the change from metals to non‑metals across a period.
Mark scheme (5 marks)
- Elements are arranged in order of increasing atomic number
- Elements are arranged in periods (rows) and groups (columns), where elements in the same group have the same number of outer electrons / similar chemical properties
- Periodicity refers to the repeating pattern of properties of elements across each period
- Across a period, the number of outer electrons increases (from 1 to 8) and the elements change from metals on the left to non-metals on the right
- Down a group, elements have one more electron shell / become more reactive (for Group 1 metals) or less reactive (for Group 7 non-metals) as you go down
Key terms in this question
periodic table · periodicity · period · group · atomic number
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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