Lithium (Li), beryllium (Be) and boron (B) are three elements found at the start of Period 2 in the periodic table. Oxygen (O) and sulfur (S) are both in Group 6. Explain how the periodic table is organised and describe the trends in properties you would expect when moving across Period 2 from left to right, and when moving down Group 6 from oxygen to sulfur.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The periodic table arranges all known elements in order of increasing atomic number. Elements in the same group share similar chemical properties.
Model answer (5 marks)
The periodic table is arranged in order of increasing atomic number (the number of protons). Elements in the same group have the same number of outer‑shell electrons, so they show similar chemical properties.
Across Period 2 from left to right the number of outer‑shell electrons increases: Li has 1, Be 2, B 3, C 4, N 5, O 6, F 7 and Ne 8 (a full outer shell). This increase leads to a gradual rise in electronegativity and ionisation energy, and a decrease in atomic radius.
Down Group 6 from O to S the number of electron shells increases (O has 2, S has 3). Both elements still have 6 valence electrons, so they retain similar chemical behaviour. However, the larger size of S reduces its electronegativity and ionisation energy compared with O, and its atomic radius is larger.
Across Period 2 from left to right the number of outer‑shell electrons increases: Li has 1, Be 2, B 3, C 4, N 5, O 6, F 7 and Ne 8 (a full outer shell). This increase leads to a gradual rise in electronegativity and ionisation energy, and a decrease in atomic radius.
Down Group 6 from O to S the number of electron shells increases (O has 2, S has 3). Both elements still have 6 valence electrons, so they retain similar chemical behaviour. However, the larger size of S reduces its electronegativity and ionisation energy compared with O, and its atomic radius is larger.
Examiner tips
- Mention atomic number and outer‑shell electrons first; link these to group similarity.
- Show the trend of increasing valence electrons across a period and explain the resulting property changes.
- Explain the increase in shell number down a group and its effect on size and electronegativity.
- Use the exact wording ‘outer‑shell electrons’ and ‘group’ as the mark scheme expects.
Common mistakes
- Confusing atomic number with mass number.
- Saying that properties change because of the number of protons instead of valence electrons.
- Failing to state that both O and S have 6 outer electrons, which is why they are in the same group.
Mark scheme (5 marks)
- Elements are arranged in order of increasing atomic number (proton number)
- Elements in the same group have the same number of outer shell electrons / the same number of electrons in their outer shell
- Across Period 2 from left to right, the number of outer shell electrons increases (from 1 in lithium to a full outer shell in neon)
- Going down Group 6 from oxygen to sulfur, the number of electron shells increases
- Going down Group 6, the elements show similar chemical properties because they have the same number of outer shell electrons / oxygen and sulfur both have 6 outer electrons
Key terms in this question
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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