Sodium (Na) and potassium (K) are both elements in Group 1 of the periodic table. Chlorine (Cl) and bromine (Br) are both elements in Group 7. Explain why elements in the same group of the periodic table have similar chemical properties, and describe one trend in reactivity as you move down Group 1 and one trend in reactivity as you move down Group 7.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The periodic table arranges elements in order of increasing atomic number. Elements in the same group have the same number of electrons in their outermost shell. Group 1 metals react vigorously with water, and Group 7 non-metals react with metals to form salts.
Model answer (5 marks)
Elements in the same group have the same number of electrons in their outermost shell, so they have the same valence configuration. Because the valence configuration determines how an element reacts, elements in the same group react in the same or similar way, giving them similar chemical properties.
In Group 1 the reactivity increases down the group. The outer electron is further from the nucleus and is shielded by more inner electrons, so it is lost more easily.
In Group 7 the reactivity decreases down the group. The outer electrons are further from the nucleus, so the attraction to the nucleus is weaker and it becomes harder for the element to gain an electron.
In Group 1 the reactivity increases down the group. The outer electron is further from the nucleus and is shielded by more inner electrons, so it is lost more easily.
In Group 7 the reactivity decreases down the group. The outer electrons are further from the nucleus, so the attraction to the nucleus is weaker and it becomes harder for the element to gain an electron.
Examiner tips
- State the common valence configuration first, then link to similar reactivity. Mention the trend (increase/decrease) before giving the reason. Use the terms ‘outer electron’, ‘shielding’, ‘loss/gain of electron’ as expected.
- common_mistakes
- :
- Saying the trend without linking it to electron loss/gain. Using ‘more electronegative’ or ‘more reactive’ without explaining the shell effect. Confusing the direction of the trend (e.g. saying Group 1 becomes less reactive down the group).
Mark scheme (5 marks)
- Elements in the same group have the same number of electrons in their outermost (outer) shell
- Because they have the same number of outer electrons, elements in the same group react in the same / similar way (giving them similar chemical properties)
- Reactivity increases as you go down Group 1
- Reactivity decreases as you go down Group 7
- A correct reason for either Group 1 or Group 7 reactivity trend linked to the outer shell / ease of losing or gaining an electron (e.g. outer shell is further from the nucleus so electron is lost more easily in Group 1, OR outer shell is further from the nucleus so gaining an electron is harder in Group 7)
Key terms in this question
group · reactivity · periodic table
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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