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2023 O Level Chem P1 Q28 — Group I / Periodic Trends Solution

Group I / Periodic Trends 2023 O Level Pure Chemistry · Paper 1 · Q28

Question

Which statement about the alkali metals is correct?

AThe outer electron is further from the nucleus in sodium than in potassium.
BThe relative atomic mass, Aᵣ, of lithium is higher than that of sodium.
CThere are more electrons in the valence shell of a potassium atom than in lithium.
DThere are more protons in an atom of potassium than in an atom of sodium.

Correct answer: D

Video Solution

Explanation

Answer: D. Down Group I (Li → Na → K), each successive element adds one occupied electron shell, so atomic radius and number of shells both increase. Potassium has 19 protons; sodium has 11 — option D states the only true comparison.

A — wrong: K is below Na, so K has more shells, and K's outer electron is further from the nucleus than Na's, not the other way round.
B — wrong: \(A_r(\text{Li}) = 7\); \(A_r(\text{Na}) = 23\). Lithium's \(A_r\) is much lower than sodium's, not higher.
C — wrong: every Group I element has exactly 1 valence electron. Li, Na, K all share this — K does not have more.

Only D is correct. Going down a group, proton number always increases — this is the fundamental ordering of the Periodic Table.

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